Population genetic assessment of two critically endangered freshwater mussel species, Tennessee bean Venustaconcha trabalis and Cumberland bean Venustaconcha troostensis

Abstract

The Cumberland bean Venustaconcha troostensis (Conrad in Academy of Natural Sciences of Philadelphia. http://biodiversitylibrary.org/page/16346317/ , 1834) and Tennessee bean V. trabalis (Lea in Proc Acad Nat Sci Phila 13:38–41, 1861) are both endangered freshwater mussel species endemic to the Cumberland and Tennessee river basins, respectively. These species are closely related and morphologically similar in appearance, and now exist only in small, fragmented populations relative to their historical abundances and distributions throughout the southeastern United States. A critical management need is to understand genetic differences among populations, so that informed decisions can be made to promote demographic and genetic viability of remaining populations. Against this background, we conducted a population genetic analysis by assessing variation at eight microsatellite DNA loci across 127 individuals. Our results showed five population clusters across five rivers, V. trabalis in the Clinch, Obed-Emory, Holston, and Hiwassee rivers; and V. troostensis in the Cumberland river. There was moderate to high differentiation among populations based on FST (range 0.09 − 0.18) and high differentiation among populations based on Jost’s D (range 0.33 − 0.67). The highest differentiation was observed between populations in the Cumberland and Tennessee River basins (mean FST = 0.155, mean D = 0.559). Analysis of allele frequencies suggest that all populations likely have lost genetic diversity and are bottlenecked to various degrees. Linkage disequilibrium-based estimates of recent-current effective population sizes (Ne) ranged from 42 in the Cumberland River population to 130 individuals in the Hiwassee River population, suggesting that populations have become small and fragmented by natural historical processes and by man-made impediments to gene flow. Using approximate Bayesian computation to estimate Ne over longer-term coalescent time, median posterior distributions of Ne ranged from 7130 for the Clinch River population to 901 for the Holston River population. The inferences gained in this study may warrant action by conservation managers to artificially cross extant stocks within basins for purposes of genetic and demographic augmentation. However, based on our initial results, we do not recommend interspecific mixing of individuals between the Cumberland and Tennessee River basins.