Hybridization, sex determination, and evolutionary diversification in freshwater fishes

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Hybridization, when closely related species produce offspring, has been an integral part of evolutionary diversification, and is deeply entwined in the evolutionary history of many lineages of plants and animals. While hybridization can promote evolutionary diversification through providing raw material for the rapid formation of new species and promoting novel traits, hybridization can also inhibit diversification by homogenizing newly differentiated lineages. My research focuses on describing how evolutionary processes, specifically hybridization, influence the evolution and maintenance of biodiversity. One relatively unexplored area is describing exactly how major genomic rearrangements, including evolutionary turnover in sex determination mechanisms, influences hybridization outcomes. Sex determining regions of the genome are believed to have been crucially important in the evolution of new species and maintenance of reproductive isolation between nascent species, but our understanding of genetic mechanisms of sex determination is still incomplete in many taxa, including fish. Fish have extremely variable sex determination mechanisms that include genetic sex determination with differentiated sex chromosomes, genetic sex determination without differentiated sex chromosomes (including polygenic systems), individuals that are hermaphrodites, and environmental sex determination. Sex determination systems in fish are known to differ within closely related groups of fish, and even across populations within a species. I hypothesize that sex determination mechanisms have shaped outcomes of hybridization in two groups of freshwater fishes in North America, specifically Catostomus suckers and Chrosomus dace. Both of these evolutionary lineages have been influenced by hybridization, but in very different ways. Catostomus suckers are part of a species-rich, phenotypically diverse, and geographically widespread family (Catostomidae) that arose from an allotetraploid hybrid ancestor approximately 50 million years ago, and extant Catostomus species hybridize extensively. In contrast, Chrosomus dace present a much different outcome of a hybridization event believed to have occurred between C. eos and C. neogaeus approximately 50,000 years ago, namely the origin of gynogenesis, a rare phenomenon where unisexual, asexual hybrid lineages reproduce clonally. I will use genomic data to describe the genetic basis of sex determination for both Catostomus and Chrosomus species, and identify how genetic sex determination mechanisms have shaped outcomes of both historical and contemporary hybridization in these lineages. By explicitly linking sex determination with hybridization outcomes for these two lineages, I will advance our understanding of how sex-determining loci might be more broadly implicated in evolutionary diversification and the maintenance of biodiversity.