Host-dependent genetic variation in freshwater pearl mussel (Margaritifera margaritifera L.)

The endangered freshwater pearl mussel (Margaritifera margaritifera) requires brown trout or Atlantic salmon as a host to complete its life cycle. In Norway, populations have been identified using almost exclusively either salmon or trout as hosts, even when both hosts are present. In this study we tested the hypothesis that the genetic variation in freshwater pearl mussel is host-dependent. A total of 587 specimens from 25 different localities were assayed for genetic variation at eight microsatellite loci. Within populations, the trout-mussel populations had significantly lower allelic richness and expected heterozygosity than the salmon-mussel populations. The genetic differentiation among trout-mussel populations was very large (F (ST) = 0.332), and significantly larger than among salmon-mussel populations (F (ST) = 0.023). Host affiliation explained more of the genetic differentiation among freshwater pearl mussel populations (F (CT) = 0.150) than geographical location (F (CT) = 0.013). Collectively these results suggest that there is strong reproductive isolation between freshwater pearl mussel populations. More importantly, our results indicate that trout- and salmon-mussel populations are differently genetically structured. Because of the mandatory parasitic larval stage on trout and salmon, the host-dependent genetic variation has implications for restoration and management plans for this species.