Life cycle of a mosquito.
An adult (imago) lays eggs which develop through several stages to
adulthood. Reproduction completes and perpetuates the cycle.
Life cycle of the single-celled parasite Babesia.
In biology, a biological life cycle (or just life cycle or lifecycle
when the biological context is clear) is a series of changes in form
that an organism undergoes, returning to the starting state. "The
concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.
In some organisms, different "generations" of the species succeed each other during the life cycle. For plants and many algae, there are two multicellular stages, and the life cycle is referred to as alternation of generations. The term life history is often used, particularly for organisms such as the red algae which have three multicellular stages (or more), rather than two.
Life cycles that include sexual reproduction involve alternating haploid (n) and diploid (2n) stages, i.e., a change of ploidy is involved. To return from a diploid stage to a haploid stage, meiosis must occur. In regard to changes of ploidy, there are 3 types of cycles:
- haplontic life cycle — the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic".
- diplontic life cycle — the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic".
- haplodiplontic life cycle (also referred to as diplohaplontic, diplobiontic, or dibiontic life cycle) — multicellular diploid and haploid stages occur, meiosis is "sporic".
The cycles differ in when mitosis (growth) occurs. Zygotic meiosis and gametic meiosis have one mitotic stage: mitosis occurs during the n phase in zygotic meiosis and during the 2n
phase in gametic meiosis. Therefore, zygotic and gametic meiosis are
collectively termed "haplobiontic" (single mitotic phase, not to be
confused with haplontic). Sporic meiosis, on the other hand, has mitosis
in two stages, both the diploid and haploid stages, termed
"diplobiontic" (not to be confused with diplontic).
Discovery
The study of reproduction and development in organisms was carried out by many botanists and zoologists.
Wilhelm Hofmeister demonstrated that alternation of generations is a feature that unites plants, and published this result in 1851.
Some terms (haplobiont and diplobiont) used for the description
of life cycles were proposed initially for algae by Nils Svedelius, and
then became used for other organisms. Other terms (autogamy and gamontogamy) used in protist life cycles were introduced by Karl Gottlieb Grell. The description of the complex life cycles of various organisms contributed to the disproof of the ideas of spontaneous generation in the 1840s and 1850s.
Haplontic life cycle
Zygotic meiosis
A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotically
to form either larger, multicellular individuals, or more haploid
cells. Two opposite types of gametes (e.g., male and female) from these
individuals or cells fuse to become a zygote.
In the whole cycle, zygotes are the only diploid cell; mitosis occurs only in the haploid phase.
The individuals or cells as a result of mitosis are haplonts,
hence this life cycle is also called haplontic life cycle. Haplonts are:
- In archaeplastidans: some green algae (e.g., Chlamydomonas, Zygnema, Chara)
- In stramenopiles: some golden algae
- In alveolates: many dinoflagellates, e.g., Ceratium, Gymnodinium, some apicomplexans (e.g., Plasmodium)
- In rhizarians: some euglyphids, ascetosporeans
- In excavates: some parabasalids
- In amoebozoans: Dictyostelium
- In opisthokonts: most fungi (some chytrids, zygomycetes, some ascomycetes, basidiomycetes)
Diplontic life cycle
Gametic meiosis
In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divides mitotically
to produce a multicellular diploid individual or a group of more
unicellular diploid cells. Cells from the diploid individuals then
undergo meiosis to produce haploid cells or gametes.
Haploid cells may divide again (by mitosis) to form more haploid cells,
as in many yeasts, but the haploid phase is not the predominant life
cycle phase. In most diplonts, mitosis occurs only in the diploid phase,
i.e. gametes usually form quickly and fuse to produce diploid zygotes.
In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.
The diploid multicellular individual is a diplont, hence a gametic meiosis is also called a diplontic life cycle. Diplonts are:
- In archaeplastidans: some green algae (e.g., Cladophora glomerata, Acetabularia)
- In stramenopiles: some brown algae (the Fucales, however, their life cycle can also be interpreted as strongly heteromorphic-diplohaplontic, with a highly reduced gametophyte phase, as in the flowering plants), some xanthophytes (e.g., Vaucheria), most diatoms, some oomycetes (e.g., Saprolegnia, Plasmopara viticola), opalines, some "heliozoans" (e.g., Actinophrys, Actinosphaerium)
- In alveolates: ciliates
- In excavates: some parabasalids
- In opisthokonts: animals, some fungi (e.g., some ascomycetes) Haplodiplontic life cycle
Sporic meiosis
In sporic meiosis (also commonly known as intermediary meiosis), the
zygote divides mitotically to produce a multicellular diploid sporophyte. The sporophyte creates spores via meiosis which also then divide mitotically producing haploid individuals called gametophytes.
The gametophytes produce gametes via mitosis. In some plants the
gametophyte is not only small-sized but also short-lived; in other
plants and many algae, the gametophyte is the "dominant" stage of the
life cycle.
Haplodiplonts are:
- In archaeplastidans: red algae (which have two sporophyte generations), some green algae (e.g., Ulva), land plants
- In stramenopiles: most brown algae
- In rhizarians: many foraminiferans, plasmodiophoromycetes
- In amoebozoa: myxogastrids
- In opisthokonts: some fungi (some chytrids, some ascomycetes like the brewer's yeast)
- Other eukaryotes: haptophytes
Some animals have a sex-determination system called haplodiploid, but this is not related to the haplodiplontic life cycle.
Vegetative meiosis
Some red algae (such as Bonnemaisonia and Lemanea) and green algae (such as Prasiola) have vegetative meiosis, also called somatic meiosis, which is a rare phenomenon.
Vegetative meiosis can occur in haplodiplontic and also in diplontic
life cycles. The gametophytes remain attached to and part of the
sporophyte. Vegetative (non-reproductive) diploid cells undergo meiosis,
generating vegetative haploid cells. These undergo many mitosis, and
produces gametes.
A different phenomenon, called vegetative diploidization, a type of apomixis, occurs in some brown algae (e.g., Elachista stellaris). Cells in a haploid part of the plant spontaneously duplicate their chromosomes to produce diploid tissue.
Parasitic life cycle
Parasites depend on the exploitation of one or more hosts. Those that must infect more than one host species
to complete their life cycles are said to have complex or indirect life
cycles, while those that infect a single species have direct life
cycles.
If a parasite has to infect a given host in order to complete its life cycle, then it is said to be an obligate parasite of that host; sometimes, infection is facultative—the
parasite can survive and complete its life cycle without infecting that
particular host species. Parasites sometimes infect hosts in which they
cannot complete their life cycles; these are accidental hosts.
A host in which parasites reproduce sexually is known as the
definitive, final or primary host. In intermediate hosts, parasites
either do not reproduce or do so asexually, but the parasite always
develops to a new stage in this type of host. In some cases a parasite
will infect a host, but not undergo any development, these hosts are
known as paratenic
or transport hosts. The paratenic host can be useful in raising the
chance that the parasite will be transmitted to the definitive host. For
example, the cat lungworm (Aelurostrongylus abstrusus) uses a
slug or snail as an intermediate host; the first stage larva enters the
mollusk and develops to the third stage larva, which is infectious to
the definitive host—the cat. If a mouse eats the slug, the third stage
larva will enter the mouse's tissues, but will not undergo any
development.
Life cycle of the apicomplexan, Babesia
Evolution
The primitive type of life cycle probably had haploid individuals with asexual reproduction. Bacteria and archaea exhibit a life cycle like this, and some eukaryotes apparently do too (e.g., Cryptophyta, Choanoflagellata, many Euglenozoa, many Amoebozoa, some red algae, some green algae, the imperfect fungi, some rotifers and many other groups, not necessarily haploid).
However, these eukaryotes probably are not primitively asexual, but
have lost their sexual reproduction, or it just was not observed yet. Many eukaryotes (including animals and plants) exhibit asexual reproduction, which may be facultative or obligate in the life cycle, with sexual reproduction occurring more or less frequently.
