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Berg, D. J., W. R. Hoeh and S. I. Guttman (1997). Alternate models of genetic structure in unionid populations: Conservation and management implications. Journal of Shellfish Research. 16:322-323.

North America is a region of immense freshwater mussel diversity. However, many of the endemic taxa are threatened with extirpation. To successfully conserve variation within taxa, management agencies must understand the genetic structure of populations. We used allozyme electrophoresis to characterize partitioning of genetic variation within-populations (w-p) and among populations (a-p) of unionids in the Ohio River system and within the Big Darby Creek system of central Ohio. Quadrula quadrula typically occupies large rivers, while Elliptio dilatata is a common resident of small streams such as Big Darby Creek. On average, populations of Q. quadrula contained greater w-p variation (2.1 alleles/locus, 61% polymorphic loci, 24% heterozygosity) than populations of E. dilatata (1.6, 32%, 10%, respectively). Patterns of a-p variation differed between species. Allele frequencies of Q. quadrula were not different among populations >1000 km apart. Populations of E. dilatata showed differences in allele frequencies between populations <100 km apart. Unionid species illustrate at least 2 models of the partitioning of genetic variation. Model I species such as Q. quadrula have a high gene flow among populations; each population contains much of the total variation present within a large geographic region. Model II species such as E. dilatata have restricted gene flow and large amounts of a-p variation; individual populations exhibit unique arrays of alleles. Large river habitats are more stable, capable of supporting larger populations of mussels, and may contain fishes with greater dispersal capability than small streams. The result of this combination is a single large metapopulation in big rivers. Preservation of several populations in big rivers will conserve most of a taxon's genetic diversity. Conservation of similar amounts of genetic diversity in small streams will require protection of a large number of populations within any geographic region. Such differences require that management agencies consider the genetic structure of mussel taxa when developing conservation plans.