The Geographic and Genomic Landscape of Divergence in Helianthus argophyllus

Many studies have looked at local adaptation and divergent selection in wild populations, and increasingly more studies are examining these processes using next-generation sequence data. With some remarkable exceptions, few studies have succeeded in unifying these two approaches by connecting genotype to phenotype to fitness. Here we present analyses begun both in the field and in the sequencer on divergent selection in Helianthus argophyllus, the silverleaf sunflower. This species is a Texas native with two radically different life history strategies in the wild: coastal populations flower earlier and are shorter than populations growing inland. These differences are observed across the species' range and persist in a common garden. This divergence in life history is particularly striking in an outcrossing species with a relatively small range and with little population structure. In 2011, we tagged seedlings at seven wild populations and monitored each plant across the growing season for flowering date and fecundity. Fitted estimates of the fitness function for flowering date reveal that, at least for the year of the study, populations on the barrier islands were under strong directional selection for early flowering, while populations further inland showed less intense, non-directional selection. These data are congruent with the observed life history differences, although the shape and mode of selection on flowering time may change across years. Next, to understand how divergent selection is acting on the genomic scale, we used RNA-Seq data to estimate Fst between coastal and inland populations across the genome. We performed a sliding window analysis that identified twelve significant regions of divergence on seven linkage groups. We then looked at genes containing SNPs with the highest Fst values within each divergent region, and found several promising candidates for targets of natural selection, including genes involved in salt tolerance and pathogen defence. None of our candidate genes, however, are known to be involved in flowering time. We discuss some possible explanations for the apparent disconnect between our two approaches and propose future analyses that may help to connect genotype to fitness in this system.