Germination studies in Arabidopsis thaliana and Sinapis arvensis : genetical and ecological perspectives

The environment can exert strong selective pressures on an organism. When selective pressures on traits differ between environments local adaptation may occur. If there is gene flow between the environments, local adaptation may be slowed or prevented. In plants, particularly weedy ephemerals, germination is a life-history trait that can be a strong determinant on fitness. In this dissertation, I explore the germination traits of two weedy Brassicaceae species, Arabidopsis thaliana and Sinapis arvensis, having populations in different habitats to determine whether germination traits within and between populations vary based on environmental conditions and to assess the extent of local adaptation. In Chapter 1, I assessed which genomic regions of A. thaliana were associated with differences in germination traits due to genotype-by-environment interactions. I performed a genome-wide association study using 100 natural accessions of A. thaliana under four light and nutrient combinations. I found 20 single nucleotide polymorphisms significantly associated with different environments, but none associated specifically with genotype-by-environment interactions. In Chapter 2, I assessed germination traits of S. arvensis collected from agricultural and non-agricultural habitats in the Bitterroot Valley of Montana. I discovered that the agricultural collection studied exhibited significantly different germination timing and amounts than the non-agricultural collections, which were statistically indistinguishable from each other. I also found evidence of a strong maternal effect on germination traits. In Chapter 3, I tested whether patterns of genetic variation between agricultural and non-agricultural collections of S. arvensis supported local adaptation to the two habitats even in the face of gene flow. While I expected to see some genetic differentiation between habitats, as seen in Chapter 2, no genetic differentiation was detected and markers putatively under selection were not associated with a particular habitat. I discuss why this might have occurred even though I have evidence for genetically-based phenotypic differentiation between agricultural and non-agricultural populations of S. arvensis.