Life cycle
A life cycle is a period involving one generation of an organism through means of reproduction, whether through asexual reproduction or sexual reproduction. In regard to its ploidy, there are three 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 phases (n and 2n). The haploid organism becomes diploid through fertilization, which joins of gametes. This results in a zygote which then germinates. To return to a haploid stage, meiosis must occur (see Cell division). 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 meiosis per phase). Sporic meiosis, on the other hand, has two meiosis events (diplobiontic): one in each phase.
Haplontic life cycle
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:
Diplontic life cycle
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 diploid cells. Cells from the diploid individuals then undergo meiosis to produce gametes. The haploid gametes do not divide mitotically, however. Without growing larger, the gametes would fuse and produce the diploid zygote with gametes of the opposite type.
In the whole cycle, gametes are the only haploid cells; mitosis 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
Diplobiontic life cycle
Main article: Alternation of generations
In sporic meiosis, also known as intermediary meiosis, mitoses occur in both the diploid and haploid phases. It exhibits alternation of generations, which features of spore-producing multicellular sporophytes and gamete-producing multicellular gametophytes. Diagramatically, sporic meiosis looks like the complex halves of gametic meiosis and zygotic meiosis are merged into one.
This type of cycle is diplobiontic (also known as diplohaplontic, haplodiplontic, or dibiontic).
Sporic meiosis occurs in plants and many algae. Having multicellular individuals in both phases means that for some seaweeds, it is difficult to determine if a macroscopic specimen is gametophytic or sporophytic unless observed under a microscope, this is called isogamy. However, not all species with sporic meiosis have both large gametophyte and sporophyte generations. The trend in higher plants is having smaller gametophytes that are more dependent and parasitic on sporophytes, a phenomena known as heterogamy.
Life history theory
In animal and human biology life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success.
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