Local Adaptation Along a Latitudinal Gradient in Wild Sunflower

Wild populations of Helianthus annuus, the progenitor of cultivated sunflower, can be found throughout much of North America. In order to understand how wild sunflowers have adapted to their local environments, we have analyzed phenotypic and population genetic differentiation across populations from a latitudinal gradient spanning the geographic range of the species. Population genetic theory predicts that loci in genomic regions that influence local adaptation will exhibit elevated population differentiation (estimated as F_ST). In total, we phenotyped 286 individuals from 15 wild populations (three populations at each of five latitudes) for a variety of traits and genotyped these same individuals using an Illumina array targeting a genome-wide collection of SNPs. Traits including flowering time, plant height, seed oil composition (i.e., the degree of saturation) differed significantly across the range of wild sunflower. Through the use of two outlier detection programs (LOSITAN and BayeScan) we were also able to identify loci that exhibited significantly elevated differentiation. We also used linear regression to test for correlations between allele frequencies and latitude. Overall, we found four loci with exceptionally high levels of F_ST as well as numerous loci that appeared to be significantly under-differentiated. Strong correlations were identified between latitude and variation in the vicinity of candidate genes (e.g., the flowering time gene FT2). Future efforts will include assaying genetic variation at much higher resolution as well as performing sequence based molecular evolutionary analyses of candidates for genes involved in local adaptation.