The Genomic and Geographic Landscapes Of Sunflower Speciation: The Transition From Local Adaptation

Reports that ecological speciation in the presence of gene flow may involve large genomic regions of divergence have stimulated work attempting to explain how genomes diverge. However, in plants there is little evidence for this, possibly because most comparisons to date have been at the species level, where long histories of gene flow, the accumulation of genome-wide divergence and population structure might erode or obscure such patterns. For example, the annual Helianthus sunflowers have been a model system for ecological speciation, but as yet we have little information on the earliest stages of the process. I will present data exploring ecotypes of the prairie sunflower, Helianthus petiolaris, occurring at Great Sand Dunes National Park (Colorado). Using restriction-associated DNA (RAD) sequencing, we have found that genomic regions of elevated divergence are large and that selective sweeps have mainly been restricted to the dune ecotype. These results stand in stark contrast with comparisons between established species in the genus.

The geographic context of speciation is also likely to remain preserved in young incipient species, and departures from isolation by distance (or isolation by landscape resistance) can be used to understand the selective pressures operating at this stage. In some parts of the genome proximity to the ancestral non-dune form appears to promote divergence of the dune ecotype. I will describe our investigations of a genomic signature reminiscent of reinforcement, which have implications for when and how this process may be detected.