S

Fig. 7.5. Lineage recombination. The genotypes of four AABB polyploid plants (a-d) are shown, based on the networks shown in Figs 7.3 and 7.4. Two cases are shown. In the first (1a-1d), there are two classes of plants, each with the same alleles at the two homoeologous loci and having the same chloroplast haplotype; these are classified as genotypes P and Q. In the second example (plants 2a-2d), these same two genotypes are also found, but lineage recombination has occurred to produce two new genotypes, R and S. In Case 1 there is evidence for two origins of the AABB polyploid. In Case 2 there is also evidence of more than one origin, but the number of different genotypes can be explained either by four separate origins or by lineage recombination following gene exchange between plants from a smaller number of origins.

is the inference that polyploids have the potential to sample extensively from the pool of genetic variation found in their progenitors. This is true spatially, through the contemporaneous formation of poly-ploids from different diploid populations with different genotypes. It is also true temporally, if polyploid formation continues over a long enough time to permit sampling of progenitors which themselves are experiencing allelic divergence from their antecedents. In such cases, the polyploid gene pool would receive periodic infusions of new allelic variation, the importance of which is not fully understood but which clearly could provide fodder for evolutionary diversification. There is as yet no documentation of the phenomenon of temporally recurrent polyploid formation, and it may be experimentally challenging to gather the quality of evidence necessary to rule out alternative explanations for an observed pattern of diversity within a given polyploid lineage. For example, one phylogenetic signature of this scenario might be that a polyploid would share some alleles with a diploid progenitor and others that are unique to the polyploid. This pattern, however, may also be produced by incomplete sampling of, or lineage extinction within, the diploid. Notwithstanding the absence of compelling empirical examples, there is no reason to suspect that polyploids do not form recurrently over time, at least in those cases where long-term sympatry among progenitors may be expected and where barriers to hybridization are relatively weak.

Multiple origins are well illustrated by Tragopogon, a genus of Asteraceae that provides the best-studied example of recent allopolyploid origin. Early work by Ownbey (1950) showed that two polyploid species, Tragopogon miscellus and Tragopogon mirus, originated through allopolyploidy in the western United States around the turn of the

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