Alternation of generations
From Freepedia
Alternation of generations is a term usually applied to the reproductive cycle of many non-vascular and vascular plants, fungi and protists. This life cycle is characterized by each generation consisting of two separate, free-living plant bodies with different numbers of chromosomes: the gametophyte (or haploid) plant and the sporophyte (or diploid) plant. There are two kinds of alternation, when the sporophyte and the gametophye are identical they are called the isomorphic generations, when they have a different appearance they are called heteromorphic generations. This kind of life cycle may also be called diplobiontic, diplohaplontic, haplodiplontic, or dibiontic.
Heterogamy is a term used to describe alteration between parthenogenesis and sexual reproduction, that occurs in some invertebrates and vertebrates. Although it is a similar concept to the alternation of generations the genetics of the two processes are significantly different.
Contents |
Plants
Non-vascular plants
The alternation of generations occurs in all marine algae. In most red algae, many green algae, and a few brown algae, the generations are isomorphic and free-living. Some species of red algae have a complex triphasic alternation of generations. The kelp are an example of a brown alga with a heteromorphic alteration 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.
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.
Vascular plants
Seed producing vascular plants also employ an alternation of generations in their reproduction. However their reproductive mechanisms are more advanced that those of the non-vascular plants. Seed plants produce pollen which is the haploid male gametophyte by meiosis. Pollen does not need water for transport, thus with the evolution of pollen plants were able to colonise dry environments. When the pollen releases the sperm and fertilization occurs in the archegonium the sporophye or seed is formed, relying on the parent for nutrition during its developemnt. Thus in the case of vascular plants the sporophyte is the dominant generation. Gymnosperm gametophytes are smaller than those from non-vascular plants, and are not free-living. The angiosperms male and female gametophytes are composed of even fewer cells than in gymnosperms. Angiosperms require two sperm to fertilize the two hapolid nuclei of the female gametophyte in a process called double fertilization.
Fungi
Fungal mycelia are typically haploid. When mycelia of different mating types meet, they produce two multinucleate ball-shaped cells, which form a mating bridge. The result is that nuclei move from one mycelium into the other, forming a heterokaryon (meaning different nuclei). This 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 very short lived sporophyte that immediately undergoes meiosis to form haploid spores. When the spores germinate they develop into new mycelia.
Protists
Tho groups of protists undergo an alternation of generations, the slime moulds and the foraminifera. The life cycle of slime moulds is similar to that of fungi. Haploid spores germinate to form swarm cells or myxamoebae, these can then 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 one to many fruiting bodies containing haploid spores depentding on species.
Foraminifera undergo an heterophasic alternation of generations between a haploid gamont and a diploid agamont. In fomaminfera the haploid generation is typically much larger that the diploid generation
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
