Small Scale Population Structure of Pityopsis ruthii and P. graminifolia var. latifolia

Pityopsis graminifolia var. latifolia (Michx.) Small var. latifolia (Fern.) or narrowleaf silkgrass is an herbaceous perennial native to the southeastern United States and northern Central America. Pityopsis ruthii (Small) Small is an endangered plant endemic to the Hiwassee and Ocoee Rivers in Polk County, Tennessee. Little is known about the genetic diversity and population structure of both Pityopsis species as well as the phylogenic relationships between the species. In this study, 16 microsatellite loci were used to assess genetic diversity and population structure of 261 individuals of P. ruthii (n=167) and P. graminifolia var. latifolia (n=76). Pityopsis ruthii is characterized as a diploid whereas P. graminifolia var. latifolia is characterized as a tetraploid. Because of unknown ploidy of P. graminifolia var. latifolia, Polysat, a package in program R, was used to infer ploidy of the individuals. Arlequin and GenAlEx were utilized to calculate genetic diversity measurements. The genetic software STRUCTURE and BAPS used Bayesian cluster analyses to group individuals based on multilocus genotypes, and were used to evaluate the genetic structure and gene flow. Samples of Pityopsis ruthii and P. graminifolia var. latifolia were analyzed as one data set and separated by species to ensure accurate results. Analyses in GenAlEx for all data sets resulted in low Shannon’s information index (I=0.13-0.14), and expected heterozygosity ranged from 0.08-0.09. Analysis of molecular variance indicated that the majority of the variation is within populations with moderate to higher genetic differentiation when P. ruthii and P. graminifolia var. latifolia are analyzed together or P. ruthii alone ([genetic differentiation] Φ_PT =0.13 and 0.18, respectively). Low levels of genetic differentiation (Φ_PT =0.02) were found when P. graminifolia var. latifolia was analyzed independently. STRUCTURE revealed three clusters with P. ruthii clustering by watershed and P. graminifolia var. latifolia resulting in one cluster. When analyzed separately, P. graminifolia var. latifolia indicated presence of three clusters with considerable admixture in each cluster. Analysis with BAPS provided similar results as found with STRUCTURE when a fixed K was selected. Understanding the population structure and genetic diversity will aid in the creation of guidelines for conservation programs of P. ruthii.