Population-genetic studies of Iris, a model genus for investigating speciation
Cornman, Robert Scott
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Empirical population-genetic studies can be used to investigate a variety of processes related to speciation. I applied population-genetic approaches to understand how genetic divergence occurs at various scales in the genus Iris, which is a model system for investigating speciation in plants. I first examined the problem of recombinant hybrid speciation, the occurrence of which is theoretically difficult but has been empirically demonstrated in Iris. Hybrid speciation requires the stabilization of a recombinant genome in sympatry with parental genotypes. Stabilization can be facilitated by factors such as assortative mating, selfing, vegetative propagation, or localized gene flow. To examine the extent to which these factors contribute to actual hybrid zone dynamics, I compared the genetic structure of established plants in a Louisiana Iris hybrid population with their mating patterns and progeny genotypes. There was no evidence that variation in phenology contributed to assortative mating, nor was there evidence of limited pollen dispersal: individuals from outside the censused population constituted one-half of all outcross paternity. Nonetheless, after controlling for clonal structure, we observed a strong spatial localization of related genotypes demonstrated by autocorrelation and cluster analyses. Furthermore, these clusters were not the product of selfing because the average inbreeding coefficient was near zero. I conclude that postzygotic selection limits successful sexual reproduction in this hybrid zone to matings between similar genotypes. Distinct hybrid lineages are therefore buffered from recombination while still in close geographic association with other recombinant or parental types, greatly facilitating hybrid speciation in this system. The second application of population-genetic approaches to the study of speciation involved the development of nuclear and chloroplast markers for I. missouriensis to determine whether strong genetic differentiation is present in this potentially cryptic species complex. I did not find evidence of genetic differentiation indicative of multiple species, but I was able to infer patterns of demographic history associated with interglacial climate change. °opulations in the southern Rocky Mountains are more closely related to coastal California than to the more biogeographically similar Sierra Nevada and Great Basin, suggesting the importance of long-distance dispersal in the history of the species.