Alternation of generations
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Alternation of generations, also known as metagenesis or heterogenesis, is the predominant type of life cycle in plants and algae. In plants, both phases are multicellular: the haploid sexual phase – the gametophyte – alternates with a diploid asexual phase – the sporophyte.
A mature sporophyte produces haploid spores by meiosis, a process which reduces the number of chromosomes to half, from two sets to one. The resulting haploid spores germinate and grow into multicellular haploid gametophytes. At maturity, a gametophyte produces gametes by mitosis, the normal process of cell division in eukaryotes, which maintains the original number of chromosomes. Two haploid gametes fuse to produce a diploid zygote, which divides repeatedly by mitosis, developing into a multicellular diploid sporophyte. This cycle, from gametophyte to sporophyte, is the way in which all land plants and most algae undergo sexual reproduction.
The relationship between the sporophyte and gametophyte phases varies among different groups of plants. In the majority of algae, 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. By contrast, in all modern vascular plants, the gametophyte is less well developed than the sporophyte. 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 generally there is no multicellular haploid phase. Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs; however the females are diploid, produced from fertilized eggs.
Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as diplohaplontic. Equivalent terms include haplodiplontic, diplobiontic and dibiontic. Life cycles 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.
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- Source / publisher: Wikipedia
- URL type: WWW
- Credits: Wikipedia
- URL: https://en.wikipedia.org/wiki/Alternation_of_generations