Genetic structure and relationships among populations of Hyalella azteca and H-montezuma (Crustacea : Amphipoda)

Allozyme analysis was used to examine the genetic structure of 12 Hyalella azteca populations collected from geographically isolated sites in the Unit ed States and Canada. Hyalella montezuma also was analyzed electrophoretica lly to determine genetic differentiation between recognized congeners. Exte nsive qualitative and quantitative genetic differentiation was observed acr oss the 9 electrophoretically identified groups, which were all presumed to be H. azteca. Levels of genetic differentiation between all pairs of H. az teca groups were higher than when any one of them was compared to H. montez uma (Nei's genetic distance, D = 0.27-2.27). These groups were considered d istinct species because of large genetic differences, probable lack of gene flow, comparatively larger allozyme differences than observed between H, a zteca and H. montezuma prominent local differentiation, and extremely low g enetic variability. Genotype frequencies in all populations markedly deviat ed from Hardy-Weinberg equilibrium with a deficiency of heterozygotes. Hyal ella populations were strongly subdivided even within a local region. No po sitive trend was observed between genetic and geographic distance. A thorou gh molecular systematic analysis of populations throughout the range of H. azteca is essential to identify the actual number of species included in th is taxon.