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How do polyploids become established?

The frequency of polyploid events is exceedingly rare (estimated to be 10-5 among offspring of diploids; Ramsey and Schemske, 1998). Although the formation of unreduced (2n) gametes is considered to be rare in general (McCoy, 1982), 2n gamete production is likely to play a major role in polyploid origins (Harlan and deWet, 1975; Vorsa and Binghm, 1979). A number of factors – genetic and environmental – have been shown to influence the frequency of 2n gamete formation (Sax 1937; Thompson and Lumaret 1991; Ramsey and Schemske 2002). Genetic factors also control unreduced gamete formation (potato, Mok and Peloquin, 1975; Veilleux et al. 1982, Peloquin, in press; alfalfa, McCoy, 1982; blueberry, Qu and Vorsa, 1999). Genes that control rates of unreduced gamete production could become fixed in small populations, and enable rare polyploids to become more frequent. Environmental factors that affect 2n gamete formation include sudden changes in temperature (heat or cold treatment), dehydration, x-rays, uv light, infections, etc., and can induce chromosome doubling (Sax, 1937). Otto and Whitton ?? Mable?? Other factors that can contribute to polyploid formation (aside from unreduced gametes) include superior vegetative (clonal) growth, perennial life history, niche separation, assortative mating, and other fitness differences. Therefore, broad generalizations may not apply to specific cases. As such, different species need to be characterized and systematically analysed to determine what mechanisms are responsible for bringing about the observed polyploid frequencies and subsequent evolutionary patterns.

A critical first step in polyploid evolution is the establishment and subsequent persistence of the neopolyploid (Fowler and Levin, 1984). A new and therefore rare polyploid in a diploid population would be at a major fertility disadvantage, since most pollinations of the polyploid will involve pollen from diploids. The predominance of one cytotype excluding the rare cytotype from reaching high frequences is known as the minority cytotype exclusion principle (Husband, 1999; 2000). A number of models have been developed to determine how polyploids may become established in a diploid population (Fowler and Levin, 1984; Felber, 1991; Rodriguez, 1996; Husband, 2000). Parameters included in these models include the production of unreduced (2n) gametes by the diploid cytotype, the frequency of tetraploids formed with each generation, multiple origins of polyploids over several generations (given perenials versus annuals). Once 2n gamete production exceeds a certain threshhold, the tetraploids are able to replace diploids. The threshold varies by modifying fertility/viability of the cytotypes. Frequency-dependent processes can be overcome by reducing inter-cytotype matings, so rare cytotypes could become established despite the minority cytotype disadvantage.

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