GENETIC DIFFERENTIATION BETWEEN UNIONICOLA-FORMOSA AND UNIONICOLA-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 imbe cillis, respectively. Cellulose acetate electrophoresis was used to ex amine the population genetics of these sibling species in order to det ermine the degree of genetic divergence. A total of four populations o f U. formosa and six populations of U. foili were scored for 19 presum ptive enzyme loci. Genetic variation among all mite populations was lo w, but genetic heterozygosity and polymorphism were greater among popu lations of U. formosa. We found a high degree of genetic differentiati on between populations of U. formosa and U. foili. Populations of thes e 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 geneti c identity and genetic distance. Coefficients of genetic identity were high among populations of the same species, but were substantially lo wer when populations from the two species were compared. A dendrogram of the distance values generated two distinct groups separated by a di stance of 0.177, corresponding to populations of U. formosa and U. foi li. The results of this study provide evidence that mite populations f rom P. cataracta and U. imbecillis are reproductively isolated sibling species of Unionicola, and complement other studies suggesting that h ost specificity is important in limiting gene flow among populations o f symbionts.