Genetic differentiation between Unionicola formosa and U-foili (Acari: Unionicolidae): Cryptic species of molluscan symbionts

The symbiotic water mites Unionicola formosa and U. foili are sibling species from the host mussels Pyganodon cataracta and Utterbackia imbecillis, respectively. Cellulose acetate electrophoresis was used to examine the population genetics of these sibling species in order to determine the degree of genetic divergence. A total of four populations of U. formosa and six populations of U. foili were scored for 19 presumptive enzyme loci. Genetic variation among all mite populations was low, but genetic heterozygosity and polymorphism were greater among populations of U. formosa. We found a high degree of genetic differentiation between populations of U. formosa and U. foili. Populations of these species were fixed for different alleles at three loci (MDH 2, MPI 2, and PEP) and exhibited significant allele frequency heterogeneity at 83% of their polymorphic loci. Genetic divergence among these species populations also was reflected in the pairwise values of Nei's genetic identity and genetic distance. Coefficients of genetic identity were high among populations of the same species, but were substantially lower when populations from the two species were compared. A dendrogram of the distance values generated two distinct groups separated by a distance of 0.177, corresponding to populations of U. formosa and U. foili. The results of this study provide evidence that mite populations from P. cataracta and U. imbecillis are reproductively isolated sibling species of Unionicola, and complement other studies suggesting that host specificity is important in limiting gene flow among populations of symbionts.