Biological life cycle

A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction (a period from one generation of organisms to the same identical). For example, a complex life cycle of Fasciola hepatica, a parasitic flatworm, includes three different multicellular generations: 1) "adult" hermaphroditic; 2) sporocyst; 3) redia.

In regard to changes of ploidy, there are 3 types of cycles:

• haplontic life cycle
• diplontic life cycle
• diplobiontic life cycle (also referred to as diplohaplontic, haplodiplontic, or dibiontic life cycle)

These three types of cycles feature alternating haploid and diploid stages. To return to a haploid stage, meiosis must occur. The cycles differ in the product of meiosis, and whether mitosis (growth) occurs. Zygotic and gametic meioses have one mitotic stage and form: during the n phase in zygotic meiosis and during the 2n phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively term haplobiontic (single mitosis per phase). Sporic meiosis, on the other hand, has two mitosis events (diplobiontic): one in each phase.

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:

• Most fungi
• Some green algae, e.g. Chlamydomonas
• Many protozoa, e.g. Plasmodium

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:

• Animals
• Some brown algae
• Some fungi, e.g. brewer's yeast

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 many plants the gametophyte is not only small-sized but also short-lived.

In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.

Haplodiplonts are:

• Land plants
• Some fungi
• Some algae

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.

Life history

The term "life history" is more accurate than "alternation of generations" for the red algae (Rhodophyta). In the Rhodophyceae there are generally three stages, rather than two.

Life history theory

In animal and human biology, life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success.

See also

• Alternation of generations
• Apomixis
• Monocarpic plants bloom once and then die
• Parasexual cycle
• Parasitic life cycles
• Parthenogenesis
• Reproductive biology

External links

• Silk worm Life cycle photos

• Life Cycles: Reflections of an Evolutionary Biologist. John Tyler Bonner. Princeton University Press, Princeton (New Jersey).
• Vegetal life cycles (in spanish)