Alternation of generations
Alternation of generations is a term applied to a reproductive cycle of certain plants, fungi, and protists. It is a bit of a misnomer, a more accurate name would be "alternation of phases of a single generation" because a generation of a species typically encompasses one complete life cycle. The life cycle of organisms with so-called "alternation of generations" is characterized by each phase consisting 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.
The sporophyte will undergo meiosis, then at the end of the diploid stage undergo meiosis to produce haploid gametophytes, which then undergo mitosis to become separate, free-living organisms. Other gametophytes which most likely have different source sporophytes from each organism then become recombine and germinate form sporophytes.
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" ^ . 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
Two groups of protists undergo an alternation of generations: the slime moulds and the foraminifera. The life cycle of slime moulds 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.
Plants
Non-vascular plants
Alternation of generations occurs in 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.
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 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.
References
^ Dettmering, C. et al. 1998. The trimorphic life cycle in foraminifera: Observations from cultures allow new evaluation. European Journal of Protistology 34:363-368 ^ Taylor, T.N. et al. 2005. Life history biology of early land plants: Understanding the gametophyte phase. PNAS 102:5892-5897