. "17 The Role of Genetic and Genomic Attributes in the Success of Polyploids." Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years after Stebbins. Washington, DC: The National Academies Press, 2000.
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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS
of sequence under study also may be important: is it coding or noncoding DNA, is it telomeric or centric in origin, and is it located near heterochromatin? Finally, what is the level of genetic differentiation between the parents?
Although unreduced gamete production and even polyploid formation may be quite common in many groups of plants (Ramsey and Schemske, 1998), there are many obstacles to establishment of a polyploid population. Minority cytotype exclusion (Levin, 1975; Fowler and Levin, 1984; Felber, 1991) may be particularly important in newly formed outcrossing polyploids where there are few potential mates unless there is substantial assortative mating (Husband, 2000); when only one or a few polyploid individuals emerge within a population of diploids, outcrossing polyploid individuals may spend most of their gametes in sterile or partially sterile matings with their diploid parents. The apparent success of polyploids is biased toward those species that have overcome the barrier(s) to establishment, and this success may ultimately derive from a number of the genetic attributes of the polyploids. Polyploids have increased heterozygosity, an attribute that may be beneficial (Mitton and Grant, 1984; Mitton, 1989). Polyploids also harbor higher levels of genetic and genomic diversity than was anticipated, with recurrent formation from genetically divergent diploid parents and possibly genome rearrangements contributing genetic diversity. This genetic diversity results in greater biochemical diversity, which also may be beneficial to the polyploid (Levin, 1983). Finally, these genetic attributes may have ecological consequences. For example, if polyploids have lower inbreeding depression and are more highly selfing, they may be better colonizers, explaining the prevalence of polyploids on the list of the world's worst weeds. Polyploids may have broader ecological amplitudes than do their diploid progenitors because of their increased genetic and biochemical diversity (Levin, 1983). Polyploids may experience new interactions with other species, such as pollinators (Segraves, 1998; Segraves and Thompson, 1999).
What are some of the future directions we see for research on the genetic attributes of polyploids? The general mode of formation of polyploids remains unknown; research into the factors that produce unreduced gametes and bring them together certainly is warranted. Additional studies, both theoretical and empirical, are needed to address expectations of inbreeding depression and outcrossing rates. Furthermore, the levels of gene flow among populations, especially those populations of separate origin, are unknown. Regarding genome rearrangements, how extensive are they within an individual or race? How widespread are they among species? How quickly do such rearrangements occur? Do populations of separate origin exhibit the same or different rearrangements? Finally, are basal angiosperms and homosporous pteridophytes