Life cycle

In biology, life cycle is the series of changes that an organism undergoes its inception through means of reproduction, whether through asexual reproduction or sexual reproduction, to the inception of the following generation in that same phase of the cyle. For some organisms, particularly samll, simple organisms such as bacteria and some protists, the life cycle is complete in one generation. This is also the case for many animals, where the male and female gamete fuse to form the new offspring. In plants, reproduction is multigenerational, also known as alternation of generations.

Sexual Reproduction

Three types of life cycles exist, depending on ploidy, a multiple of the number of chromosomes in a cell:

• 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. 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.

Alternation of Generations

Sporic or diplohaplontic life cycle. A diploid (2n) sporophyte undergoes meiosis to produce haploid (1n) reproductive cells, often called spores. Haploid cells undergo mitosis to produce a gametophyte. The gametophyte produces haploid gametes which fuse to form a diploid zygotic sporophyte.

Alternation of generations is a term applied to a reproductive cycle of certain vascular plants, fungi, and protists. The term is a bit confusing for people familiar only with the life cycle of a typical animal. A more understandable name would be "alternation of phases of a single generation" because we usually consider a generation of a species to encompass one complete life cycle. The life cycle of organisms with "alternation of generations" is characterized by each phase consisting of one of two separate, free-living organisms: a gametophyte (thallus or plant), which is genetically haploid, and a sporophyte (thallus or plant), which is genetically diploid.

A haploid plant of the gametophyte generation produces gametes by mitosis. Two gametes (originating from different organisms of the same species or from the same organism) combine to produce a zygote, which develops into a diploid plant of the sporophyte generation. This sporophyte produces spores by meiosis, which germinate and develop into a gametophyte of the next generation. This cycle, from gametophyte to gametophyte, is the way in which plants and many algae undergo sexual reproduction.

Distinctions

The distinction of "free-living" is important, because all sexually reproducing organisms can be thought to involve alternating phases, at least at the cellular level as meiosis. However, not all biologists agree. It is often stated that alternation of generations refers to both the diploid and haploid stages being "multicellular" and this is more important than "free-living" (Taylor T.N. et al. 2005). Such a distinction changes the concept to one separating animals and plants.

All plants have diploid sporophyte and haploid gametophyte stages that are multicellular, and the differences between plant groups are in the relative sizes, forms, and trophic abilities of the gametophyte or sporophyte forms, as well as the level of differentiation in the gametophytes. An example would be comparing pollen and ovules to bisexual gametophyte thalli. Both approaches are discussed in this article.

Biologists recognize two categories of alternation: the first if the sporophyte and the gametophye forms are more or less identical, alternation is called isomorphic; and second if the forms have very different appearances, alternation is called heteromorphic. Other terms applied to this kind of life cycle are diplobiontic, diplohaplontic, haplodiplontic, or dibiontic.

Heterogamy is a term used to describe alternation between parthenogenic and sexually reproductive phases that occurs in some invertebrates and vertebrates. Although conceptually similar to "alternation of generations", the genetics of heterogamy is significantly different.

Fungi

Fungal mycelia are typically haploid. When mycelia of different mating types meet, they produce two multinucleate ball-shaped cells, which join via a "mating bridge". Nuclei move from one mycelium into the other, forming a heterokaryon (meaning "different nuclei"). This process is called plasmogamy. Actual fusion to form diploid nuclei is called karyogamy, and may not occur until sporangia are formed. Karogamy produces a diploid zygote, which is a short-lived sporophyte that soon undergoes meiosis to form haploid spores. When the spores germinate, they develop into new mycelia.

Protists

Some protists undergo an alternation of generations, including the slime molds, foraminifera, and many marine algae.

The life cycle of slime molds is very similar to that of fungi. Haploid spores germinate to form swarm cells or myxamoebae. These fuse in a process referred to as plasmogamy and karyogamy to form a diploid zygote. The zygote develops into a plasmodium, and the mature plasmodium produces, depending on the species, one to many fruiting bodies containing haploid spores.

Foraminifera undergo a heteromorphic alternation of generations between a haploid gamont and a diploid agamont phases. The single-celled haploid organism is typically much larger than the diploid organism.

Alternation of generations occurs in almost all marine seaweeds. In most red algae, many green algae, and a few brown algae, the phases are isomorphic and free-living. Some species of red algae have a complex triphasic alternation of generations. Kelp are an example of a brown alga with a heteromorphic alternation of generations. Species from the genus Laminaria have a large sporophytic thallus that produces haploid spores which germinate to produce free-living microscopic male and female gametophytes.

Plants

Non-vascular plants

Nontracheophyte plants including the liverworts, hornworts and mosses undergo an alternation of generations; the gametophyte generation is the most common. The haploid gametophyte produces haploid gametes in multicellular gametangia. Female gametangia are called archegonium and produce eggs, while male structures called antheridium produce sperm. Water is required so that the sperm can swim to the archegonium, where the eggs are fertilized to form the diploid zygote. The zygote develops into a sporophyte that is dependent on the parent gametophyte. Mature sporophytes produce haploid spores by meiosis in sporangia. When a spore germinates, it grows into another gametophyte.

Vascular plants

Ferns and their allies, including clubmoss and horsetails, reproduce via an alteration of generations. The conspicuous plant observed in the field is the diploid sporophyte. This plant creates by meiosis single-celled haploid spores which are shed and dispersed by the wind (or in some cases, by floating on water). If conditions are right, a spore will germinate and grow into a rather inconspicuous plant body called a prothallus.

The underside of a Dicksonia antarctica frond showing the sori, or spore-holding structures.

The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations. The prothallus is short-lived, but carries out sexual reproduction, producing the diploid zygote that then grows out of the prothallus as the sporophyte.

Haplontic life cycle

Zygotic meiosis

A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, 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:

• All fungi
• Some green algae
• Many protozoa

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 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

Sporic meiosis

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.

Asexual Reproduction

In cases of asexual reproduction, the life cycle is complete in one generation, where an individual inherits all of its chromosomes from one parent and is genetically identical to its parents. Prokaryotes, such as bacteria, undergo binary fission, where each cell divides in half to form two cells with identical DNA to the original cell. In order for the original cell divide, the prokaryotic chromosome that is a single DNA molecule must first replicate and then attaches itself to a different part of the cell membrane. Most protists, unicellular eukaryotes, also reproduce asexually, except under stress they reproduce sexually.

Many multicellular organisms also have the ability to reproduce asexually. Many such organisms will bud off a localized cluster of cells, which then grows through mitosis to form a new individual. Animals such as sponges can reproduce by fragmenting their bodies. Many plants have the ability to reproduce asexually as well.

Life history theory

In animal and human biology life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success.Life history theory is an analytical framework widely used in animal and human biology, psychology, and evolutionary anthropology which postulates that many of the physiological traits and behaviors of individuals may be best understood in terms of the key maturational and reproductive characteristics that define the life course.

Examples of these characteristics include:

• Age at weaning
• Age of sexual maturity or puberty
• Adult body size
• Age specific mortality schedules
• Age specific fecundity
• Time to first sexual activity or mating
• Time to first reproduction
• Duration of gestation
• Litter size
• Interbirth interval

Variations in these characteristics reflect differing allocations of an individual's resources (i.e., time, effort, and energy expenditure) to competing life functions, especially growth, body maintenance, and reproduction. For any given individual, available resources in any particular environment are finite. Time, effort, and energy used for one purpose diminishes the time effort, and energy available for another. For example, resources spent growing to a larger body size cannot be spent increasing the number of offspring. In general terms the costs of reproduction may be paid in terms of energy being diverted away from body repair and maintenance and by reducing investment in immunological competence.

References

ISBN links support NWE through referral fees

• Dettmering, C. et al. 1998. The trimorphic life cycle in foraminifera: Observations from cultures allow new evaluation. European Journal of Protistology.34:363-368.

• Graham, L., J. Graham, & L. Wilcox. 2003. Plant Biology. Pearson Education, Inc., Upper Saddle River, N.J.: pp. 258-259.

• Raven, Peter H. and George B. Johnson. 1996. Biology. Dubuque, IA: Wn.C. Brown Publishers.

• Taylor, T.N. and others. 2005. Life history biology of early land plants: Understanding the gametophyte phase. Proceedings of the National Academy of Sciences. 102:5892-5897.

Links

Cell Division: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookmito.html

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