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Alternation of generations

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**1. Alternation of Generations in Plants:**

– Definition: Alternation of generations involves the alternation of multicellular diploid and haploid forms in the organism’s life cycle.
– History: Descriptions by Chamisso, Steenstrup, Hofmeister, and Čelakovský led to the understanding of morphological alternation of generations in plants.
– Relationship between sporophyte and gametophyte: Varies among plant groups, with some showing independence while others demonstrate dependence of sporophyte on gametophyte.
– Variations: Differentiation of gametes and spores, complexity of life cycles, and variations in the dominance of sporophyte and gametophyte.
– Life cycles of different plant groups: Red and green algae, bryophytes, ferns, and seed plants exhibit varying patterns of alternation of generations.

**2. Alternation of Generations in Animals:**

– Animals lack alternation between multicellular diploid and haploid generations.
– Some animals exhibit alternation between parthenogenic and sexually reproductive phases.
– Examples include salps and doliolids in the class Thaliacea.
– Animal life cycles do not typically involve haploid-diploid alternation.

**3. Evolution of Dominant Diploid Phase:**

– Diploidy in plants masks deleterious mutations, increases genome size, and encodes new adaptations.
– Diploid phase dominance is seen in vascular plants due to genetic advantages.
– Selection effectiveness in haploid vs. diploid phases is debated, with diploid phase favored for genetic benefits.

**4. Alternation of Generations in Fungi and Slime Moulds:**

– Fungi and slime moulds exhibit alternation of haploid and diploid stages in their life cycles.
– Fungal mycelia are typically haploid, with mating types forming heterokaryon via plasmogamy.
– Slime moulds develop haploid spores that germinate into new cells, forming diploid zygotes through plasmogamy and karyogamy.

**5. Evolutionary History and Studies on Alternation of Generations:**

Evolutionary history of plants and the origin of the alternation of phases.
Ploidy, apomixis, and the importance of DNA repair and variation in the evolution of sexual reproduction.
– Studies on the physiological evaluation of plant sporophytes, new gametophytes, and the efficiency of selection in haploid and diploid life stages.

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.

Diagram showing the alternation of generations between a diploid sporophyte (bottom) and a haploid gametophyte (top)

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 (originating from different organisms of the same species or from the same organism) fuse to produce a diploid zygote, which divides repeatedly by mitosis, developing into a multicellular diploid sporophyte. This cycle, from gametophyte to sporophyte (or equally from sporophyte to gametophyte), 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. 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 generally there is no multicellular haploid phase. Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs; however females produced from fertilized eggs 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 and dibiontic are also in use, as is describing such an organism as having a diphasic ontogeny. 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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