Alternation of Generations - Other Groups of Organisms

Other Groups of Organisms

Some organisms currently classified in the Chromalveolata, and thus not plants in the sense used here, exhibit alternation of generations. Kelp are an example of a brown alga with a heteromorphic alternation 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. Foraminifera undergo a heteromorphic alternation of generations between haploid gamont and diploid agamont forms. The single-celled haploid organism is typically much larger than the diploid organism.

Fungal mycelia are typically haploid. When mycelia of different mating types meet, they produce two multinucleate ball-shaped cells, which join via a "mating bridge". Nuclei move from one mycelium into the other, forming a heterokaryon (meaning "different nuclei"). This process 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 short-lived sporophyte that soon undergoes meiosis to form haploid spores. When the spores germinate, they develop into new mycelia.

The life cycle of slime moulds is very similar to that of fungi. Haploid spores germinate to form swarm cells or myxamoebae. These 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, depending on the species, one to many fruiting bodies containing haploid spores.

In some animals, there is an alternation between parthenogenic and sexually reproductive phases (heterogamy). Both phases are diploid. This has also been called "alternation of generations", although as alternation between a diploid and a haploid generation is never found in animals, most modern sources seem to avoid the term in this context.

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