Extensive allozyme monomorphism in a threatened species of freshwater mussel, Margaritifera hembeli Conrad (Bivalvia : Margaritiferidae)

A threatened but under-studied component of freshwater biodiversity in North America is the native freshwater mussels (Bivalvia: Unionoidea: Margaritiferidae and Unionidae). Genetic data suggest that these mussel species generally exhibit levels of variability similar to other invertebrates. We surveyed allozyme variation in the Louisiana Pearlshell, Margaritifera hembeli (Margaritiferidae), a threatened freshwater mussel. Five examined populations are monomorphic for 25 allozyme loci, the first report of a native freshwater mussel species with extensive allozyme monomorphism. Low genetic diversity appears to be characteristic of margaritiferids, as an ANOVA indicated that mussels of the family Margaritiferidae have significantly lower levels of heterozygosity than the mussels of the family Unionidae. Margaritiferids have strong habitat preferences and modification of habitat leads to rapid loss of populations. Although bottlenecks are known to cause low genetic variability, margaritiferids may exhibit metapopulation structure with extinction/re-colonization dynamics leading to low genetic variability. Margaritiferids generally exhibit a patchy distribution with a colonization rate that is approximately twice that of extinction. Tests of the metapopulation hypothesis will require additional allozyme population genetic data as well as hypervariable microsatellite loci.