Eusociality

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Eusociality

 

Co-operative brood rearing, seen here in honeybees, is a condition of eusociality.

 

Eusociality (from Greek εὖ eu "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society which are sometimes called castes. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform at least one behavior characteristic of individuals in another caste.

Eusociality exists in certain insects, crustaceans and mammals. It is mostly observed and studied in the Hymenoptera (ants, bees, and wasps) and in Isoptera (termites). A colony has caste differences: Queens and reproductive males take the roles of the sole reproducers, while soldiers and workers work together to create a living situation favorable for the brood. In addition to Hymenoptera and Isoptera, there are two known eusocial vertebrates among rodents: the naked mole-rat and the Damaraland mole-rat. Some shrimps, such as Synalpheus regalis, are also eusocial. E. O. Wilson and others have claimed that humans have evolved a weak form of eusociality (e.g., with menopause), but these arguments have been disputed.

History

The term "eusocial" was introduced in 1966 by Suzanne Batra, who used it to describe nesting behavior in Halictine bees. Batra observed the cooperative behavior of the bees, males and females alike, as they took responsibility for at least one duty (i.e., burrowing, cell construction, oviposition) within the colony. The cooperativeness was essential as the activity of one labor division greatly influenced the activity of another.

For example, the size of pollen balls, a source of food, depended on when the egg-laying females oviposited. If the provisioning by pollen collectors was incomplete by the time the egg-laying female occupied a cell and oviposited, the size of the pollen balls would be small, leading to small offspring. Batra applied this term to species in which a colony is started by a single individual. Batra described other species, wherein the founder is accompanied by numerous helpers—as in a swarm of bees or ants—as "hypersocial". 

In 1969, Charles D. Michener further expanded Batra’s classification with his comparative study of social behavior in bees. He observed multiple species of bees (Apoidea) in order to investigate the different levels of animal sociality, all of which are different stages that a colony may pass through. Eusociality, which is the highest level of animal sociality a species can attain, specifically had three characteristics that distinguished it from the other levels:

1. "Egg-layers and worker-like individuals among adult females" (division of labor)
2. The overlap of generations (mother and adult offspring)
3. Cooperative work on the cells of the bees' honeycomb

Weaver ants, here collaborating to pull nest leaves together, can be considered primitively eusocial, as they do not have permanent division of labour.

 

E. O. Wilson then extended the terminology to include other social insects, such as ants, wasps, and termites. Originally, it was defined to include organisms (only invertebrates) that had the following three features:

1. Reproductive division of labor (with or without sterile castes)
2. Overlapping generations
3. Cooperative care of young

As eusociality became a recognized widespread phenomenon, however, it was also discovered in a group of chordates, the mole-rats. Further research also distinguished another possibly important criterion for eusociality known as "the point of no return". This is characterized by eusocial individuals that become fixed into one behavioral group, which usually occurs before reproductive maturity. This prevents them from transitioning between behavioral groups and creates an animal society that is truly dependent on each other for survival and reproductive success. For many insects, this irreversibility has changed the anatomy of the worker caste, which is sterile and provides support for the reproductive caste.

Taxonomic range

Most eusocial societies exist in arthropods, while a few are found in mammals.

In insects

A swarming meat-eater ant colony

The order Hymenoptera contains the largest group of eusocial insects, including ants, bees, and wasps—those with reproductive "queens" and more or less sterile "workers" and/or "soldiers" that perform specialized tasks. For example, in the well-studied social wasp Polistes versicolor, dominant females perform tasks such as building new cells and ovipositing, while subordinate females tend to perform tasks like feeding the larvae and foraging. The task differentiation between castes can be seen in the fact that subordinates complete 81.4% of the total foraging activity, while dominants only complete 18.6% of the total foraging. Eusocial species with a sterile caste are sometimes called hypersocial.

While only a moderate percentage of species in bees (families Apidae and Halictidae) and wasps (Crabronidae and Vespidae) are eusocial, nearly all species of ants (Formicidae) are eusocial. Some major lineages of wasps are mostly or entirely eusocial, including the subfamilies Polistinae and Vespinae. The corbiculate bees (subfamily Apinae of family Apidae) contain four tribes of varying degrees of sociality: the highly eusocial Apini (honey bees) and Meliponini (stingless bees), primitively eusocial Bombini (bumble bees), and the mostly solitary or weakly social Euglossini (orchid bees). Eusociality in these families is sometimes managed by a set of pheromones that alter the behavior of specific castes in the colony. These pheromones may act across different species, as observed in Apis andreniformis (black dwarf honey bee), where worker bees responded to queen pheromone from the related Apis florea (red dwarf honey bee). Pheromones are sometimes used in these castes to assist with foraging. Workers of the Australian stingless bee Tetragonula carbonaria, for instance, mark food sources with a pheromone, helping their nest mates to find the food.

Reproductive specialization generally involves the production of sterile members of the species, which carry out specialized tasks to care for the reproductive members. It can manifest in the appearance of individuals within a group whose behavior or morphology is modified for group defense, including self-sacrificing behavior ("altruism"). An example of a species whose sterile caste displays this altruistic behavior is Myrmecocystus mexicanus, one of the species of honey ant. Select sterile workers fill their abdomens with liquid food until they become immobile and hang from the ceilings of the underground nests, acting as food storage for the rest of the colony. Not all social species of insects have distinct morphological differences between castes. For example, in the Neotropical social wasp Synoeca surinama, social displays determine the caste ranks of individuals in the developing brood. These castes are sometimes further specialized in their behavior based on age. For example, Scaptotrigona postica workers assume different roles in the nest based on their age. Between approximately 0–40 days old, the workers perform tasks within the nest such as provisioning cell broods, colony cleaning, and nectar reception and dehydration. Once older than 40 days, Scaptotrigona postica workers move outside of the nest to practice colony defense and foraging.

In Lasioglossum aeneiventre, a halictid bee from Central America, nests may be headed by more than one female; such nests have more cells, and the number of active cells per female is correlated with the number of females in the nest, implying that having more females leads to more efficient building and provisioning of cells. In similar species with only one queen, such as Lasioglossum malachurum in Europe, the degree of eusociality depends on the clime in which the species is found.

Termites (order Blattodea, infraorder Isoptera) make up another large portion of highly advanced eusocial animals. The colony is differentiated into various castes: the queen and king are the sole reproducing individuals; workers forage and maintain food and resources; and soldiers defend the colony against ant attacks. The latter two castes, which are sterile and perform highly specialized, complex social behaviors, are derived from different stages of pluripotent larvae produced by the reproductive caste. Some soldiers have jaws so enlarged (specialized for defense and attack) that they are unable to feed themselves and must be fed by workers.

Austroplatypus incompertus is a species of ambrosia beetle native to Australia, and is the first beetle (order Coleoptera) to be recognized as eusocial. This species forms colonies in which a single female is fertilized, and is protected by many unfertilized females, which also serve as workers excavating tunnels in trees. This species also participates in cooperative brood care, in which individuals care for juveniles that are not their own.

Some species of gall-inducing insects, including the gall-forming aphid, Pemphigus spyrothecae (order Hemiptera), and thrips (order Thysanoptera), were also described as eusocial. These species have very high relatedness among individuals due to their partially asexual mode of reproduction (sterile soldier castes being clones of the reproducing female), but the gall-inhabiting behavior gives these species a defensible resource that sets them apart from related species with similar genetics. They produce soldier castes capable of fortress defense and protection of their colony against both predators and competitors. In these groups, therefore, high relatedness alone does not lead to the evolution of social behavior, but requires that groups occur in a restricted, shared area. These species have morphologically distinct soldier castes that defend against kleptoparasites (parasitism by theft) and are able to reproduce parthenogenetically (without fertilization).

In crustaceans

Eusociality has also arisen three different times among some crustaceans that live in separate colonies. Synalpheus regalis, Synalpheus filidigitus, and Synalpheus chacei, three species of parasitic shrimp that rely on fortress defense and live in groups of closely related individuals in tropical reefs and sponges, live eusocially with a single breeding female and a large number of male defenders, armed with enlarged snapping claws. As with other eusocial societies, there is a single shared living space for the colony members, and the non-breeding members act to defend it.

The fortress defense hypothesis additionally points out that because sponges provide both food and shelter, there is an aggregation of relatives (because the shrimp do not have to disperse to find food), and much competition for those nesting sites. Being the target of attack promotes a good defense system (soldier caste); soldiers therefore promote the fitness of the whole nest by ensuring safety and reproduction of the queen.

Eusociality offers a competitive advantage in shrimp populations. Eusocial species were found to be more abundant, occupy more of the habitat, and use more of the available resources than non-eusocial species. Other studies add to these findings by pointing out that cohabitation was more rare than expected by chance, and that most sponges were dominated by one species, which was frequently eusocial.

In nonhuman mammals

Naked mole-rat, one of two eusocial species in the Bathyergidae

 

Among mammals, eusociality is known in two species in the Bathyergidae, the naked mole-rat (Heterocephalus glaber) and the Damaraland mole-rat (Fukomys damarensis), both of which are highly inbred. Usually living in harsh or limiting environments, these mole-rats aid in raising siblings and relatives born to a single reproductive queen. However, this classification is controversial owing to disputed definitions of 'eusociality'. To avoid inbreeding, mole rats sometimes outbreed and establish new colonies when resources are sufficient. Most of the individuals cooperatively care for the brood of a single reproductive female (the queen) to which they are most likely related. Thus, it is uncertain whether mole rats classify as true eusocial organisms, since their social behavior depends largely on their resources and environment.

Some mammals in the Carnivora and Primates exhibit eusocial tendencies, especially meerkats (Suricata suricatta) and dwarf mongooses (Helogale parvula). These show cooperative breeding and marked reproductive skews. In the dwarf mongoose, the breeding pair receives food priority and protection from subordinates and rarely has to defend against predators.

In humans

An early 21st century debate focused on whether humans are prosocial or eusocial. Edward O. Wilson called humans eusocial apes, arguing for similarities to ants, and observing that early hominins cooperated to rear their children while other members of the same group hunted and foraged. Wilson argued that through cooperation and teamwork, ants and humans form superorganisms. Wilson's claims were vigorously rejected because they were based on group selection and reproductive division of labour in humans. However, it has been claimed that suicide, male homosexuality, and female menopause evolved through kin selection, which, if true, would by some definitions make humans eusocial.

Evolution

Phylogenetic distribution

Eusociality is a rare but widespread phenomenon in species in at least seven orders in the animal kingdom, as shown in the phylogenetic tree (non-eusocial groups not shown). All species of termites are eusocial, and it is believed that they were the first eusocial animals to evolve, sometime in the upper Jurassic period (~150 million years ago). The other orders shown also contain non-eusocial species, including many lineages where eusociality was inferred to be the ancestral state. Thus the number of independent evolutions of eusociality is still under investigation.

Paradox

Prior to the gene-centered view of evolution, eusociality was seen as an apparent evolutionary paradox: if adaptive evolution unfolds by differential reproduction of individual organisms, how can individuals incapable of passing on their genes evolve and persist? In On the Origin of Species, Darwin referred to the existence of sterile castes as the "one special difficulty, which at first appeared to me insuperable, and actually fatal to my theory". Darwin anticipated that a possible resolution to the paradox might lie in the close family relationship, which W.D. Hamilton quantified a century later with his 1964 inclusive fitness theory. After the gene-centered view of evolution was developed in the mid 1970s, non-reproductive individuals were seen as an extended phenotype of the genes, which are the primary beneficiaries of natural selection.

Inclusive fitness and haplodiploidy

According to inclusive fitness theory, organisms can gain fitness not just through increasing their own reproductive output, but also via increasing the reproductive output of other individuals that share their genes, especially their close relatives. Individuals are selected to help their relatives when the cost of helping is less than the benefit gained by their relative multiplied by the fraction of genes that they share, i.e. when Cost < relatedness * Benefit. Under inclusive fitness theory, the necessary conditions for eusociality to evolve are more easily fulfilled by haplodiploid species because of their unusual relatedness structure. 

In haplodiploid species, females develop from fertilized eggs and males develop from unfertilized eggs. Because a male is haploid, his daughters share 100% of his genes and 50% of their mother's. Therefore, they share 75% of their genes with each other. This mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters" which are more related to their sisters than they would be to their own offspring. Even though workers often do not reproduce, they can potentially pass on more of their genes by helping to raise their sisters than they would by having their own offspring (each of which would only have 50% of their genes). This unusual situation, where females may have greater fitness when they help rear siblings rather than producing offspring, is often invoked to explain the multiple independent evolutions of eusociality (arising at least nine separate times) within the haplodiploid group Hymenoptera. While females share 75% of genes with their sisters in haplodiploid populations, they only share 25% of their genes with their brothers. Accordingly, the average relatedness of an individual to their sibling is 50%. Therefore, helping behavior is only advantageous if it is biased to helping sisters, which would drive the population to a 1:3 sex ratio of males to females. At this ratio, males, as the rarer sex, increase in reproductive value, negating the benefit of female-biased investment. 

However, not all eusocial species are haplodiploid (termites, some snapping shrimps, and mole rats are not). Conversely, many bees are haplodiploid yet are not eusocial, and among eusocial species many queens mate with multiple males, resulting in a hive of half-sisters that share only 25% of their genes. The association between haplodiploidy and eusociality is below statistical significance. Haplodiploidy alone is thus neither necessary nor sufficient for eusociality to emerge. However relatedness does still play a part, as monogamy (queens mating singly) has been shown to be the ancestral state for all eusocial species so far investigated. If kin selection is an important force driving the evolution of eusociality, monogamy should be the ancestral state, because it maximizes the relatedness of colony members. 

Ecology

Many scientists citing the close phylogenetic relationships between eusocial and non-eusocial species are making the case that environmental factors are especially important in the evolution of eusociality. The relevant factors primarily involve the distribution of food and predators.

Increased parasitism and predation rates are the primary ecological drivers of social organization. Group living affords colony members defense against enemies, specifically predators, parasites, and competitors, and allows them to gain advantage from superior foraging methods.

With the exception of some aphids and thrips, all eusocial species live in a communal nest which provides both shelter and access to food resources. Mole rats, many bees, most termites, and most ants live in burrows in the soil; wasps, some bees, some ants, and some termites build above-ground nests or inhabit above-ground cavities; thrips and aphids inhabit galls (neoplastic outgrowths) induced on plants; ambrosia beetles and some termites nest together in dead wood; and snapping shrimp inhabit crevices in marine sponges. For many species the habitat outside the nest is often extremely arid or barren, creating such a high cost to dispersal that the chance to take over the colony following parental death is greater than the chance of dispersing to form a new colony. Defense of such fortresses from both predators and competitors often favors the evolution of non-reproductive soldier castes, while the high costs of nest construction and expansion favor non-reproductive worker castes.

The importance of ecology is supported by evidence such as experimentally induced reproductive division of labor, for example when normally solitary queens are forced together. Conversely, female Damaraland mole-rats undergo hormonal changes that promote dispersal after periods of high rainfall, supporting the plasticity of eusocial traits in response to environmental cues. 

Climate also appears to be a selective agent driving social complexity; across bee lineages and Hymenoptera in general, higher forms of sociality are more likely to occur in tropical than temperate environments. Similarly, social transitions within halictid bees, where eusociality has been gained and lost multiple times, are correlated with periods of climatic warming. Social behavior in facultative social bees is often reliably predicted by ecological conditions, and switches in behavioral type have been experimentally induced by translocating offspring of solitary or social populations to warm and cool climates. In H. rubicundus, females produce a single brood in cooler regions and two or more broods in warmer regions, so the former populations are solitary while the latter are social. In another species of sweat bees, L. calceatum, social phenotype has been predicted by altitude and micro-habitat composition, with social nests found in warmer, sunnier sites, and solitary nests found in adjacent, cooler, shaded locations. Facultatively social bee species, however, which comprise the majority of social bee diversity, have their lowest diversity in the tropics, being largely limited to temperate regions.

Multilevel selection

Once pre-adaptations such as group formation, nest building, high cost of dispersal, and morphological variation are present, between-group competition has been cited as a quintessential force in the transition to advanced eusociality. Because the hallmarks of eusociality will produce an extremely altruistic society, such groups will out-reproduce their less cooperative competitors, eventually eliminating all non-eusocial groups from a species. Multilevel selection has however been heavily criticized by some for its conflict with the kin selection theory.

Reversal to solitarity

A reversal to solitarity is an evolutionary phenomenon in which descendants of a eusocial group evolve solitary behavior once again. Bees have been model organisms for the study of reversal to solitarity, because of the diversity of their social systems. Each of the four origins of eusociality in bees was followed by at least one reversal to solitarity, giving a total of at least nine reversals.^[6][7] This suggests that eusociality is costly to maintain, and can only persist when ecological variables favor it. Disadvantages of eusociality include the cost of investing in non-reproductive offspring, and an increased risk of disease.

All reversals to solitarity have occurred among primitively eusocial groups; none have followed the emergence of advanced eusociality. The "point of no return" hypothesis posits that the morphological differentiation of reproductive and non-reproductive castes prevents highly eusocial species such as the honeybee from reverting to the solitary state.

Physiological and developmental mechanisms

An understanding of the physiological causes and consequences of the eusocial condition has been somewhat slow; nonetheless, major advancements have been made in learning more about the mechanistic and developmental processes that lead to eusociality.

Involvement of pheromones

Pheromones are thought to play an important role in the physiological mechanisms underlying the development and maintenance of eusociality. In fact the evolution of enzymes involved both in the production and perception of pheromones has been shown to be important for the emergence of eusociality both within termites and in Hymenoptera. The most well-studied queen pheromone system in social insects is that of the honey bee Apis mellifera. Queen mandibular glands were found to produce a mixture of five compounds, three aliphatic and two aromatic, which have been found to control workers. Mandibular gland extracts inhibit workers from constructing queen cells in which new queens are reared which can delay the hormonally based behavioral development of workers and can suppress ovarian development in workers. Both behavioral effects mediated by the nervous system often leading to recognition of queens (releaser) and physiological effects on the reproductive and endocrine system (primer) are attributed to the same pheromones. These pheromones volatilize or are deactivated within thirty minutes, allowing workers to respond rapidly to the loss of their queen.

The levels of two of the aliphatic compounds increase rapidly in virgin queens within the first week after eclosion (emergence from the pupal case), which is consistent with their roles as sex attractants during the mating flight. It is only after a queen is mated and begins laying eggs, however, that the full blend of compounds is made. The physiological factors regulating reproductive development and pheromone production are unknown.

In several ant species, reproductive activity has also been associated with pheromone production by queens. In general, mated egg laying queens are attractive to workers whereas young winged virgin queens, which are not yet mated, elicit little or no response. However, very little is known about when pheromone production begins during the initiation of reproductive activity or about the physiological factors regulating either reproductive development or queen pheromone production in ants.

Among ants, the queen pheromone system of the fire ant Solenopsis invicta is particularly well studied. Both releaser and primer pheromones have been demonstrated in this species. A queen recognition (releaser) hormone is stored in the poison sac along with three other compounds. These compounds were reported to elicit a behavioral response from workers. Several primer effects have also been demonstrated. Pheromones initiate reproductive development in new winged females, called female sexuals. These chemicals also inhibit workers from rearing male and female sexuals, suppress egg production in other queens of multiple queen colonies and cause workers to execute excess queens. The action of these pheromones together maintains the eusocial phenotype which includes one queen supported by sterile workers and sexually active males (drones). In queenless colonies that lack such pheromones, winged females will quickly shed their wings, develop ovaries and lay eggs. These virgin replacement queens assume the role of the queen and even start to produce queen pheromones. There is also evidence that queen weaver ants Oecophylla longinoda have a variety of exocrine glands that produce pheromones, which prevent workers from laying reproductive eggs.

Similar mechanisms are used for the eusocial wasp species Vespula vulgaris. In order for a Vespula vulgaris queen to dominate all the workers, usually numbering more than 3000 in a colony, she exerts pheromone to signal her dominance. The workers were discovered to regularly lick the queen while feeding her, and the air-borne pheromone from the queen's body alerts those workers of her dominance.

The mode of action of inhibitory pheromones which prevent the development of eggs in workers has been convincingly demonstrated in the bumble bee Bombus terrestris. In this species, pheromones suppress activity of the corpora allata and juvenile hormone (JH) secretion. The corpora allata is an endocrine gland that produces JH, a group of hormones that regulate many aspects of insect physiology. With low JH, eggs do not mature. Similar inhibitory effects of lowering JH were seen in halictine bees and polistine wasps, but not in honey bees.

Other strategies

A variety of strategies in addition to the use of pheromones have evolved that give the queens of different species of social insects a measure of reproductive control over their nest mates. In many Polistes wasp colonies, monogamy is established soon after colony formation by physical dominance interactions among foundresses of the colony including biting, chasing and food soliciting. Such interactions created a dominance hierarchy headed by individuals with the greatest ovarian development. Larger, older individuals often have an advantage during the establishment of dominance hierarchies. The rank of subordinates is positively correlated with the degree of ovarian development and the highest ranking individual usually becomes queen if the established queen disappears. Workers do not oviposit when queens are present because of a variety of reasons: colonies tend to be small enough that queens can effectively dominate workers, queens practice selective oophagy or egg eating, or the flow of nutrients favors queen over workers and queens rapidly lay eggs in new or vacated cells. However, it is also possible that morphological differences favor the worker. In certain species of wasps, such as Apoica flavissima queens are smaller than their worker counterparts. This can lead to interesting worker-queen dynamics, often with the worker policing queen behaviors. Other wasps, like Polistes instabilis have workers with the potential to develop into reproductives, but only in cases where there are no queens to suppress them. 

In primitively eusocial bees (where castes are morphologically similar and colonies usually small and short-lived), queens frequently nudge their nest mates and then burrow back down into the nest. This behavior draws workers into the lower part of the nest where they may respond to stimuli for cell construction and maintenance. Being nudged by the queen may play a role in inhibiting ovarian development and this form of queen control is supplemented by oophagy of worker laid eggs. Furthermore, temporally discrete production of workers and gynes (actual or potential queens) can cause size dimorphisms between different castes as size is strongly influenced by the season during which the individual is reared. In many wasp species worker caste determination is characterized by a temporal pattern in which workers precede non-workers of the same generation. In some cases, for example in the bumble bee, queen control weakens late in the season and the ovaries of workers develop to an increasing extent. The queen attempts to maintain her dominance by aggressive behavior and by eating worker laid eggs; her aggression is often directed towards the worker with the greatest ovarian development.

In highly eusocial wasps (where castes are morphologically dissimilar), both the quantity and quality of food seem to be important for caste differentiation. Recent studies in wasps suggest that differential larval nourishment may be the environmental trigger for larval divergence into one of two developmental classes destined to become either a worker or a gyne. All honey bee larvae are initially fed with royal jelly, which is secreted by workers, but normally they are switched over to a diet of pollen and honey as they mature; if their diet is exclusively royal jelly, however, they grow larger than normal and differentiate into queens. This jelly seems to contain a specific protein, designated as royalactin, which increases body size, promotes ovary development and shortens the developmental time period. Furthermore, the differential expression in Polistes of larval genes and proteins (also differentially expressed during queen versus caste development in honey bees) indicate that regulatory mechanisms may occur very early in development.

Ploidy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Ploidy

A haploid set that consists of a single complete set of chromosomes (equal to the monoploid set), as shown in the picture above, must belong to a diploid species. If a haploid set consists of two sets, it must be of a tetraploid (four sets) species.

 

Ploidy (/ˈplɔɪdi/) is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Somatic cells, tissues, and individual organisms can be described according to the number of sets of chromosomes present (the "ploidy level"): monoploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term polyploid is often used to describe cells with three or more chromosome sets.

Virtually all sexually reproducing organisms are made up of somatic cells that are diploid or greater, but ploidy level may vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism's life cycle. Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid. Many animals are uniformly diploid, though polyploidy is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in the social insects), and in others entire tissues and organ systems may be polyploid despite the rest of the body being diploid (as in the mammalian liver). For many organisms, especially plants and fungi, changes in ploidy level between generations are major drivers of speciation. In mammals and birds, ploidy changes are typically fatal. There is, however, evidence of polyploidy in organisms now considered to be diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive rounds of polyploidization and rediploidization.

Humans are diploid organisms, carrying two complete sets of chromosomes in their somatic cells: one set of 23 chromosomes from their father and one set of 23 chromosomes from their mother. The two sets combined provide a full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called the chromosome number. The number of chromosomes found in a single complete set of chromosomes is called the monoploid number (x). The haploid number (n) refers to the total number of chromosomes found in a gamete (a sperm or egg cell produced by meiosis in preparation for sexual reproduction). Under normal conditions, the haploid number is exactly half the total number of chromosomes present in the organism's somatic cells. For diploid organisms, the monoploid number and haploid number are equal; in humans, both are equal to 23. When a human germ cell undergoes meiosis, the diploid 46-chromosome complement is split in half to form haploid gametes. After fusion of a male and a female gamete (each containing 1 set of 23 chromosomes) during fertilization, the resulting zygote again has the full complement of 46 chromosomes: 2 sets of 23 chromosomes.

Etymology

The term ploidy is a back-formation from haploidy and diploidy. "Ploid" is a combination of Ancient Greek -πλόος (-plóos, “-fold”) and -ειδής (-eidḗs), from εἶδος (eîdos, "form, likeness"). The principal meaning of the Greek word ᾰ̔πλόος (haplóos) is "single", from ἁ- (ha-, “one, same”). διπλόος (diplóos) means "duplex" or "two-fold". Diploid therefore means "duplex-shaped" (compare "humanoid", "human-shaped").

Polish botanist Eduard Strasburger coined the terms haploid and diploid in 1905. Some authors suggest that Strasburger based the terms on August Weismann's conception of the id (or germ plasm), hence haplo-id and diplo-id. The two terms were brought into the English language from German through William Henry Lang's 1908 translation of a 1906 textbook by Strasburger and colleagues.

Types of ploidy

Haploid and monoploid

A comparison of sexual reproduction in predominantly haploid organisms and predominantly diploid organisms.

1) A haploid organism is on the left and a diploid organism is on the right.
2 and 3) Haploid egg and sperm carrying the dominant purple gene and the recessive blue gene, respectively. These gametes are produced by simple mitosis of cells in the germ line.
4 and 5) Diploid sperm and egg carrying the recessive blue gene and the dominant purple gene, respectively. These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells.
6) The short-lived diploid state of haploid organisms, a zygote generated by the union of two haploid gametes during sex.
7) The diploid zygote which has just been fertilized by the union of haploid egg and sperm during sex.
8) Cells of the diploid structure quickly undergo meiosis to produce spores containing the meiotically halved number of chromosomes, restoring haploidy. These spores express either the mother's dominant gene or the father's recessive gene and proceed by mitotic division to build a new entirely haploid organism.
9) The diploid zygote proceeds by mitotic division to build a new entirely diploid organism. These cells possess both the purple and blue genes, but only the purple gene is expressed since it is dominant over the recessive blue gene.

The term haploid is used with two distinct but related definitions. In the most generic sense, haploid refers to having the number of sets of chromosomes normally found in a gamete. Because two gametes necessarily combine during sexual reproduction to form a single zygote from which somatic cells are generated, healthy gametes always possess exactly half the number of sets of chromosomes found in the somatic cells, and therefore "haploid" in this sense refers to having exactly half the number of sets of chromosomes found in a somatic cell. By this definition, an organism whose gametic cells contain a single copy of each chromosome (one set of chromosomes) may be considered haploid while the somatic cells, containing two copies of each chromosome (two sets of chromosomes), are diploid. This scheme of diploid somatic cells and haploid gametes is widely used in the animal kingdom and is the simplest to illustrate in diagrams of genetics concepts. But this definition also allows for haploid gametes with more than one set of chromosomes. As given above, gametes are by definition haploid, regardless of the actual number of sets of chromosomes they contain. An organism whose somatic cells are tetraploid (four sets of chromosomes), for example, will produce gametes by meiosis that contain two sets of chromosomes. These gametes might still be called haploid even though they are numerically diploid. 

An alternative usage defines "haploid" as having a single copy of each chromosome – that is, one and only one set of chromosomes. In this case, the nucleus of a eukaryotic cell is only said to be haploid if it has a single set of chromosomes, each one not being part of a pair. By extension a cell may be called haploid if its nucleus has one set of chromosomes, and an organism may be called haploid if its body cells (somatic cells) have one set of chromosomes per cell. By this definition haploid therefore would not be used to refer to the gametes produced by the tetraploid organism in the example above, since these gametes are numerically diploid. The term monoploid is often used as a less ambiguous way to describe a single set of chromosomes; by this second definition, haploid and monoploid are identical and can be used interchangeably.

Gametes (sperm and ova) are haploid cells. The haploid gametes produced by most organisms combine to form a zygote with n pairs of chromosomes, i.e. 2n chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be homologous. Cells and organisms with pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. During meiosis, sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another.

All plants and many fungi and algae switch between a haploid and a diploid state, with one of the stages emphasized over the other. This is called alternation of generations. Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants like mosses. More recently evolved plants, like the gymnosperms and angiosperms, spend the majority of their life cycle in the diploid stage. Most animals are diploid, but male bees, wasps, and ants are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their system haplodiploid. 

In some cases there is evidence that the n chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes. This "base" number – the number of apparently originally unique chromosomes in a haploid set – is called the monoploid number, also known as basic or cardinal number, or fundamental number. As an example, the chromosomes of common wheat are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. The monoploid number is thus 7 and the haploid number is 3 × 7 = 21. In general n is a multiple of x. The somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes. As a formula, for wheat 2n = 6x = 42, so that the haploid number n is 21 and the monoploid number x is 7. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes (n = 3x).

In the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated. In many other organisms, although the number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number. Thus in humans, x = n = 23.

Diploid

Diploid cells have two homologous copies of each chromosome, usually one from the mother and one from the father. All or nearly all mammals are diploid organisms. The suspected tetraploid (possessing four chromosome sets) plains viscacha rat (Tympanoctomys barrerae) and golden viscacha rat (Pipanacoctomys aureus) have been regarded as the only known exceptions (as of 2004). However, some genetic studies have rejected any polyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents. All normal diploid individuals have some small fraction of cells that display polyploidy. Human diploid cells have 46 chromosomes (the somatic number, 2n) and human haploid gametes (egg and sperm) have 23 chromosomes (n). Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid. Examples include human foamy virus, human T-lymphotropic virus, and HIV.

Polyploidy

Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms are triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), octoploid (8 sets), nonaploid (9 sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc. Some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets), though Greek terminology may be set aside for readability in cases of higher ploidy (such as "16-ploid"). Polytene chromosomes of plants and fruit flies can be 1024-ploid. Ploidy of systems such as the salivary gland, elaiosome, endosperm, and trophoblast can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm Bombyx mori.

The chromosome sets may be from the same species or from closely related species. In the latter case, these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from the hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreduced gametes, and not by diploid–diploid hybridization followed by chromosome doubling. The so-called Brassica triangle is an example of allopolyploidy, where three different parent species have hybridized in all possible pair combinations to produce three new species.

Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms, many somatic cells are polyploid due to a process called endoreduplication, where duplication of the genome occurs without mitosis (cell division). The extreme in polyploidy occurs in the fern genus Ophioglossum, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand.

It is possible for polyploid organisms to revert to lower ploidy by haploidisation.

In bacteria and archaea

Polyploidy is a characteristic of the bacterium Deinococcus radiodurans and of the archaeon Halobacterium salinarum. These two species are highly resistant to ionizing radiation and desiccation, conditions that induce DNA double-strand breaks. This resistance appears to be due to efficient homologous recombinational repair.

Variable or indefinite ploidy

Depending on growth conditions, prokaryotes such as bacteria may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because under exponential growth conditions the cells are able to replicate their DNA faster than they can divide.

In ciliates, the macronucleus is called ampliploid, because only part of the genome is amplified.

Mixoploidy

Mixoploidy is the case where two cell lines, one diploid and one polyploid, coexist within the same organism. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology.

Dihaploidy and polyhaploidy

Not to be confused with haplodiploidy (where diploid and haploid individuals are different sexes).

 

Dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i.e., by halving the chromosome constitution.

Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion.

The term "dihaploid" was coined by Bender to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense, but it has also been used for doubled monoploids or doubled haploids, which are homozygous and used for genetic research.

Euploidy and aneuploidy

Euploidy (Greek eu, "true" or "even") is the state of a cell or organism having one or more than one set of the same set of chromosomes, possibly excluding the sex-determining chromosomes. For example, most human cells have 2 of each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. Euploid karyotypes would consequentially be a multiple of the haploid number, which in humans is 23.

Aneuploidy is the state where one or more individual chromosomes of a normal set are absent or present in more than their usual number of copies (excluding the absence or presence of complete sets, which is considered euploidy). Unlike euploidy, aneuploid karyotypes will not be a multiple of the haploid number. In humans, examples of aneuploidy include having a single extra chromosome (as in Down syndrome, where affected individuals have three copies of chromosome 21) or missing a chromosome (as in Turner syndrome, where affected individuals are missing an X chromosome). Aneuploid karyotypes are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy and monosomy.

Homoploid

Homoploid means "at the same ploidy level", i.e. having the same number of homologous chromosomes. For example, homoploid hybridization is hybridization where the offspring have the same ploidy level as the two parental species. This contrasts with a common situation in plants where chromosome doubling accompanies or occurs soon after hybridization. Similarly, homoploid speciation contrasts with polyploid speciation.

Zygoidy and azygoidy

Zygoidy is the state in which the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid. In the azygoid state the chromosomes are unpaired. It may be the natural state of some asexual species or may occur after meiosis. In diploid organisms the azygoid state is monoploid. (See below for dihaploidy.)

Special cases

More than one nucleus per cell

In the strictest sense, ploidy refers to the number of sets of chromosomes in a single nucleus rather than in the cell as a whole. Because in most situations there is only one nucleus per cell, it is commonplace to speak of the ploidy of a cell, but in cases in which there is more than one nucleus per cell, more specific definitions are required when ploidy is discussed. Authors may at times report the total combined ploidy of all nuclei present within the cell membrane of a syncytium, though usually the ploidy of each nucleus is described individually. For example, a fungal dikaryon with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together.

Ancestral ploidy levels

It is possible on rare occasions for ploidy to increase in the germline, which can result in polyploid offspring and ultimately polyploid species. This is an important evolutionary mechanism in both plants and animals and is known as a primary driver of speciation. As a result, it may become desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. The number of chromosomes in the ancestral (non-homologous) set is called the monoploid number (x), and is distinct from the haploid number (n) in the organism as it now reproduces.

Common wheat (Triticum aestivum) is an organism in which x and n differ. Each plant has a total of six sets of chromosomes (with two sets likely having been obtained from each of three different diploid species that are its distant ancestors). The somatic cells are hexaploid, 2n = 6x = 42 (where the monoploid number x = 7 and the haploid number n = 21). The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, einkorn wheat.

Tetraploidy (four sets of chromosomes, 2n = 4x) is common in many plant species, and also occurs in amphibians, reptiles, and insects. For example, species of Xenopus (African toads) form a ploidy series, featuring diploid (X. tropicalis, 2n=20), tetraploid (X. laevis, 4n=36), octaploid (X. wittei, 8n=72), and dodecaploid (X. ruwenzoriensis, 12n=108) species.

Over evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less apparent by karyotype – for example, humans are generally regarded as diploid, but the 2R hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors.

Haplodiploidy

Ploidy can also vary between individuals of the same species or at different stages of the life cycle. In some insects it differs by caste. In humans, only the gametes are haploid, but in many of the social insects, including ants, bees, and termites, certain individuals develop from unfertilized eggs, making them haploid for their entire lives, even as adults. In the Australian bulldog ant, Myrmecia pilosula, a haplodiploid species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes. In Entamoeba, the ploidy level varies from 4n to 40n in a single population. Alternation of generations occurs in most plants, with individuals "alternating" ploidy level between different stages of their sexual life cycle.

Tissue-specific polyploidy

In large multicellular organisms, variations in ploidy level between different tissues, organs, or cell lineages are common. Because the chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote by mitosis. However, in many situations somatic cells double their copy number by means of endoreduplication as an aspect of cellular differentiation. For example, the hearts of two-year-old human children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei.

Adaptive and ecological significance of variation in ploidy

There is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels. A study comparing the karyotypes of endangered or invasive plants with those of their relatives found that being polyploid as opposed to diploid is associated with a 14% lower risk of being endangered, and a 20% greater chance of being invasive. Polyploidy may be associated with increased vigor and adaptability. Some studies suggest that selection is more likely to favor diploidy in host species and haploidy in parasite species.

When a germ cell with an uneven number of chromosomes undergoes meiosis, the chromosomes cannot be evenly divided between the daughter cells, resulting in aneuploid gametes. Triploid organisms, for instance, are usually sterile. Because of this, triploidy is commonly exploited in agriculture to produce seedless fruit such as bananas and watermelons. If the fertilization of human gametes results in three sets of chromosomes, the condition is called triploid syndrome.

Glossary of ploidy numbers

Term	Description
Ploidy number	Number of chromosome sets
Monoploid number (x)	Number of chromosomes found in a single complete set
Chromosome number	Total number of chromosomes in all sets combined
Zygotic number	Number of chromosomes in zygotic cells
Haploid or gametic number (n)	Number of chromosomes found in gametes
Diploid number	Chromosome number of a diploid organism
Tetraploid number	Chromosome number of a tetraploid organism

The common potato (Solanum tuberosum) is an example of a tetraploid organism, carrying four sets of chromosomes. During sexual reproduction, each potato plant inherits two sets of 12 chromosomes from the pollen parent, and two sets of 12 chromosomes from the ovule parent. The four sets combined provide a full complement of 48 chromosomes. The haploid number (half of 48) is 24. The monoploid number equals the total chromosome number divided by the ploidy level of the somatic cells: 48 chromosomes in total divided by a ploidy level of 4 equals a monoploid number of 12. Hence, the monoploid number (12) and haploid number (24) are distinct in this example.

However, commercial potato crops (as well as many other crop plants) are commonly propagated vegetatively (by asexual reproduction through mitosis), in which case new individuals are produced from a single parent, without the involvement of gametes and fertilization, and all the offspring are genetically identical to each other and to the parent, including in chromosome number. The parents of these vegetative clones may still be capable of producing haploid gametes in preparation for sexual reproduction, but these gametes are not used to create the vegetative offspring by this route.

Zygosity

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Zygosity

 

Homozygous and heterozygous

 

Zygosity (the noun, zygote, is from the Greek zygotos "yoked," from zygon "yoke") (/zaɪˈɡɒsɪti/) is the degree of similarity of the alleles for a trait in an organism. 

Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system. If both alleles of a diploid organism are the same, the organism is homozygous at that locus. If they are different, the organism is heterozygous at that locus. If one allele is missing, it is hemizygous, and, if both alleles are missing, it is nullizygous.

The DNA sequence of a gene often varies from one individual to another. Those variations are called alleles. While some genes have only one allele because there is low variation, others have only one allele because deviation from that allele can be harmful or fatal. But most genes have two or more alleles. The frequency of different alleles varies throughout the population. Some genes may have two alleles with equal distribution. For other genes, one allele may be common, and another allele may be rare. Sometimes, one allele is a disease-causing variation while the other allele is healthy. Sometimes, the different variations in the alleles make no difference at all in the function of the organism.

In diploid organisms, one allele is inherited from the male parent and one from the female parent. Zygosity is a description of whether those two alleles have identical or different DNA sequences. In some cases the term "zygosity" is used in the context of a single chromosome.

Types

The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous describes a genotype consisting of two identical alleles at a given locus, heterozygous describes a genotype consisting of two different alleles at a locus, hemizygous describes a genotype consisting of only a single copy of a particular gene in an otherwise diploid organism, and nullizygous refers to an otherwise-diploid organism in which both copies of the gene are missing.

Homozygous

A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant.

An individual that is homozygous-dominant for a particular trait carries two copies of the allele that codes for the dominant trait. This allele, often called the "dominant allele", is normally represented by a capital letter (such as "P" for the dominant allele producing purple flowers in pea plants). When an organism is homozygous-dominant for a particular trait, the genotype is represented by a doubling of the symbol for that trait, such as "PP".

An individual that is homozygous-recessive for a particular trait carries two copies of the allele that codes for the recessive trait. This allele, often called the "recessive allele", is usually represented by the lowercase form of the letter used for the corresponding dominant trait (such as, with reference to the example above, "p" for the recessive allele producing white flowers in pea plants). The genotype of an organism that is homozygous-recessive for a particular trait is represented by a doubling of the appropriate letter, such as "pp".

Heterozygous

A diploid organism is heterozygous at a gene locus when its cells contain two different alleles (one wild-type allele and one mutant allele) of a gene. The cell or organism is called a heterozygote specifically for the allele in question, and therefore, heterozygosity refers to a specific genotype. Heterozygous genotypes are represented by a capital letter (representing the dominant/wild-type allele) and a lowercase letter (representing the recessive/mutant allele), such as "Rr" or "Ss". Alternatively, a heterozygote for gene "R" is assumed to be "Rr". The capital letter is usually written first.

If the trait in question is determined by simple (complete) dominance, a heterozygote will express only the trait coded by the dominant allele, and the trait coded by the recessive allele will not be present. In more complex dominance schemes the results of heterozygosity can be more complex.

A heterozygous genotype can have a higher relative fitness than either the homozygous dominant or homozygous recessive genotype – this is called a heterozygote advantage.

Hemizygous

A chromosome in a diploid organism is hemizygous when only one copy is present.^[2] The cell or organism is called a hemizygote. Hemizygosity is also observed when one copy of a gene is deleted, or, in the heterogametic sex, when a gene is located on a sex chromosome. Hemizygosity must not be confused with haploinsufficiency, which describes a mechanism for producing a phenotype. For organisms in which the male is heterogametic, such as humans, almost all X-linked genes are hemizygous in males with normal chromosomes, because they have only one X chromosome and few of the same genes are on the Y chromosome. Transgenic mice generated through exogenous DNA microinjection of an embryo's pronucleus are also considered to be hemizygous, because the introduced allele is expected to be incorporated into only one copy of any locus. A transgenic individual can later be bred to homozygosity and maintained as an inbred line to reduce the need to confirm the genotype of each individual.

In cultured mammalian cells, such as the Chinese hamster ovary cell line, a number of genetic loci are present in a functional hemizygous state, due to mutations or deletions in the other alleles.

Nullizygous

A nullizygous organism carries two mutant alleles for the same gene. The mutant alleles are both complete loss-of-function or 'null' alleles, so homozygous null and nullizygous are synonymous. The mutant cell or organism is called a nullizygote.

Autozygous and allozygous

Zygosity may also refer to the origin(s) of the alleles in a genotype. When the two alleles at a locus originate from a common ancestor by way of nonrandom mating (inbreeding), the genotype is said to be autozygous. This is also known as being "identical by descent", or IBD. When the two alleles come from different sources (at least to the extent that the descent can be traced), the genotype is called allozygous. This is known as being "identical by state", or IBS.

Because the alleles of autozygous genotypes come from the same source, they are always homozygous, but allozygous genotypes may be homozygous too. Heterozygous genotypes are often, but not necessarily, allozygous because different alleles may have arisen by mutation some time after a common origin. Hemizygous and nullizygous genotypes do not contain enough alleles to allow for comparison of sources, so this classification is irrelevant for them.

Monozygotic and dizygotic twins

As discussed above, "zygosity" can be used in the context of a specific genetic locus (example). The word zygosity may also be used to describe the genetic similarity or dissimilarity of twins. Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are dizygotic because they develop from two separate eggs that are fertilized by two separate sperms.

Heterozygosity in population genetics

Heterozygosity values of 51 worldwide human populations. Sub-Saharan Africans have the highest values in the world.

 

In population genetics, the concept of heterozygosity is commonly extended to refer to the population as a whole, i.e., the fraction of individuals in a population that are heterozygous for a particular locus. It can also refer to the fraction of loci within an individual that are heterozygous.

Typically, the observed (${\displaystyle H_{o}}$) and expected (${\displaystyle H_{e}}$) heterozygosities are compared, defined as follows for diploid individuals in a population:

Observed

${\displaystyle H_{o}={\frac {\sum \limits _{i=1}^{n}{(1\ {\textrm {if}}\ a_{i1}\neq a_{i2})}}{n}}}$

where ${\displaystyle n}$ is the number of individuals in the population, and ${\displaystyle a_{i1},a_{i2}}$ are the alleles of individual ${\displaystyle i}$ at the target locus.

Expected

${\displaystyle H_{e}=1-\sum \limits _{i=1}^{m}{(f_{i})^{2}}}$

where ${\displaystyle m}$ is the number of alleles at the target locus, and ${\displaystyle f_{i}}$ is the allele frequency of the ${\displaystyle i^{th}}$ allele at the target locus.

Supercritical water reactor

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Supercritical_water_reactor

 

Supercritical water reactor scheme.

 

The supercritical water reactor (SCWR) is a concept Generation IV reactor, mostly designed as light water reactor (LWR) that operates at supercritical pressure (i.e. greater than 22.1 MPa). The term critical in this context refers to the critical point of water, and must not be confused with the concept of criticality of the nuclear reactor. 

The water heated in the reactor core becomes a supercritical fluid above the critical temperature of 374 °C, transitioning from a fluid more resembling liquid water to a fluid more resembling saturated steam (which can be used in a steam turbine), without going through the distinct phase transition of boiling.

In contrast, the well-established pressurized water reactors (PWR) have a primary cooling loop of liquid water at a subcritical pressure, transporting heat from the reactor core to a secondary cooling loop, where the steam for driving the turbines is produced in a boiler (called the steam generator). Boiling water reactors (BWR) operate at even lower pressures, with the boiling process to generate the steam happening in the reactor core.

The supercritical steam generator is a proven technology. The development of SCWR systems is considered a promising advancement for nuclear power plants because of its high thermal efficiency (~45 % vs. ~33 % for current LWRs) and simpler design. As of 2012 the concept was being investigated by 32 organizations in 13 countries.

History

The super-heated steam cooled reactors operating at subcritical-pressure were experimented with in both Soviet Union and in the United States as early as the 1950s and 1960s such as Beloyarsk Nuclear Power Station, Pathfinder and Bonus of GE's Operation Sunrise program. These are not SCWRs. SCWRs were developed from the 1990s onwards. Both a LWR-type SCWR with a reactor pressure vessel and a CANDU-type SCWR with pressure tubes are being developed.

A 2010 book includes conceptual design and analysis methods such as core design, plant system, plant dynamics and control, plant startup and stability, safety, fast reactor design etc.

A 2013 document saw the completion of a prototypical fueled loop test in 2015. A Fuel Qualification Test was completed in 2014.

A 2014 book saw reactor conceptual design of a thermal spectrum reactor (Super LWR) and a fast reactor (Super FR) and experimental results of thermal hydraulics, materials and material-coolant interactions.

Design

Moderator-coolant

The SCWR operates at supercritical pressure. The reactor outlet coolant is supercritical water. Light water is used as a neutron moderator and coolant. Above the critical point, steam and liquid become the same density and are indistinguishable, eliminating the need for pressurizers and steam generators (PWR), or jet/recirculation pumps, steam separators and dryers (BWR). Also by avoiding boiling, SCWR does not generate chaotic voids (bubbles) with less density and moderating effect. In a LWR this can affect heat transfer and water flow, and the feedback can make the reactor power harder to predict and control. Neutronic and thermal hydraulic coupled calculation is needed to predict the power distribution. SCWR's simplification should reduce construction costs and improve reliability and safety. A LWR type SCWR adopts water rods with thermal insulation and A CANDU type SCWR keeps water moderator in a Calandria tank. A fast reactor core of the LWR type SCWR adopts tight fuel rod lattice as a high conversion LWR. The fast neutron spectrum SCWR has advantages of a higher power density, but needs plutonium and uranium mixed oxides fuel which will be available from reprocessing.　

Control

SCWRs would likely have control rods inserted through the top, as is done in PWRs.

Material

The conditions inside an SCWR are harsher than those in LWRs, LMFBRs, and supercritical fossil fuel plants (with which much experience has been gained, though this does not include the combination of harsh environment and intense neutron radiation). SCWRs need a higher standard of core materials (especially fuel cladding) than either of these. R&D focuses on:

• The chemistry of supercritical water under radiation (preventing stress corrosion cracking, and maintaining corrosion resistance under neutron radiation and high temperatures)
• Dimensional and microstructural stability (preventing embrittlement, retaining strength and creep resistance also under radiation and high temperatures)
• Materials that both resist the harsh conditions and do not absorb too many neutrons, which affects fuel economy

Advantages

• Supercritical water has excellent heat transfer properties allowing a high power density, a small core, and a small containment structure.
• The use of a supercritical Rankine cycle with its typically higher temperatures improves efficiency (would be ~45 % versus ~33 % of current PWR/BWRs).
• This higher efficiency would lead to better fuel economy and a lighter fuel load, lessening residual (decay) heat.
• SCWR is typically designed as a direct-cycle, whereby steam or hot supercritical water from the core is used directly in a steam turbine. This makes the design simple. As a BWR is simpler than a PWR, a SCWR is a lot simpler and more compact than a less-efficient BWR having the same electrical output. There are no steam separators, steam dryers, internal recirculation pumps, or recirculation flow inside the pressure vessel. The design is a once-through, direct-cycle, the simplest type of cycle possible. The stored thermal and radiologic energy in the smaller core and its (primary) cooling circuit would also be less than that of either a BWR's or a PWR's.
• Water is liquid at room temperature, cheap, non-toxic and transparent, simplifying inspection and repair (compared to liquid metal cooled reactors).
• A fast SCWR could be a breeder reactor, like the proposed Clean And Environmentally Safe Advanced Reactor, and could burn the long-lived actinide isotopes.
• A heavy-water SCWR could breed fuel from thorium (4x more abundant than uranium), with increased proliferation resistance over plutonium breeders.

Disadvantages

• Lower water inventory (due to compact primary loop) means less heat capacity to buffer transients and accidents (e.g. loss of feedwater flow or large break loss-of-coolant accident) resulting in accident and transient temperatures that are too high for conventional metallic cladding.

However, Safety analysis of LWR type SCWR showed that safety criteria are met at accidents and abnormal transients including total loss of flow and loss of coolant accident. No double ended break occurs because of the once-through coolant cycle. Core is cooled by the induced flow at the loss of coolant accident.

• Higher pressure combined with higher temperature and also a higher temperature rise across the core (compared to PWR/BWRs) result in increased mechanical and thermal stresses on vessel materials that are difficult to solve. A LWR type design, reactor pressure vessel inner wall is cooled by the inlet coolant as a PWR. Outlet coolant nozzles are equipped with thermal sleeves. A pressure-tube design, where the core is divided up into smaller tubes for each fuel channel, has potentially fewer issues here, as smaller diameter tubing can be much thinner than massive single pressure vessels, and the tube can be insulated on the inside with inert ceramic insulation so it can operate at low (calandria water) temperature.

The coolant greatly reduces its density at the end of the core, resulting in a need to place extra moderator there. A LWR type SCWR design adopts water rods in the fuel assemblies. Most designs of CANDU type SCWR use an internal calandria where part of the feedwater flow is guided through top tubes through the core, that provide the added moderation (feedwater) in that region. This has the added advantage of being able to cool the entire vessel wall with feedwater, but results in a complex and materially demanding (high temperature, high temperature differences, high radiation) internal calandria and plena arrangement. Again a pressure-tube design has potentially fewer issues, as most of the moderator is in the calandria at low temperature and pressure, reducing the coolant density effect on moderation, and the actual pressure tube can be kept cool by the calandria water.

• Extensive material development and research on supercritical water chemistry under radiation is needed
• Special start-up procedures needed to avoid instability before the water reaches supercritical conditions. Instability is managed by power to coolant flow rate ratio as a BWR.
• A fast SCWR needs a relatively complex reactor core to have a negative void coefficient. But single coolant flow pass core is feasible.