Genetic structuring in the freshwater mussel Anodonta corresponds with major hydrologic basins in the western United States.

Freshwater mussels (unionids) are increasingly recognized as important providers of ecosystem services, yet are among the most endangered fauna in the world. Because unionids are generally sessile and require specific fish hosts for development and dispersal, they are particularly vulnerable to habitat degradation. Surprisingly, little is known about the distribution of genetic diversity in freshwater mussels and this gap has a negative impact on taxonomy, monitoring, conservation and ecological research in these species. Here, we focus on western North American Anodonta, one of only three genera known to exist in this broad landscape and which contains three highly divergent lineages. We describe phylogeographical subdivision in the most widespread and diverse of these lineages, which includes Anodonta californiensis and Anodonta nuttalliana and occurs from Canada to Mexico. Using mitochondrial and nuclear data, we found that genetic structuring within this clade is inconsistent with morphologically based species designations, but instead follows patterns of vicariance among major hydrogeologic basins. Furthermore, there was a strong tendency for population diversity within drainage systems to increase downstream, implying greater habitat or host fish availability in this direction. Microsatellite results indicated that sampling locations were all genetically distinct, even at short distances. Many of our sample populations showed evidence of a recent demographic bottleneck, although this effect seemed to be very local and not drainage or basin-specific. This study provides a foundation for the establishment of appropriate management units and future research on adaptive differentiation and host fish relationships.

Isolation and characterization of microsatellite loci in western North American Anodonta species.

We have developed and characterized 13 microsatellite loci from a group of Anodonta species in western North America, and demonstrated their utility in populations representing two major clades in this genus. Allelic diversity and polymorphic information content were high for all loci, although these characteristics varied across populations. Deviations from Hardy-Weinberg genotypic ratios were not detected, although the estimated frequency of null alleles was high in one population for one locus. This is the first set of microsatellite loci to be developed for freshwater mussels in western North America, and will be useful for describing gene flow patterns among populations.

Isolation and characterization of microsatellite loci in the western pearlshell mussel, Margaritifera falcata (Gould).

Ten microsatellite loci were isolated from the western pearlshell, Margaritifera falcata (Gould, 1850) and characterized in populations from Washington and Montana, USA. We also assessed eight microsatellite loci developed in M. margaritifera, two of which showed utility. Both of our test populations showed significant heterozygote deficiencies at most loci, consistent with a hermaphroditic life history. Populations differed markedly with respect to allelic richness, allele frequencies and numbers of identical multilocus genotypes. This panel of loci should prove useful in describing gene flow and genetic diversity patterns among M. falcata populations, information that should aid future conservation efforts.