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Briana L. Gross

Washington University in St. Louis
The origin and evolution of the homoploid hybrid species Helianthus deserticola
Interspecific hybridization is frequent in plants, but the evolutionary forces that determine the outcome(s) of hybridization are poorly understood. My research has focused on one important consequence of hybridization: the origin of new species, in this case the homoploid hybrid sunflower, Helianthus deserticola. The hybrid species inhabits the desert floor, an environment that is both extreme and divergent compared to those of its two parental species, H. annuus and H. petiolaris. I am exploring the origin and subsequent evolution of this species using phylogeographic, quantitative genetic, and genomic techniques. A phylogeographic study indicated that H. deserticola may be the product of multiple hybrid speciation events, emphasizing the importance of deterministic, rather than stochastic, forces in the speciation process. An analysis of phenotypic selection documented strong selection acting on early generation hybrids between the parental species grown in the H. deserticola habitat. Selection on several traits was in the direction of H. deserticola, suggesting that ecological selection shaped the morphology and life history of the hybrid species. Genotyping the BC2Pet cross from the field experiment showed that the strength of selection on individual QTLs would allow divergence in the face of gene flow. Finally, a genomic scan for the signature of selection revealed that some loci showed a selective sweep across the entire species range with the same allele fixed in all populations. This pattern is consistent with concerted evolution of H. deserticola as a species, despite geographical isolation and potential independent origins of different populations.
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