Population isolation results in unexpectedly high differentiation in Carolina hemlock (Tsuga caroliniana), an imperiled southern Appalachian endemic conifer

Carolina hemlock ( Tsuga caroliniana Engelm.) is a rare conifer species that exists in small, isolated populations within a limited area of the Southern Appalachian Mountains of the USA. As such, it represents an opportunity to assess whether population size and isolation can affect the genetic diversity and differentiation of a species capable of long-distance gene flow via wind-dispersed pollen and seed. This information is particularly important in a gene conservation context, given that Carolina hemlock is experiencing mortality throughout its range as a result of infestation by hemlock wooly adelgid ( Adelges tsugae Annand), an exotic insect. In this study, 439 Carolina hemlock trees from 29 areas (analyzed as populations) were sampled, representing an extensive range-wide sampling of the species. Data from 12 polymorphic nuclear microsatellite loci were collected and analyzed for these samples. The results show that populations of Carolina hemlock are extremely inbred ( F IS = 0.713) and surprisingly highly differentiated from each other ( F ST = 0.473) with little gene flow (N m = 0.740). Additionally, most populations contained at least one unique allele. This level of differentiation is unprecedented for a North American conifer species. Numerous genetic clusters were inferred using two different clustering approaches. The results clearly demonstrate that, existing as a limited number of small and isolated populations, Carolina hemlock has insufficient gene flow to avoid widespread genetic drift and inbreeding, despite having the capacity to disperse pollen and seed relatively long distances by wind. These results have important conservation implications for this imperiled species.