Biochemical Systematics of the Atherinid Genus Menidia

Abstract

Species of the atherinid genus Menidia were compared electrophoretically for enzymes representing an apparent 24 gene loci and for nonenzymatic skeletal muscle proteins. Average allelic frequency similarities between conspecific populations range from 0.84 to 0.99, averaging 0.94. The greatest allelic frequency similarity between two populations definitely representing separate species is 0.82. At a given locus, pairs of species tend to have either essentially identical or completely different allelic frequencies. Consistent with current taxonomic treatment, M. menidia and M. extensa were found to be quite distinctive from each other and from each of the other species in the genus. Populations of M. peninsulae were consistently distinct from M. beryllina, with which they were formerly considered conspecific. These two species co-occur from Florida to Texas, and genetic discontinuity was verified in four areas of local sympatry. Morphological and ecological differences also were found between M. peninsulae and M. beryllina. Contrasting with all other species comparisons in the genus, no electrophoretic differences were found between M. audens and M. beryllina. Morphological analysis revealed an intermediate population in Lake Palourde, southern Louisiana, the region where the ranges of these two species approach one another. Although the structure of this population suggests hybridization, the reproductive relationships of M. audens and M. beryllina remain in doubt. Hybridization is possible between all three of the coastal species, M. beryllina, M. peninsulae and M. menidia, but is common only in the zone of parapatry of M. menidia and M. peninsulae in northeastern Florida. Very little or no gene exchange between species occurs in this hybrid zone. Hybridization in Menidia appears to be related more to ecological similarity than to genetic similarity.