Alternation of generations is a reproductive cycle of those
plants,
fungi and
protists in which a multicellular
diploid phase alternates with a multicellular
haploid phase. The term can be 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 distinct organisms a
gametophyte (
thallus (tissue) 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 all land plants and many algae undergo
sexual reproduction.
It is often stated that 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, alternation of generations implies that both the diploid and haploid stages are multicellular and this is more important than "free-living" [1]. Such a distinction changes the concept to one separating animals and plants. The gametophyte and sporophyte are usually separate, independent organisms in basal algae such as Ulva lactuca, where the gametes are free-swimming, and the zygote is formed in the water. By contrast, in land plants the sporophytes are to a greater or lesser extent dependent on the gametophytes, and vice-versa, especially in the gymnosperms and angiosperms. Indeed it is a defining characteristic of the land plants, or embryophytes (and hence the name), that a developing multicellular sporophyte is, for at least the first stages of its development, nurtured by the gametophyte, as can be seen most clearly in the bryophytes.
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.
Life cycles in which there is no multicellular diploid phase are referred to as haplontic. Life cycles with alternating haploid and diploid phases are diplohaplontic, but the equivalent terms diplobiontic, haplodiplontic, or dibiontic are also in use. Two main types of diplohaplontic (alternating) life-cycles are recognized if the sporophyte and the gametophyte generations are more or less identical in form, the life cycle is said to be isomorphic, meaning "same form". If the generations have very different morphology, the life cycle is called heteromorphic meaning "different forms".
