Alternation of generations

Alternation of generations (also known as metagenesis) is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct sexual haploid and asexual diploid stages. In these groups, a multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n. Diagram of alternation of generations in liverworts.Moss life cycle diagramHornwort life cycle diagramAngiosperm life cycleTip of tulip stamen showing pollen (microgametophytes)Plant ovules (megagametophytes): Gymnosperm ovule on left, angiosperm ovule (inside ovary) on rightDouble fertilization Alternation of generations (also known as metagenesis) is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct sexual haploid and asexual diploid stages. In these groups, a multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n. The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. 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 gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many algae undergo sexual reproduction. The relationship between the sporophyte and gametophyte varies among different groups of plants. In those algae which have alternation of generations, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. In liverworts, mosses and hornworts, the sporophyte is less well developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen. By contrast, in all modern vascular plants the gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity. In ferns the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is briefly dependent for its nutrition. In flowering plants, the reduction of the gametophyte is much more extreme; it consists of just a few cells which grow entirely inside the sporophyte. Animals develop differently. They directly produce haploid gametes. No haploid spores capable of dividing are produced, so they do not have a haploid gametophyte alternating with a diploid sporophyte. (Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs; however the females are diploid.) Life cycles of plants and algae with alternating haploid and diploid multicellular stages are 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 multicellular stage are referred to as diplontic. Life cycles in which there is only a haploid multicellular stage are referred to as haplontic. Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether or not these forms are free-living. 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 and therefore said to be isomorphic. The free-swimming, haploid gametes form a diploid zygote which germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by meiosis that germinate into haploid gametophytes. However, in some other groups, either the sporophyte or the gametophyte is very much reduced and is incapable of free living. For example, in all bryophytes the gametophyte generation is dominant and the sporophyte is dependent on it. By contrast, in all modern vascular land plants the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors. In seed plants, the female gametophyte develops totally within the sporophyte which protects and nurtures it and the embryo sporophyte that it produces. The pollen grains, which are the male gametophytes, are reduced to only a few cells (just three cells in many cases). Here the notion of two generations is less obvious; as Bateman & Dimichele say 'porophyte and gametophyte effectively function as a single organism'. The alternative term 'alternation of phases' may then be more appropriate. Debates about alternation of generations in the early twentieth century can be confusing because various ways of classifying 'generations' co-exist (sexual vs. asexual, gametophyte vs. sporophyte, haploid vs. diploid, etc.). Initially, Chamisso and Steenstrup described the succession of differently organized generations (sexual and asexual) in animals as 'alternation of generations', while studying the development of tunicates, cnidarians and trematode animals. This phenomenon is also known as heterogamy. Presently, the term 'alternation of generations' is almost exclusively associated with the life cycles of plants, specifically with the alternation of haploid gametophytes and diploid sporophytes.