Population genetics and phylogeography of freshwater mussels in North America, Elliptio dilatata and Actinonaias ligamentina (Bivalvia: Unionidae)

Abstract

Extrinsic and intrinsic forces combined shape the population structure of every species differently. Freshwater mussels are obligate parasites to a host fish during a juvenile stage (glochidia). Elliptio dilatata (ED) and Actinonaias ligamentina (AL) are co-occurring freshwater mussel taxa with similar North American distribution and share some potential host fish. Using mitochondrial DNA, we determined the genotypes of 190 + individuals from collection sites in at least two tributaries in the Lake Erie and Ohio River watersheds, along with the Ouachita and Strawberry rivers in the southeast. Both species had followed a stepping-stone model of dispersal, with greater pairwise genetic structure among collection sites of ED. Also, phylogeographical analysis for ED found significant geographical structuring of haplotype diversity. Overall, within-population variation increased significantly from north to south, with low genetic diversity in the Strawberry River. We calculated significant among-population structure for both species (ED: Phi(ST) = 0.62, P < 0.001; AL: Phi(ST) = 0.16, P < 0.001). Genetic analysis identified the Ouachita River as an area of significant reproductive isolation for both species. Results for AL indicated dispersal into northern areas from two genetically distinct glacial refugia, where results for ED indicated dispersal followed by low gene flow in northern areas. The conservation strategies for mussels that co-occur in the same 'bed' could be species specific. Species such as ED have management units on the population scale, where AL has a more homogeneous genetic structure across its range.

Phylogeographic and population genetic analyses reveal Pleistocene isolation followed by high gene flow in a wide ranging, but endangered, freshwater mussel

“…However, the two refugia scenario is further supported by the existence of two COI lineages indicating that at some point in the past, these lineages were likely isolated in two discrete geographic areas. Multiple glacial refugia are a common pattern found in large river mussel and fish species (Elderkin et al, 2008;April et al, 2013). Furthermore, estimates of divergence times in the mid-Pleistocene (0.81 Mya) lend support to a hypothesis of Pleistocene isolation followed by rapid post-Pleistocene dispersal consistent with that described for fishes (Gagnon and Angers, 2006;April et al, 2013).…”

“…This trend was also found in a previous allozyme study of C. monodonta (Monroe, 2008). Within-population variation was not negatively correlated with latitude, unlike the results for other mussels (Elderkin et al, 2008) and fish (Bernatchez and Wilson, 1998); thus our results do not support the hypothesis that the one extant population north of the last glacial maximum has lower genetic variation than southern unglaciated populations due to post-glacial founder effects. In fact, the northernmost population (St Croix) had almost the same genetic diversity as the Gasconade and Meramec populations, and greater overall diversity (y) than the Clinch and Gasconade populations.…”

“…Release of such large numbers of glochidia increases the probability of successfully infesting host species. If the host is a highly migratory fish species or the riverscape is nonfragmented, this can result in dispersal across long river distances (Elderkin et al, 2008;Grobler et al, 2011). Unfortunately, the host species for C. monodonta are still unknown (Butler, 2002); our results suggest studies need to test fish species whose movements might account for the lack of differentiation in contemporary populations of C. monodonta.…”

“…For mussel species studied to date, most have met the prediction of dispersal by the stepping-stone model (Berg et al, 1998;Elderkin et al, 2007;Elderkin et al, 2008), which is expected, as mussels live in linear river systems with patchy distributions and use host fishes that are confined to the same linear systems. However, our results indicate a panmictic population across most of the range of C. monodonta.…”

“…Populations in glaciated regions typically have less genetic diversity than those in southern unglaciated areas. However, co-occurring mussel species can have different genetic structure at various spatial scales, presumably due to variation in life history traits such as demography and use of different fish hosts (Elderkin et al, 2008). This variation among mussel species becomes important when developing conservation strategies for threatened and endangered species, a status that applies to B70% of all mussel taxa in North America .…”