Parasitoid

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

 

A parasitoid wasp (Trioxys complanatus, Aphidiidae) ovipositing into the body of a spotted alfalfa aphid, a behaviour that is used in biological pest control

 

A parasitoid is an organism that lives in close association with its host, at the host's expense and results in the death of the host. Parasitoidism is one of six major evolutionary strategies within parasitism, distinguished by the fatal prognosis for the host, which makes the strategy close to predation. 

Among parasitoids, strategies range from living inside the host, allowing it to go on growing until the parasitoid emerges as an adult, to paralysing the host and living outside it. Hosts include other parasitoids, resulting in hyperparasitism; in the case of oak galls, up to five levels of parasitism are possible. Some parasitoids influence their host's behaviour in ways that favour the propagation of the parasitoid.

Parasitoids are found in a variety of taxa across the endopterygote insects, whose complete metamorphosis may have pre-adapted them for a split lifestyle, with parasitoid larvae and freeliving adults. Most are in the Hymenoptera, where the ichneumons and many other parasitoid wasps are highly specialised for a parasitoidal way of life. Other parasitoids are in the Diptera, Coleoptera and other orders of endopterygote insects. Some of these, usually but not only wasps, are used in biological pest control.

The biology of parasitoidism has inspired science fiction authors and scriptwriters to create numerous parasitoidal aliens that kill their human hosts, such as the alien species in Ridley Scott's 1979 film Alien.

History

Maria Sibylla Merian (1647–1717) was one of the first naturalists to study and depict parasitoids. The term "parasitoid" was coined in 1913 by the Swedo-Finnish writer Odo Reuter, and adopted in English by his reviewer, the entomologist William Morton Wheeler. Reuter used it to describe the strategy where the parasite develops in or on the body of a single host individual, eventually killing that host, while the adult is free-living. Since that time, the concept has been generalised and widely applied.

Strategies

A perspective on the evolutionary options can be gained by considering four questions: the effect on the fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, proposed by K. D. Lafferty and A. M. Kunis, the major evolutionary strategies of parasitism emerge, alongside predation.

Evolutionary strategies in parasitism and predation

Host fitness	Single host, stays alive	Single host, dies	Multiple hosts
Able to reproduce (fitness > 0)	Conventional parasite    Pathogen	Trophically transmitted parasite    Trophically transmitted pathogen	Micropredator    Micropredator
Unable to reproduce (fitness = 0)	-----    Parasitic castrator	Trophically transmitted parasitic castrator    Parasitoid	Social predator    Solitary predator

Parasitoidism, in the view of R. Poulin and H. S. Randhawa, is one of six main evolutionary strategies within parasitism, the others being parasitic castrator, directly transmitted parasite, trophically transmitted parasite, vector-transmitted parasite, and micropredator. These are adaptive peaks, with many possible intermediate strategies, but organisms in many different groups have consistently converged on these six.

Parasitoids feed on a living host which they eventually kill, typically before it can produce offspring, whereas conventional parasites usually do not kill their hosts, and predators typically kill their prey immediately.

Basic concepts

A hyperparasitoid chalcid wasp on the cocoons of its host, a braconid wasp, itself a koinobiont parasitoid of Lepidoptera

Parasitoids can be classified as either endo- or ectoparasitoids with idiobiont or koinobiont developmental strategies. Endoparasitoids live within their host's body, while ectoparasitoids feed on the host from outside. Idiobiont parasitoids prevent further development of the host after initially immobilizing it, whereas koinobiont parasitoids allow the host to continue its development while feeding upon it. Most ectoparasitoids are idiobiont, as the host could damage or dislodge the external parasitoid if allowed to move and moult. Most endoparasitoids are koinobionts, giving them the advantage of a host that continues to grow larger and avoid predators.

Primary parasitoids have the simplest parasitic relationship, involving two organisms, the host and the parasitoid. Hyperparasitoids are parasitoids of parasitoids; secondary parasitoids have a primary parasitoid as their host, so there are three organisms involved. Hyperparasitoids are either facultative (can be a primary parasitoid or a hyperparasitoid depending on the situation) or obligate (always develop as a hyperparasitoid). Levels of parasitoids beyond secondary also occur, especially among facultative parasitoids. In oak gall systems, there can be up to five levels of parasitism. Cases in which two or more species of parasitoids simultaneously attack the same host without parasitizing each other are called multi- or multiple parasitism. In many cases, multiple parasitism still leads to the death of one or more of the parasitoids involved. If multiple parasitoids of the same species coexist in a single host, it is called superparasitism. Gregarious species lay multiple eggs or polyembryonic eggs which lead to multiple larvae in a single host. The end result of gregarious superparasitism can be a single surviving parasitoid individual or multiple surviving individuals, depending on the species. If superparasitism occurs accidentally in normally solitary species the larvae often fight among themselves until only one is left.

Influence on host behaviour

Female phorid fly Apocephalus borealis (centre left) ovipositing into the abdomen of a worker honey bee, altering its behaviour

In another strategy, some parasitoids influence the host's behaviour in ways that favour the propagation of the parasitoid, often at the cost of the host's life. A spectacular example is the lancet liver fluke, which causes host ants to die clinging to grass stalks, where grazers or birds may be expected to eat them and complete the parasitoidal fluke's life cycle in its definitive host. Similarly, as strepsipteran parasitoids of ants mature, they cause the hosts to climb high on grass stalks, positions that are risky, but favour the emergence of the strepsipterans. Among pathogens of mammals, the rabies virus affects the host's central nervous system, eventually killing it, but perhaps helping to disseminate the virus by modifying the host's behaviour. Among the parasitic wasps, Glyptapanteles modifies the behaviour of its host caterpillar to defend the pupae of the wasps after they emerge from the caterpillar's body. The phorid fly Apocephalus borealis oviposits into the abdomen of its hosts, including honey bees, causing them to abandon their nest, flying from it at night and soon dying, allowing the next generation of flies to emerge outside the hive.

Taxonomic range

About 10% of described insects are parasitoids, in the orders Hymenoptera, Diptera, Coleoptera, Neuroptera, Lepidoptera, Strepsiptera, and Trichoptera. The majority are wasps within the Hymenoptera; most of the others are Dipteran flies. Parasitoidism has evolved independently many times: once each in Hymenoptera, Strepsiptera, Neuroptera, and Trichoptera, twice in the Lepidoptera, 10 times or more in Coleoptera, and no less than 21 times among the Diptera. These are all holometabolous insects (Endopterygota, which form a single clade), and it is always the larvae that are parasitoidal. The metamorphosis from active larva to an adult with a different body structure permits the dual lifestyle of parasitic larva, freeliving adult in this group. These relationships are shown on the phylogenetic tree; groups containing parasitoids are shown in boldface, e.g. Coleoptera, with the number of times parasitoidism evolved in the group in parentheses, e.g. (10 clades). The approximate number (estimates can vary widely) of parasitoid species out of the total is shown in square brackets, e.g. [2,500 of 400,000].

Hymenoptera

Potter wasp, an idiobiont, building a mud nest; she will provision it with paralysed insects, on which she will lay her eggs; she will then seal the nest and provide no further care for her young

 

Within the Hymenoptera, parasitoidism evolved just once, and the many described species of parasitoid wasps represent the great majority of species in the order, barring those like the ants, bees, and Vespidae wasps that have secondarily lost the parasitoid habit. The parasitoid wasps include some 25,000 Ichneumonoidea, 22,000 Chalcidoidea, 5,500 Vespoidea, 4,000 Platygastroidea, 3,000 Chrysidoidea, 2,300 Cynipoidea, and many smaller families. These often have remarkable life cycles. They can be classified as either endoparasitic or ectoparasitic according to where they lay their eggs. Endoparasitic wasps insert their eggs inside their host, usually as koinobionts, allowing the host to continue to grow (thus providing more food to the wasp larvae), moult, and evade predators. Ectoparasitic wasps deposit theirs outside the host's body, usually as idiobionts, immediately paralysing the host to prevent it from escaping or throwing off the parasite. They often carry the host to a nest where it will remain undisturbed for the wasp larva to feed on. Most species of wasps attack the eggs or larvae of their host, but some attack adults. Oviposition depends on finding the host and on evading host defenses; the ovipositor is a tube-like organ used to inject eggs into hosts, sometimes much longer than the wasp's body. Hosts such as ants often behave as if aware of the wasps' presence, making violent movements to prevent oviposition. Wasps may wait for the host to stop moving, and then attack suddenly.

Parasitoid wasps face a range of obstacles to oviposition, including behavioural, morphological, physiological and immunological defenses of their hosts. To thwart this, some wasps inundate their host with their eggs so as to overload its immune system's ability to encapsulate foreign bodies; others introduce a virus which interferes with the host's immune system. Some parasitoid wasps locate hosts by detecting the chemicals that plants release to defend against insect herbivores.

Other orders

The head of a sessile female strepsipteran protruding (lower right) from the abdomen of its wasp host; the male (not shown) has wings

 

The true flies (Diptera) include several families of parasitoids, the largest of which is the Tachinidae (some 9,200 species), followed by the Bombyliidae (some 4,500 species), along with the Pipunculidae and the Conopidae, which includes parasitoidal genera such as Stylogaster. Other families of flies include some protelean species. Some Phoridae are parasitoids of ants. Some flesh flies are parasitoids: for instance Emblemasoma auditrix is parasitoidal on cicadas, locating its host by sound.

The Strepsiptera (twisted-wing parasites) consist entirely of parasitoids; they usually sterilise their hosts.

Two beetle families, Ripiphoridae (450 species) and Rhipiceridae, are largely parasitoids, as are Aleochara Staphylinidae; in all, some 400 staphylinids are parasitoidal. Some 1,600 species of the large and mainly freeliving family Carabidae are parasitoids.

A few Neuroptera are parasitoidal; they have larvae that actively search for hosts. The larvae of some Mantispidae, subfamily Symphrasinae, are parasitoids of other arthropods including bees and wasps.

Although nearly all Lepidoptera (butterflies and moths) are herbivorous, a few species are parasitic. The larvae of Epipyropidae feed on Homoptera such as leafhoppers and cicadas, and sometimes on other Lepidoptera. The larvae of Cyclotornidae parasitise first Homoptera and later ant brood. The pyralid moth Chalcoela has been used in biological control of the wasp Polistes in the Galapagos Islands.

Parasitism is rare in the Trichoptera (caddisflies), but it is found among the Hydroptilidae (purse-case caddisflies), probably including all 10 species in the Orthotrichia aberrans group; they parasitise the pupae of other trichopterans.

In biological pest control

Encarsia formosa, an endoparasitic chalcid wasp, bred commercially to control whitefly in greenhouses

Parasitoids are among the most widely used biological control agents. Classic biological pest control using natural enemies of pests (parasitoids or predators) is extremely cost effective, the cost/benefit ratio for classic control being 1:250, but the technique is more variable in its effects than pesticides; it reduces rather than eliminates pests. The cost/benefit ratio for screening natural enemies is similarly far higher than for screening chemicals: 1:30 against 1:5 respectively, since the search for suitable natural enemies can be guided accurately with ecological knowledge. Natural enemies are more difficult to produce and to distribute than chemicals, as they have a shelf life of weeks at most; and they face a commercial obstacle, namely that they cannot be patented.

From the point of view of the farmer or horticulturalist, the most important groups are the ichneumonid wasps, which prey mainly on caterpillars of butterflies and moths; braconid wasps, which attack caterpillars and a wide range of other insects including greenfly; chalcid wasps, which parasitise eggs and larvae of greenfly, whitefly, cabbage caterpillars, and scale insects; and tachinid flies, which parasitize a wide range of insects including caterpillars, adult and larval beetles, and true bugs. Commercially, there are two types of rearing systems: short-term seasonal daily output with high production of parasitoids per day, and long-term year-round low daily output with a range in production of 4–1000 million female parasitoids per week, to meet demand for suitable biological control agents for different crops.

In culture

Charles Darwin

Parasitoids influenced the thinking of Charles Darwin, who wrote in an 1860 letter to the American naturalist Asa Gray: "I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars." The palaeontologist Donald Prothero notes that religiously minded people of the Victorian era, including Darwin, were horrified by this instance of evident cruelty in nature, particularly noticeable in the Ichneumonidae wasps.

In science fiction

A 1990s gargoyle at Paisley Abbey, Scotland, resembling a Xenomorph parasitoid from the film Alien

 

Parasitoids have inspired science fiction authors and screenwriters to create terrifying parasitic alien species that kill their human hosts. One of the best-known is the Xenomorph in Ridley Scott's 1979 film Alien, which runs rapidly through its lifecycle from violently entering a human host's mouth to bursting fatally from the host's chest. The molecular biologist Alex Sercel, writing in Signal to Noise Magazine, compares "the biology of the [Alien] Xenomorphs to parasitoid wasps and nematomorph worms from Earth to illustrate how close to reality the biology of these aliens is and to discuss this exceptional instance of science inspiring artists". Sercel notes that the way the Xenomorph grasps a human's face to implant its embryo is comparable to the way a parasitoid wasp lays its eggs in a living host. He further compares the Xenomorph life cycle to that of the nematomorph Paragordius tricuspidatus which grows to fill its host's body cavity before bursting out and killing it. Alistair Dove, on the science website Deep Sea News, writes that there are multiple parallels with parasitoids, though there are in his view more disturbing life cycles in real biology. In his view, the parallels include the placing of an embryo in the host; its growth in the host; the resulting death of the host; and alternating generations, as in the Digenea (trematodes). The social anthropologist Marika Moisseeff argues that "The parasitical and swarming aspects of insect reproduction make these animals favored bad-guy characters in Hollywood science fiction. The battle of culture against nature is depicted as an unending combat between humanity and insect-like extraterrestrial species that tend to parasitize human beings in order to reproduce." The Encyclopedia of Science Fiction lists many instances of "parasitism", often causing the host's death.

Behavior-altering parasite

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Behavior-altering_parasite 

 

Behavior-altering parasites are parasites with two or more hosts, capable of causing changes in the behavior of one of their hosts to enhance their transmission, sometimes directly affecting the hosts' decision-making and behavior control mechanisms. They do this by making the intermediate host, where they may reproduce asexually, more likely to be eaten by a predator at a higher trophic level which becomes the definitive host where the parasite reproduces sexually; the mechanism is therefore sometimes called parasite increased trophic facilitation or parasite increased trophic transmission. Examples can be found in bacteria, protozoa, viruses, and animals. Parasites may also alter the host behaviour to increase the protection to the parasites or their offspring. The term bodyguard manipulation is used for such mechanisms.

Among the behavioral changes caused by parasites is carelessness, making their hosts easier prey. The protozoan Toxoplasma gondii, for example, infects small rodents and causes them to become careless and attracted to the smell of feline urine, which increases their risk of predation and the parasite's chance of infecting a cat, its definitive host.

Parasites may alter the host's behavior by infecting the host's central nervous system, or by altering its neurochemical communication, studied in neuro-parasitology.

Behavioral change

Types

Parasite manipulations can be either direct or indirect. Indirect manipulation is the most frequent method used by behavior-altering parasites, while the direct approach is far less common. Direct manipulation is when the parasite itself affects the host and induces a behavioral response, for example by creating neuroactive compounds that stimulate a response in the host's central nervous system (CNS), a method mostly practiced by parasites that reside within the CNS. Affecting the host's neural system is complicated and manipulation includes initiating immune cascades in the host. However, determination of the causative factor is difficult, especially whether the behavioral change is the result of direct manipulation from the parasite, or an indirect response of the host's immune system. A direct approach to behavioral manipulation is often very costly for the parasite, which results in a trade-off between the benefits of the manipulation (e.g., fitness increase) and the energy it costs. The more common approach for parasites is to indirectly induce behavioral responses by interacting with the host's immune system to create the necessary neuroactive compounds to induce a desired behavioral response. Parasites can also indirectly affect the behavior of their hosts by disturbing their metabolism, development or immunity. Parasitic castrators drastically modify their hosts' metabolism and reproduction, sometimes by secreting castrating hormones, changing their behavior and physiology to benefit the parasite.

Parasites may alter hosts' behaviors in ways that increase their likelihood of transmission (e.g. by the host being ingested by a predator); result in the parasite's release at appropriate sites (e.g. by changes in the host's preferences for habitats); increase parasite survival or increase the host's likelihood of being infected with more parasites.

By viruses

Rabies causes the host to be aggressive and prone to biting others. This along with increased salivation, which carries the virus, increases the chances of it spreading to new hosts.

Viruses from the family Baculoviridae induce in their hosts changes to both feeding behavior and environment selection. They infect moth and butterfly caterpillars, who some time following infection begin to eat incessantly, providing nutrients for the virus's replication. When the virions (virus "units") are ready to leave the host, the caterpillar climbs higher and higher, until its cells are made to secrete enzymes that "dissolve the animal into goo", raining down clumps of tissue and viral material for ingestion by future hosts.

By protozoa

The protozoan Toxoplasma gondii infects animals from the family Felidae (its definitive host), and its oocysts are shed with the host's feces. When a rodent consumes the fecal matter it gets infected with the parasite (becoming its intermediate host). The rodent subsequently becomes more extroverted and less fearful of cats, increasing its chance of predation and the parasite's chance of completing its lifecycle. There is some evidence that T. gondii, when infecting humans, alters their behavior in similar ways to rodents; it has also been linked to cases of schizophrenia. Other parasites that increase their host's risk of predation include Euhaplorchis californiensis, Dicrocoelium dendriticum, Myrmeconema neotropicum and Diplostomum pseudospathaceum. 

The malaria parasite Plasmodium falciparum, carried by the Anopheles gambiae mosquito, changes its host's attraction to sources of nectar in order to increase its sugar intake and enhance the parasite's chance of survival. It also decreases the host's attraction to human blood while gestating, only to increase it when it is ready to transmit to a human host.

By helminths

A long horsehair worm shortly after emerging from its cricket host, now drowned

 

Making the host careless increases the risk of it being eaten by a non-host predator, interrupting the parasite's life-cycle. Some parasites manipulate their intermediate host to reduce this risk. For example, the parasitic trematode Microphallus sp., uses the snail Potamopyrgus antipodarum as an intermediate host. The parasite manipulates the snail's foraging behavior to increase the chance of it being preyed upon by the parasite's definitive hosts (waterfowl). The infected snail forages on the upper side of rocks during the period of the day when waterfowl feed most intensely. During the rest of the day, the snail forages at the bottom of rocks to reduce the risk of being eaten by fish (non-hosts for the parasitic trematode).

The lancet liver fluke (Dicrocoelium dendriticum) is a parasitic trematode with a complex life cycle. In its adult state it occurs in the liver of its definitive host (ruminants), where it reproduces. The parasite eggs are passed with the feces of the host, which then are eaten by a terrestrial snail (first intermediate host). The fluke matures into a juvenile stage in the snail, which in an attempt to protect itself excretes the parasites in "slime-balls". The "slime-balls" are then consumed by ants (second intermediate hosts). The fluke manipulates the ant to move up to the top of grass, where they have a higher chance of being eaten by grazing ruminants.

The parasitic nematode Myrmeconema neotropicum infects the intermediate ant host Cephalotes atratus. The nematode then induces a morphological change in the ant, which turns the gaster color from black to red, making it resemble fruit. This color transition makes the ant susceptible to predation by frugivorous birds, which act as the parasite's definitive hosts. The parasitic eggs are deposited in the bird's feces and are eaten by ants, which complete the cycle.

Snail with its left eye stalk parasitized by Leucochloridium paradoxum

Crickets infected by horsehair worms exhibit light-seeking behavior and increased walking speed, leading them to open spaces and ponds (the surface of which reflects moonlight); the crickets will eventually find and enter a body of water, where the worm will wiggle out of the cricket's abdomen and swim away. While crickets often drown in the process, those who survive exhibit a partial recovery and return to normal activities in as little as 20 hours.

The trematode Leucochloridium paradoxum matures inside snails of the genus Succinea. When ready to switch to its definitive host, a bird, the parasite travels to the eye stalks of its host and begins to pulsate, attracting birds with its striking resemblance to an insect larva. It also influences the normally nocturnal snail to climb out into the open during the day for an increased chance of being consumed by a bird.

Schistocephalus solidus is a parasitic tapeworm with three different hosts, two intermediate and one definitive. In its adult stage the tapeworm resides in the intestine of piscivorous birds, where they reproduce and release eggs through the bird's feces. Free-swimming larvae hatch from the eggs, which are in turn ingested by copepods (the first intermediate host). The parasite grows and develops in the crustacean into a stage that can infect the second intermediate host, the three-spined stickleback (Gasterosteus aculeatus). The parasite's definitive host, a bird, then consumes the infected three-spined stickleback and the cycle is complete. It has been observed that S. solidus alters the behavior of the fish in a manner that impedes its escape response when faced with a predatorial bird. This parasite-induced behavioral manipulation effectively increases the chance of it being consumed by its definitive bird host. It has also been observed that the parasite does not induce this behavior until it has reached a developed stage that can survive in the host bird and therefore effectively reduce its own mortality rate, due to premature transmission.

By insects

Emerald cockroach wasp "walking" a paralyzed cockroach to its burrow

The emerald cockroach wasp (Ampulex compressa) parasitises its host, the American cockroach (Periplaneta americana) as a food source and for its growing larvae. The wasp stings the cockroach twice: First in the thoracic ganglion, paralyzing its front legs and enabling the wasp to deliver a second, more difficult sting, directly into the cockroach's brain; this second sting makes the cockroach groom itself excessively before sinking into a state of hypokinesia – "a... lethargy characterized by lack of spontaneous movement or response to external stimuli". The wasp then pulls the idle cockroach into its burrow, where it deposits an egg onto its abdomen and buries it for the growing larva to feed on. Keeping the cockroach in a hypokinetic state at this stage, rather than simply killing it, allows it to stay "fresh" for longer for the larva to feed on. The adult wasp emerges after 6 weeks, leaving behind nothing but an empty cockroach "shell".

The parasitic wasp Hymenoepimecis argyraphaga grows its larvae on spiders of the species Leucauge argyra. Shortly before killing its host the larva injects it with a chemical that changes its weaving behavior, causing it to weave a strong, cocoon-like structure. The larva then kills the spider and enters the cocoon to pupate.

Ladybug guarding a Dinocampus coccinellae cocoon. The ladybug will remain stationary until the adult wasp emerges from its cocoon, and die some time afterwards

The wasp Dinocampus coccinellae is both an endoparasite and ectoparasite of ladybugs. The wasp injects an egg into the beetle's abdomen, where the larva feeds on its haemolymph. When grown and ready to pupate the larva exits its host, which remains immobile, and weaves a cocoon on its underside, where it pupates. Were a predator to approach, the beetle would twitch its limbs, scaring the predator off. This use of the host as a protection has been termed as bodyguard manipulation. Similarly several parasitic wasps induce their spider hosts to build stronger webs to protect the growing parasites.

Strepsiptera of the family Myrmecolacidae can cause their ant host to linger on the tips of grass leaves, increasing the chance of being found by the parasite's males (in case of females) and putting them in a good position for male emergence (in case of males). A similar, but much more intricate behavior is exhibited by ants infected with the fungus Ophiocordyceps unilateralis: irregularly-timed body convulsions cause the ant to drop to the forest floor, from which it climbs a plant up to a certain height before locking its jaws into the vein of one of its leaves answering certain criteria of direction, temperature and humidity. After several days the fruiting body of the fungus grows from the ant's head and ruptures, releasing the fungus's spores.

Several species of fly in the family Phoridae parasitise fire ants. The fly injects an egg into the ant's thorax; upon hatching, the larva migrates into the ant's head, where it feeds on the ant's haemolymph, muscle and nerve tissue. During this period some larvae direct the ant up to 50 meters away from the nest and towards a moist, leafy place where they can hatch safely. Eventually the larva completely devours the ant's brain, which often falls off (hence the species nickname: "decapitating fly"). The larva then pupates in the empty head capsule, emerging as an adult fly after two weeks.

By crustaceans

Members of the order Rhizocephala such as S. carcini alter male hosts' hormonal balance, to encourage nurturing behavior similar to that seen in females. The parasite usually spends its entire life within the host; however, if it is removed from the host in a laboratory setting, male hosts will subsequently grow partial or complete female gonads.

Evolutionary perspective

Addition of intermediate hosts

For complex life cycles to emerge in parasites, the addition of intermediate host species must be beneficial, e.g., result in a higher fitness. It is probable that most parasites with complex life cycles evolved from simple life cycles. The transfer from simple to complex life cycles has been analyzed theoretically, and it has been shown that trophically transmitted parasites can be favored by the addition of an intermediate prey host if the population density of the intermediate host is higher than that of the definitive host. Additional factors that catalyze this transfer are high predation rates, and a low natural mortality rate of the intermediate host.

Parasites with a single host species are faced with the problem of not being able to survive in higher trophic levels and therefore dying with its prey host. The development of complex life cycles is most likely an adaptation of the parasite to survive in the predator. The development of parasite increased trophic transmission is a further adaptation in relation to a complex life cycle, where the parasite increases its transmission to a definitive host by manipulating its intermediate host.

Evolution of induced behaviors

The adaptive manipulation hypothesis posits that specific behavioral alterations induced in a host can be used by parasites to increase their fitness. Under this hypothesis, induced behaviors are the result of natural selection acting upon the parasite's extended phenotype (in this case its host's behavior). Many behaviors induced by obligate parasites to complete their lifecycles are examples of adaptive manipulation because of their clear relationship to parasite fitness. For example, evidence has shown that infection by the parasitic worm Pomphorhynchus laevis leads to altered drifting behavior in its intermediate host, the amphipod Gammarus pulex; this altered behavior increases its host's predation risk by fish which are P. laevis's definitive hosts. The induced behavioral change in the host thus leads to the parasite's increased success in completing its life cycle. In general, whether a specific behavioral change serves an adaptive purpose for the parasite, the host, or both, depends on the entire "host-parasite system": The life cycle of the pathogen, its virulence and the host's immune response.

Conversely, evolved behaviors of the host may be a result of adaptations to parasitism.

Mechanisms

The way in which parasites induce behavioral changes in hosts has been compared to the way a neurobiologist would effect a similar change in a lab. A scientist may stimulate a certain pathway in order to produce a specific behavior, such as increased appetite or lowered anxiety; parasites also produce specific behavioral changes in their hosts, but rather than stimulate specific neurological pathways, they appear to target broader areas of the central nervous system. While the proximate mechanisms underlying this broad targeting have not been fully characterized, two mechanisms used by parasites to alter behavior in vertebrate hosts have been identified: infection of the central nervous system and altered neurochemical communication.

Infection of the central nervous system

Some parasites alter host behavior by infecting neurons in the host's central nervous system. The host's central nervous system responds to the parasite as it would to any other infection. The hallmarks of such response include local inflammation and the release of chemicals such as cytokines. The immune response itself is responsible for induced behavioral changes in many cases of parasitic infection. Parasites that are known to induce behavioral changes through central nervous system inflammation in their hosts include Toxoplasma gondii in rats, Trypanosoma cruzi in mice and Plasmodium mexicanum in the Mexican lizard.

Toxoplasma gondii induces behavioral changes in rats by infecting central nervous system neurons.

 

While some parasites exploit their hosts' typical immune responses, others seem to alter the immune response itself. For example, the typical immune response in rodents is characterized by heightened anxiety. Infection with Toxoplasma gondii inhibits this response, increasing the risk of predation by T. gondii's subsequent hosts. Research suggests that the inhibited anxiety-response could be the result of immunological damage to the limbic system.

Altered neurochemical communication

Parasites that induce behavioral changes in their hosts often exploit the regulation of social behavior in the brain. Social behavior is regulated by neurotransmitters, such as dopamine and serotonin, in the emotional centers of the brain – primarily the amygdala and the hypothalamus, and although parasites may be capable of stimulating specific neurochemical pathways to induce behavioral changes, evidence suggests that they alter neurochemical communication through broad rather than specific targeting. For example, Toxoplasma gondii attaches to the hypothalamus rather than target a specific cellular pathway; this broad targeting leads to a widespread increase in host dopamine levels, which may in turn account for the loss of aversion to cat odor. In some cases, T. gondii is believed to cause increases in dopamine levels by secreting another compound, L-Dopa, which may trigger a rise in dopamine levels, though concrete evidence for this mechanism has not yet been demonstrated. This rise in dopamine levels induces a loss of aversion to cat odor in the rats, increasing the risk of predation by cats, T. gondii's definitive host. The mechanistic details underlying the increase in dopamine levels and the way it affects the rat's behavioral change remain elusive.

The emerald cockroach wasp alters behavior through the injection of venom directly into the host's brain, causing hypokinesia. This is achieved by a reduction in dopamine and octopamine activity, which affects the transmission of interneurons involved in the escape response; so while the host's brain circuitry responsible for movement control is still functional – and indeed it will slog along when pulled by the wasp – the nervous system is in a depressed state. Put differently: the wasp's toxin affects not the host's ability to move, but its motivation to do so.

The original function of such secretions may have been to suppress the immune system of the host, as described above. The trematode Schistosoma mansoni secretes opioid peptides into the host's bloodstream, influencing both its immune response and neural function. Other sources suggest a possible origin in molecular mimicry.

Other mechanisms

Mermithid nematodes infect arthropods, residing in their haemocoel (circulatory cavity) and manipulating their hemolymph osmolality to trigger water-seeking behavior. The means by which they do so are unknown.

Alkylation

From Wikipedia, the free encyclopedia

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

 

Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene (or their equivalents). An alkyl group is a piece of a molecule with the general formula C_nH_2n+1, where n is the integer depicting the number of carbons linked together. For example, a methyl group (n = 1, CH₃) is a fragment of a methane molecule (CH₄). Alkylating agents use selective alkylation by adding the desired aliphatic carbon chain to the previously chosen starting molecule. This is one of many known chemical syntheses. Alkyl groups can also be removed in a process known as dealkylation. Alkylating agents are often classified according to their nucleophilic or electrophilic character.

In oil refining contexts, alkylation refers to a particular alkylation of isobutane with olefins. For upgrading of petroleum, alkylation produces a premium blending stock for gasoline.

In medicine, alkylation of DNA is used in chemotherapy to damage the DNA of cancer cells. Alkylation is accomplished with the class of drugs called alkylating antineoplastic agents.

Typical route for alkylation of benzene with ethylene and ZSM-5 as a heterogeneous catalyst

Nucleophilic alkylating agents

Nucleophilic alkylating agents deliver the equivalent of an alkyl anion (carbanion). The formal "alkyl anion" attacks an electrophile, forming a new covalent bond between the alkyl group and the electrophile. The counterion, which is a cation such as lithium, can be removed and washed away in the work-up. Examples include the use of organometallic compounds such as Grignard (organomagnesium), organolithium, organocopper, and organosodium reagents. These compounds typically can add to an electron-deficient carbon atom such as at a carbonyl group. Nucleophilic alkylating agents can displace halide substituents on a carbon atom through the SN2 mechanism. With a catalyst, they also alkylate alkyl and aryl halides, as exemplified by Suzuki couplings. 

The Kumada coupling employs both a nucleophilic alkylation step subsequent to the oxidative addition of the aryl halide (L = Ligand, Ar = Aryl).

 

The SN2 mechanism is not available for aryl substituents, where the trajectory to attack the carbon atom would be inside the ring. Thus only reactions catalyzed by organometallic catalysts are possible.

Electrophilic alkylating agents

C-alkylation

C-alkylation is a process for the formation of carbon-carbon bonds. For alkylation at carbon, the electrophilicity of alkyl halides is enhanced by the presence of a Lewis acid such as aluminium trichloride. Lewis acids are particularly suited for C-alkylation. C-alkylation can also be effected by alkenes in the presence of acids.

N-and P-alkylation

N- and P-alkylation are important processes for the formation of carbon-nitrogen and carbon-phosphorus bonds. 

Amines are readily alkylated. The rate of alkylation follows the order tertiary amine < secondary amine < primary amine. Typical alkylating agents are alkyl halides. Industry often relies on green chemistry methods involving alkylation of amines with alcohols, the byproduct being water. Hydroamination is another green method for N-alkylation.

In the Menshutkin reaction, a tertiary amine is converted into a quaternary ammonium salt by reaction with an alkyl halide. Similar reactions occur when tertiary phosphines are treated with alkyl halides, the products being phosphonium salts.

S-alkylation

Thiols are readily alkylated to give thioethers. The reaction is typically conducted in the presence of a base or using the conjugate base of the thiol. Thioethers undergo alkylation to give sulfonium ions.

O-alkylation

Alcohols alkylate to give ethers:

ROH + R'X → ROR'

When the alkylating agent is an alkyl halide, the conversion is called the Williamson ether synthesis. Alcohols are also good alkylating agents in the presence of suitable acid catalysts. For example, most methyl amines are prepared by alkylation of ammonia with methanol. The alkylation of phenols is particularly straightforward since it is subject to fewer competing reactions.

${\displaystyle \mathrm {Ph{-}O^{-}\ +\ Me_{2}{-}SO_{4}\ \longrightarrow \ Ph{-}O{-}Me\ +\ Me{-}SO_{4}^{-}} }$

More complex alkylation of a alcohols and phenols involve ethoxylation. Ethylene oxide is the alkylating group in this reaction.

Oxidative addition to metals

In the process called oxidative addition, low-valent metals often react with alkylating agents to give metal alkyls. This reaction is one step in the Cativa process for the synthesis of acetic acid from methyl iodide. Many cross coupling reactions proceed via oxidative addition as well.

Electrophilic alkylating agents

Electrophilic alkylating agents deliver the equivalent of an alkyl cation. Alkyl halides are typical alkylating agents. Trimethyloxonium tetrafluoroborate and triethyloxonium tetrafluoroborate are particularly strong electrophiles due to their overt positive charge and an inert leaving group (dimethyl or diethyl ether). Dimethyl sulfate is intermediate in electrophilicity. 

Triethyloxonium tetrafluoroborate is one of the most electrophilic alkylating agents.

Hazards

Electrophilic, soluble alkylating agents are often toxic and carcinogenic, due to their tendency to alkylate DNA. This mechanism of toxicity is relevant to the function of anti-cancer drugs in the form of alkylating antineoplastic agents. Some chemical weapons such as mustard gas function as alkylating agents. Alkylated DNA either does not coil or uncoil properly, or cannot be processed by information-decoding enzymes.

Catalysts

Friedel-Crafts alkylation of benzene is often catalyzed by aluminium trichloride.

 

Electrophilic alkylations use Lewis acids and Brønsted acids, sometimes both. Classically, Lewis acids, e.g., aluminium trichloride, are employed when the alkyl halide are used. Brønsted acids are used when alkylating with olefins. Typical catalysts are zeolites, i.e. solid acid catalysts, and sulfuric acid. Silicotungstic acid is used to manufacture ethyl acetate by the alkylation of acetic acid by ethylene:

C₂H₄ + CH₃CO₂H → CH₃CO₂C₂H₅

In biology

Methylation is the most common type of alkylation. Methylation in nature is often effected by vitamin B12- and radical-SAM-based enzymes.

The SN2-like methyl transfer reaction in DNA methylation. Only the SAM cofactor and cytosine base are shown for simplicity.

In methanogenesis, coenzyme M is methylated by tetrahydromethanopterin.

Commodity chemicals

Several commodity chemicals are produced by alkylation. Included are several fundamental benzene-based feedstocks such as ethylbenzene (precursor to styrene), cumene (precursor to phenol and acetone), linear alkylbenzene sulfonates (for detergents).

Sodium dodecylbenzene, obtained by alkylation of benzene with dodecene, is a precursor to linear alkylbenzene sulfonate detergents.

Oil refining

Typical acid-catalyzed route to 2,4-dimethylpentane.

 

In a conventional oil refinery, isobutane is alkylated with low-molecular-weight alkenes (primarily a mixture of propene and butene) in the presence of a Brønsted acid catalyst, which can include solid acids (zeolites). The catalyst protonates the alkenes (propene, butene) to produce carbocations, which alkylate isobutane. The product, called "alkylate", is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons (mostly isoheptane and isooctane). Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. Alkylate is also a key component of avgas. By combining fluid catalytic cracking, polymerization, and alkylation refineries can obtain a gasoline yield of 70 percent. The widespread use of sulfuric acid and hydrofluoric acid in refineries poses significant environmental risks.