Genetic variation and reproductive isolation among phenotypically divergent amphipod populations
Limnol. Oceanogr., 43(6), 1998, 1162-1169 | DOI: 10.4319/lo.19126.96.36.1992
ABSTRACT: We examined the degree of reproductive isolation and patterns of genetic structure and diversification within and among seven populations of the freshwater amphipod Hyalella azteca. Hyalella occurs across ecologically dissimilar habitats in southeastern Michigan and exhibits substantial, and apparently adaptive, genetically based phenotypic variation among populations. Habitats with predatory centrarchid fish contain a small-bodied Hyalella ecotype, whereas habitats lacking these predators contain a large-bodied ecotype. Hierarchical F statistics showed that allele frequency variation at six electrophoretic loci is greater among populations within an ecotype than between ecotypes, a result also reflected in a cluster analysis based on genetic distances among the seven populations. Despite the lack of clear genetic differentiation between ecotypes, gene flow between ecotypes appeared restricted or absent. Physically adjacent large- and small-ecotype populations in the same drainage differed appreciably in allele frequencies. Within-population genetic structuring also differed between ecotypes. Whereas genotype frequencies never differed from Hardy-Weinberg expectations in large-ecotype populations, all small-ecotype populations exhibited a significant deficiency of heterozygous genotypes at one or more loci. Interbreeding trials demonstrated that individuals from different populations of the same ecotype readily interbreed, but no interbreeding was observed in crosses involving individuals of dissimilar ecotypes. Interbreeding results, considered together with between-ecotype differences in allele and genotype structure, suggest that Hyalella ecotypes in southeast Michigan form at least two separate species. Furthermore, the lack of distinct allelic differences and the relative levels of allele frequency differentiation within and between ecotypes suggest recent local speciation in this taxon.