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How do polyploids acquire variation?

Polyploids can acquire variation both through mechanisms of population genetics (gene flow with diploids and multiple origins of polyploids), and through mechanisms that generate “de novo variation” such as chromosomal rearrangements and epigenetic phenomena.

Polyploidy has long been considered an important example of instant or sympatric speciation, since polyploid species are mostly reproductively isolated from their diploid progenitors (Stebbins 1950, 1971; Levin 1983). An interesting aspect related to allopolyploidization or hybridization of different species is the question of the “species barrier” when using a biological species concept. Members of the same biological species are commonly defined as related individuals of a population that can interbreed and whose offspring are fertile. Thus, the horse and a donkey are considered separate species because their hybrid offspring are viable but infertile. In plants, hybridization of different species is quite common and many of the well-known crop plants are allopolyploids resulting from inter-species hybrids. Such allopolyploids pose a challenge to phylogenetic species concepts, which define species on strict monophyletic criteria. Over the last decade this challenge has taken on additional relevance as “polyploid species” have been found to form repeatedly in close proximity to one another (Soltis and Soltis, 1993; 1999; 2000). The polyphyly of “polyploid species” calls into question the very definition of “species.” Allopolyploids – like other organisms with reticulate evolutionary histories (e.g., eukaryotes, lichens) – give biologists important examples when theorizing about evolutionary entities. Aside from philosophical considerations about species definitions, there are many implications for the multiplicity of origins for polyploids. Multiple origins of polyploid species have been reported for mosses, ferns, and many angiosperms (reviewed in Vogel et al., 1999; Soltis and Soltis, 2000).

Allopolyploidy presents a paradox because it is both a diversifying force and a genetic bottleneck (Stebbins, 1971). However, the genetic bottleneck problem may be solved by the fact that population-level genetic studies of polyploid plants and animals indicate that polyploidy is not a rare event leading to unique and uniform genotypes. Rather, the multiple independent formations of polyploid species from heterozygous diploid progenitors may provide a significant source of genetic variation (reviewed in Soltis and Soltis, 1993; 1999; 2000).

Many new polyploids also are genetically unstable, and the next section describes mechanisms that can lead to novel variation.

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