A Fine Scale Population Genetics Study of the Rare <i>Helianthus verticillatus</i>

Helianthus verticillatus, the whorled sunflower, is a rare plant endemic to only four locations: one in Cave Springs, Georgia, one in Cherokee County, Alabama and two in Madison County, Tennessee. The species can grow up to three meters tall and has multiple showy yellow blooms making it a prime ornamental plant that is attractive to many different pollinators. This plant was designated as a federally endangered species in 2014 due to habitat loss. Currently, there is no recovery plan for H. verticillatus. Still, there are large gaps in knowledge related to basic biology of this plant and its importance in ecosystem services. In this study, microsatellite loci were utilized to investigate fine-scale population structure and clonal diversity of 206 H. verticillatus individuals found on two sampling sites within the Georgia population. Our results indicated the presence of two distinct genetic clusters that correlated with respective sampling site. However, admixture was present at the collection zones closest to the forested barrier separating the sites. Analyses of molecular variance indicated that the majority of variance (51%, P<0.001) was individually based, thus confirming high genetic differentiation (Fst=0.20) and limited gene flow between H. verticillatus collection zones. The evidence of a population bottleneck in these sites suggests a recent reduction in population size that could possibly be due to habitat loss. In addition, high levels of linkage disequilibrium were found, implying that individuals within these sites are primarily reproducing asexually. Based on our results, although populations of H. verticillatus are limited and highly fragmented, they are still harboring moderate levels of genetic diversity (H_exp = 0.50) and, in contrast to previous studies, high numbers of distinct genets. However, because of self-incompatibility and the ability to reproduce vegetatively, sexual reproduction is extremely limited in these populations. Results presented here provided a better understanding of fine-scale genetic diversity and spatial distribution of H. verticillatus populations in Georgia. Combined with previous research findings, our results can underpin a novel recovery plan for H. verticillatus that could be utilized for conservation of this endangered species to promote its persistence in the wild.