Alternation of generations (also known as alternation of phases or metagenesis) is a term primarily used in describing the life cycle of plants (taken here to mean the Archaeplastida). A multicellular diploid sporophyte, with N paired chromosomes (i.e. 2N in total), alternates with a multicellular haploid gametophyte, with N unpaired chromosomes. A mature sporophyte produces spores by meiosis, a process which results in a reduction of the number of chromosomes by a half. Spores germinate and grow into a gametophyte. At maturity, the gametophyte produces gametes by mitosis. Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a zygote, which develops into a diploid sporophyte. This cycle, from sporophyte to sporophyte (or equally from gametophyte to gametophyte), is the way in which all land plants and many algae undergo sexual reproduction.
All animals develop differently. A mature animal is diploid and so is, in one sense, equivalent to a sporophyte. However, an animal directly produces haploid gametes by meiosis. No haploid spores capable of dividing are produced, so neither is a haploid gametophyte. There is no alternation between diploid and haploid forms.
Other organisms, such as fungi, can have life-cycles in which different kinds of organism alternate. The term 'alternation of generations' has also been applied to these cases.
Life cycles, such as those of plants, with alternating haploid and diploid phases can be referred to as diplohaplontic (the equivalent terms haplodiplontic, diplobiontic or dibiontic are also in use). Life cycles, such as those of animals, in which there is only a diploid phase are referred to as diplontic. (Life cycles in which there is only a haploid phase are referred to as haplontic.)
The discussion of 'alternation of generations' above treats the alternation of a multicellular diploid form with a multicellular haploid form as the defining characteristic, regardless of whether these forms are free-living or not. In some species, such as the alga Ulva lactuca, the diploid and haploid forms are indeed both free-living independent organisms, essentially identical in appearance. The free-swimming gametes form a zygote which germinates into a diploid sporophyte; the free-swimming spores germinate into a haploid gametophyte. Alternation of generations is an appropriate term.
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