Endangered Rough Pigtoe Pearlymussel: Assessment of Phylogenetic Status and Genetic Differentiation of Two Disjunct Populations

We conducted a genetic characterization of two extant populations of an endangered freshwater mussel, the rough pigtoe Pleurobema plenum, in the Clinch River, Tennessee of the Tennessee River basin, and the Green River, Kentucky, of the Ohio River basin for purposes of conservation recovery planning. First, phylogenetic status of this species within the Pleurobema cordatum species complex (P. cordatum, plenum, rubrum, sintoxia) was assessed using mitochondrial ND1 deoxyribonucleic acid (DNA) sequences, which showed that all investigated species of Pleurobema are genetically distinct. Our results indicated that P. cordatum, P. plenum, P. rubrum, and P. sintoxia each represent monophyletic clades; however, the latter two species were closely related, separated by only one to three nucleotide differences. Further, DNA sequence haplotypes from both populations of P. plenum grouped together into one monophyletic clade and did not support characterizing the populations as separate species. Thus, our phylogenetic analysis confirms that populations of P. plenum in the Clinch and Green rivers are the same species. Second, we assessed genetic differentiation between P. plenum populations in each river by analyzing variation at eight nuclear DNA microsatellite loci, where F-ST (= 0.023) and Jost's D (= 0.175) indicated genetically differentiated populations. Because these populations are geographically and demographically independent, and differentiated at nuclear microsatellite loci, recognition as management units is recommended. Additional studies are needed to determine whether there are differences at adaptiveor life-history traits, such as fish host usage, shell morphology, and soft anatomy between populations, and to further investigate the phylogenetic relationship of P. rubrum and P. sintoxia.