Conservation genetics of Hill's thistle (Cirsium hillii)

Populations at the edges of their geographical ranges are often characterized by reduced genetic diversity and increased genetic differentiation. These patterns may be exacerbated by specialized associations between plants and uncommon habitats. Together, reduced diversity and increased subdivision may increase the risk of population extirpation, an issue that is particularly important for plants classified as rare or threatened. Hill's thistle (Cirsium hillii (Canby) Fernald),is a perennial herb endemic to the Great Lakes region and is considered threatened in Canada where its northernmost distribution occurs. We used microsatellite markers to genotype plants from 11 populations of Hill's thistle that span the range of the species distribution within Canada. These data were used to investigate levels of genetic diversity within populations and genetic differentiation between populations, and to evaluate associations between patterns of genetic variation and those expected under ongoing habitat loss. Genetic differentiation among populations was low, although significant pairwise F(ST) values, along with the presence of private alleles in most populations, was consistent with moderate to low gene flow. Nevertheless, in comparison with a congeneric rare species (Cirsium pitcheri) that occurs in the same geographical region in Canada, genetic diversity within most populations was high, and we suggest that this is at least partially attributable to this species' combination of sexual and clonal reproduction. Despite high levels of genetic diversity, Hill's thistle should probably remain a designated species at risk because of ongoing habitat loss; however, our data suggest that Hill's thistle is unlikely to suffer from reduced genetic diversity in the foreseeable future.