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polyploid. Perhaps most important to Stebbins (1950) was the availability of new ecological niches. Additional hypotheses for the success of polyploids include broader ecological amplitude of the polyploid relative to its diploid parents, better colonizing ability, higher selfing rates, and increased heterozygosity.

In fact, many aspects of the genetic systems of polyploids may contribute to the success of polyploid plants. These characteristics range from the molecular level to the population level and include increased heterozygosity, reduced inbreeding depression and an associated increase in selfing rates, increased genetic diversity through multiple formations of a polyploid species, genome rearrangements, and ancient polyploidy and gene silencing. But what role, if any, do these factors really play in the success of polyploids? In this paper, we will explore the evidence for the role of these genetic attributes in the evolutionary success of polyploid plant species.

ALLO- VERSUS AUTOPOLYPLOIDY

We will distinguish among types of polyploids by using Stebbins' (1947, 1950) classification: Allopolyploids are those polyploids that have arisen through the processes of interspecific hybridization and chromosome doubling (not necessarily in that order), autopolyploids are those polyploids that have arisen from conspecific parents, and segmental allopolyploids are those that have arisen from parents with partially divergent chromosome arrangements such that some chromosomal regions are homologous between the parents and others are homoeologous; segmental allopolyploids will not be considered further in this paper. Allopolyploids are characterized by fixed (i.e., nonsegregating) heterozygosity, resulting from the combination of divergent parental genomes; bivalent formation occurs at meiosis, and disomic inheritance operates at each locus. Autopolyploids may exhibit multivalent formation at meiosis and are characterized by polysomic inheritance. Allopolyploids are considered much more prevalent in nature than are autopolyploids, but even a cursory glance at any flora (for example, see Hickman, 1993) or list of plant chromosome numbers (for example, see Federov, 1969) will reveal multiple cytotypes within many species, even though these additional ploidal levels are not typically accorded species status. Thus, autopolyploids in nature likely are much more common than typically is recognized.

INCREASED HETEROZYGOSITY

Roose and Gottlieb (1976) showed that allotetraploids in Tragopogon had fixed heterozygosity at isozyme loci, representing the combination of



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