The Maintenance of Genetic Variation by Environmental Selection
Understanding forces creating or maintaining the vast amount of biodiversity has been a major task of biologists. Genetic variation plays a major role in the creation of biodiversity because in contrast to environmental influence, genetic variants can be inherited. For a species in natural environments, genetic variation is generated by mutation, eliminated by genetic drift or selective sweep, and maintained by balancing selection that favors different alleles in different environments or time. In my dissertation, I will address how spatially heterogeneous environmental selection maintains genetic variation in two aspects.
Genes in the genome vary vastly in their level of polymorphism. Previous studies have used features within the genome, such as recombination rate or expression level, to explain the variation in gene polymorphism. One factor, however, that has often been overlooked is the effect of environmental adaptation on gene polymorphism. Specifically, if different alleles of a gene are responsible for local adaptation to distinct environments, the polymorphism of this gene will be actively maintained by spatially heterogeneous environmental selection. In the first part (Chapter 2) of my dissertation, I used publicly available genomic data from Arabidopsis thaliana to address this question. I found that environmental relevance of a gene has a significantly positive relationship with the variation in polymorphism level among genes in the Arabidopsis genome, consistent with the hypothesis that environmental selection actively maintains the polymorphism of environmentally responsive genes.
A biological species is formed by a mating pool of individuals, and for two populations of the same species, differentiation is often homogenized by gene flow. Reproductive isolation between populations allows genetic differentiation, and therefore speciation, the process in which full reproductive isolation is achieved between populations, plays important role in generating biodiversity. In the second part of my dissertation I used Boechera stricta to address how environmental selection contributes to speciation. In Chapter 3, I used niche modeling to show that environmental factors have more important roles than geographical distance in the genetic differentiation of EAST and WEST subspecies, and local water availability is the most important factor. In Chapter 4, I performed large-scale greenhouse experiments to identify key traits responsible for the EAST-WEST local adaptation, and that those traits have significantly larger differentiation between subspecies than neutral expectation. In Chapter 5, I performed quantitative trait loci mapping for those important traits and fitness in both parental environments and greenhouse. In summary, the second part of my dissertation provides an example to study ecological speciation from the environment, trait, to the genetic level.
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