Population Genetics of Winter-flowering and Summer-flowering Cohorts of Helianthus annuus in South Texas

The common sunflower, Helianthus annuus, is the most common sunflower in North America. It is normally an annual plant that flowers in the summer.  However, in Nueces County, TX, we have observed what appear to be separate winter- and summer-flowering cohorts of this species.  These cohorts do not appear to occur in other parts of the native range of this species.  Understanding these cohorts is important because as climate changes they may be able to become established in locations farther north.  To evaluate genetic connectivity between the summer- and winter-flowering cohorts, we are studying variation in species specific genetic markers.  Our aims are to determine if (1) cohorts collected from different locations and (2) the winter- and summer-flowering cohorts are genetically differentiated, as might be expected if winter and summer cohorts do not interbreed due to differences in flowering time.  We began genotyping winter-flowering sunflowers by collecting 30 samples from two locations and extracting DNA.  Summer-flowering plants from the same locations will be collected and processed in the same way this summer.  We are using PCR amplification, fragment analysis, and appropriate statistical analyses of at least 10 microsatellites to fully evaluate possible genetic differentiation of locations and cohorts.  We expect much greater differentiation between cohorts than locations within a cohort.  If these predictions are met, then we would have evidence of seasonally separated cohorts that may perform better under different conditions (e.g., cool vs. hot conditions).  This study has implications for understanding possible responses of plant populations to climate change.