A
life cycle is a period involving all different
generationGeneration , also known as procreation in biological sciences, is the act of producing offspring....
s of a
speciesIn biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
succeeding each other through means of reproduction, whether through
asexual reproductionAsexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
or
sexual reproductionSexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
(a period from one generation of organisms to the same identical). For example, a complex life cycle of
Fasciola hepaticaFasciola hepatica, also known as the common liver fluke or sheep liver fluke, is a parasitic flatworm of the class Trematoda, phylum Platyhelminthes that infects the livers of various mammals, including humans. The disease caused by the fluke is called fascioliasis . F...
includes three different multicellular generations: 1) "adult" hermaphroditic; 2)
sporocystSporocyst is a common name of a lifecycle stage in two unrelated groups of species:*Apicomplexa parasites: see Apicomplexa lifecycle stages*Trematode flatworms: see Trematode lifecycle stages...
; 3)
rediaRedia or redia can be:-*A synonym for the plant genus Cleidion of the family Euphorbiaceae*A stage in the development of a trematode*A dipstick for urinalysis, determining levels of glucose, albumin and erythrocytes...
.
In regard to changes of
ploidyPloidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
, 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 all
germinatesGermination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
. To return to a haploid stage,
meiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
must occur (see
Cell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
). The cycles differ in the product of meiosis, and whether
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
(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
A
zygotic meiosis is a
meiosisMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
of a
zygoteA zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
immediately after
karyogamyKaryogamy is the fusion of pronuclei of two cells, as part of syngamy, fertilization, or true bacterial conjugation.It is one of the two major modes of reproduction in fungi...
, which is the fusion of two
cell nucleiIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
. 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
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
- Many protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
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 unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or
gameteA gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s. 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:
- Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s
- Some brown algae
The Phaeophyceae or brown algae , is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments, both as food and for the habitats they form...
- Some fungi, e.g. brewer's yeast
Haplodiplontic life cycle
In
sporic meiosis (also commonly known as
intermediary meiosis), the zygote divides mitotically to produce a multicellular diploid "
sporophyteAll land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
". The sporophyte creates spores via meiosis which
also then divide mitotically producing haploid individuals called "
gametophyteA gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
s". Gametophytes now 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:
- Most plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s
- Some fungi
Life history
The term "Life history" is better than "life cycle" especially for the
red algaeThe red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds...
(Rhodophyta). In the Rhodophyceae there are generally three stages and thus "alternation of generations" is not suitable.
Life history theory
In animal and human biology,
life history theoryLife history theory posits that the schedule and duration of key events in an organism's lifetime are shaped by natural selection to produce the largest possible number of surviving offspring...
is a method of understanding evolved behaviors and strategies to
optimizeOptimality theory is a linguistic model proposing that the observed forms of language arise from the interaction between conflicting constraints. OT models grammars as systems that provide mappings from inputs to outputs; typically, the inputs are conceived of as underlying representations, and...
reproductive success.