DIVERGENCE AMONG MOBILE BASIN POPULATIONS OF THE PLEUROCERID SNAIL GENUS, LEPTOXIS, ESTIMATED BY ALLOZYME ELECTROPHORESIS

Although the Mobile River Basin of Alabama was historically a center o f great pleurocerid diversity, populations today are small and scatter ed. We obtained samples of all four nominal species of Mobile Basin Le ptoxis currently extant: L., ampla (3 populations), L. picta (1 popula tion), L. plicata (2 populations), and L. taeniata (2 populations). Ge ne frequencies at nine variable enzyme loci were determined for about 30 individuals from each population using horizontal starch gel electr ophoresis. Samples of about 30 individuals from three populations of t he widespread Leptoxis praerosa were analyzed as controls. Within popu lations, 18 of 99 loci were polymorphic, none showing genotype frequen cies significantly different from Hardy-Weinberg expectation. Between populations within species, statistically significant divergence was a pparent at most loci. Comparisons among the nominal species showed L. praerosa and L. plicata to be quite distinct from each other, and from all other populations. Much lower levels of divergence among populati ons nominally L. picta, L. ampla, and L. taeniata seem more consistent with a hypothesis of geographic isolation than reproductive isolation . We refer to these three taxa as the ''Leptoxis picta group.'' Our re sults compare favorably in most respects with previously published dat a on mitochondrial 16S rRNA gene sequence divergence among these taxa, especially in the genetic distinction of L. plicata. The relationship s within the L. picta group warrant further study.