DISSERTATION RESEARCH: Identifying the genetic basis of adaptation to a freshwater environment using admixture in natural populations

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This research will determine the genetic changes that have permitted the Atlantic killifish, Fundulus heteroclitus, to move from saltwater to freshwater. Adaptation to different environments is important for generating biological diversity, and for fish, environmental salinity is a major factor limiting where they can live. Adaptation to different salinities is therefore thought to have been critical in the generation of the amazing biodiversity of fishes. However little is known about the genetic and physiological changes that enable species to adapt to different salinities. The Atlantic killifish mainly occupies marine habitats, but some populations have recently moved into fresh water. The freshwater populations provide an ideal study system for discovering the genetic changes that have enabled life in a fresh water environment. This research is important because it will shed light on the types of evolutionary changes that have permitted this species, and perhaps many species, to colonize new environments resulting in the biodiversity of the fishes. The project will have broader societal impacts through the training of undergraduates in the techniques and research methodology of the project. In addition, the research will provide material in college preparatory mentorship of disadvantaged youth through the AggieMentors program at UC Davis. The main goal of this research is to identify the genetic loci that are associated with tolerance of low salinity. Admixture mapping is a method that takes advantage of natural hybridization between genetically and phenotypically distinct populations in order to determine the loci that influence a particular phenotypic trait. Admixture mapping will be applied to two replicate hybrid zones: one within the Chesapeake Bay consisting of a freshwater native and a brackish water native population, and the other along the east coast where a northern and southern population are both marine. Sampling in these zones will provide genetically similar populations that experience vastly different selective pressures. It is therefore possible to disentangle the effects of genetic ancestry versus the effects of selection on the fitness of fish adapted to low salinity environments. Parental populations and their hybrids will be tested for low salinity tolerance and genotyped using restriction site associated genetic markers. Admixture mapping will identify loci contributing to the specific phenotypes. Genome-wide analysis will identify loci that are under selection only in the freshwater environment. This combination will not only allow for the identification of phenotypes involved in general salinity tolerance, but also will reveal those loci driving recent adaptation to fresh water.