High genetic diversity in a rare and endangered sunflower as compared to a common congener

Determining the genetic structure of isolated or fragmented species is of critical importance when planning a suitable conservation strategy. In this study, we use nuclear and chloroplast SSRs (simple sequence repeats) to investigate the population genetics of an extremely rare sunflower, Helianthus verticillatus Small, which is known from only three locations in North America. We investigated levels of genetic diversity and population structure compared to a more common congener, Helianthus angustifolius L., using both nuclear and chloroplast SSRs. We also investigated its proposed hybrid origin from Helianthus grosseserratus Martens and H. angustifolius. Twenty- two nuclear SSRs originating from the cultivated sunflower (Helianthus annuus L.) expressed sequence tag (EST) database, and known to be transferable to H. verticillatus and its putative parental taxa, were used in this study thereby allowing for statistical control of locus-specific effects in population genetic analyses. Despite its rarity, H. verticillatus possessed significantly higher levels of genetic diversity than H. angustifolius at nuclear loci and equivalent levels of chloroplast diversity. Significant levels of population subdivision were observed in H. verticillatus but of a magnitude comparable to that of H. angustifolius. Inspection of multilocus genotypes also revealed that clonal spread is highly localized. Finally, we conclude that H. verticillatus is not of hybrid origin as it does not exhibit a mixture of parental alleles at nuclear loci, and it does not share a chloroplast DNA haplotype with either of its putative parents.