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Berg, D. J., E. G. Cantonwine, W. R. Hoeh and S. I. Guttman (1998). Genetic structure of Quadrula quadrula (Bivalvia: Unionidae): Little variation across large distances. Journal of Shellfish Research. 17:1365-1373.

North American freshwater bivalves of the families Unionidae and Margaritiferidae represent one of the endangered faunas of the world. Effective management of threatened and endangered species requires knowledge not only of abundances of these species but also the degree of variation within species and the geographic distribution of this intraspecific variation. We used allozyme electrophoresis to examine the genetic structure of seven Quadrula quadrula populations from the Ohio, Tennessee, and Tensas Rivers. We then considered the implications of our results for the development of effective bivalve conservation strategies. Descriptive measures of genetic variation within populations are quite high (2.1 plus or minus 0.1(se) alleles per locus; 61.4 plus or minus 2.6% polymorphic loci; 0.24 plus or minus 0.01 heterozygosity) relative to other unionids. Genotype frequencies met Hardy-Weinberg expectations at all polymorphic loci. Among-population variation was low and mostly confined to differences between the Tensas River population (lower Mississippi River basin) and the Ohio River basin populations. Significant differences in allele frequencies among populations were only detected at 3 of 10 loci; no differences in allele frequencies were found among Ohio River basin populations. Genetic distances, though all small, were significantly correlated with geographic distance. Estimated gene flow was high among populations, but variation among populations did tend to follow the predictions of an isolation-by-distance model of dispersal. The low levels of among-population genetic variation are remarkable given that these populations are separated by distances as great as 2,500+ river kilometers. High levels of gene flow may ensure that within- population variation remains high and that populations do not become differentiated due to genetic drift. An optimum conservation strategy for this species in the mainstem of the Ohio River would center on the protection of a number of large populations and maintenance of corridors for dispersal of host fishes. Successful protection of threatened and endangered species requires conservation of both abundance and genetic diversity of unionids. Further work is needed to characterize general patterns of genetic structure within freshwater bivalve species.