Allozymes, Mitchondrial DNA, and Levels of Phylogenetic Resolution Among Four Minnow Species (Notropis: Cyprinidae)

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Abstract
Comparison of allozymes and restriction endonuclease cleavage maps of mitochondrial DNA (mtDNA) was used to infer phylogenetic relationships among four species of minnows in the cyprinid genus Notropis. Statistical analysis of the allozyne data supported the monophyly of N. chrysocephalus and N. cornutus (subgenus Luxilus) relative to N. photogenis and N. rubellus (subgenera Notropis and Hydrophlox, respectively). In contrast, statistical analysis of the mtDNA cleavage site data provided no resolution for these relationships. Given the rapid rate of vertebrate mtDNA evolution, mtDNA data were expected to provide greater taxonomic resolution than allozyme data. Four possible reasons for this discrepancy were considered; two of these, rate heterogeneity between genes and lineages and introgressive hybridization, were analyzed in detail. Comparison of sequence divergence estimates among genetically different regions of the same mtDNA and among mtDNAs of different species revealed significant variation in the relative rates of change. This variation can lead to intramolecular and interspecific differences in levels of convergence, and, thus, to a loss of phyogenetic resolution. Because of the propensity for these species to hybridize and the possibility of mtDNA introgression, the potential impact of hybridization and introgression was also considered. Given the internal factors affecting the evolution of mtDNA and the population genetic processes affecting its transmission, assumptions of rate homogeneity and of strict vertical inheritance should be closely scrutinized when constructing phylogenies with mtDNA data.