CYTOCHROME-B SEQUENCE VARIATION AND A MOLECULAR PHYLOGENY OF THE LIVE-BEARING FISH GENUS GAMBUSIA (CYPRINODONTIFORMES, POECILIIDAE)

Nucleotide sequences for a 402-base segment of the mitochondrial cytoc hrome b gene were determined from 25 species of live-bearing fishes. A total of 34 sequences representing 24 species of the genus Gambusia a nd 1 species of Belonesox were generated via the polymerase chain reac tion. The levels of overall variation were consistent with those from other genera of fishes. In total, 137 of 402 (34.1%) nucleotides exhib ited variation within or among the species. Observed differences at 24 (17.9%) of the 134 cytochrome b codons would result in amino acid rep lacements. Phylogenetic analyses employing various weighting schemes r esulted in several clades representing traditionally recognized taxono mic groups. However, precise relationships among species-groups remain ed uncertain. Randomization tests indicated that these topologies cont ained significant nonrandom phylogenetic information. As with other fi shes, the overall rate of divergence appeared to be slower than that o f other vertebrates and the overall replacement/substitution pattern w as suggestive of nonrandom evolutionary input.