UCLA

Characterizing the genomic basis of climate adaptation is essential both for understanding the process of evolution and for conserving populations under climate change. Here, I use several methods to investigate the genomic basis of adaptation across oak species and populations. In Chapter 1, we compared the gene expression response to a simulated drought stress across six oak species, two species from each of the three taxonomic sections of Quercus in California, which varied in their drought tolerance. We found that drought tolerant species had a less plastic response to leaf drying, suggesting that phenotypic traits and non-plastic patterns of gene expression contribute to their drought tolerance more than plasticity. We also found that the two deciduous trees, which were the most drought sensitive species, responded to drying with 22% of the same genes, indicating these responses had evolved in parallel across distantly related clades. In Chapter 2, we used whole-genome sequencing to characterize the rangewide genetic structure of a rare island endemic oak species, Quercus tomentella, from the California Channel Islands and Guadalupe Island in Mexico. We found evidence for widespread hybridization with a related oak species, isolation of the Guadalupe Island trees, and some gene flow among the California islands that is likely mediated by wind pollination. We also identified putatively adaptive SNPs that were associated with climate variables, compared the spatial patterns of neutral SNPs and candidate SNPs, and used this information to make recommendations for choosing seed sources for restoration projects in the light of climate change. In Chapter 3, we characterized the heat stress response across species and populations. We performed a heat wave experiment and compared the gene expression responses among three oak species and two populations within each species, one from a southern, warmer site and one from a northern, cooler site. We found shared responses to heat stress among all species and populations, including the responses of individual genes as well as genes with related functions. We found limited evidence of differences in stress response among populations, suggesting a lack of local adaptation in the plastic heat stress response.