Morphological and Genetic Variation Among Cutthroat Trout Subspecies Provide Clues in Evaluating Population Distinctiveness

Cutthroat trout are one of the most widely distributed species of freshwater fish in western North America. Occupying a diverse range of habitats, they exhibit significant morphological variability that is often assumed to be recognized by sub-specific taxonomy. Recent molecular phylogenies have described phylogenetic diversification across cutthroat trout populations but no study has provided a morphological assessment of taxonomic divisions. In this study we used linear and geometric-based morphometrics to determine if phylogenetic and subspecies divisions correspond to significant morphological divisions in cutthroat trout using replicate populations from throughout the geographic range of this species. Our data indicate significant morphological divergence based on sub-specific categories in some, but not all, cutthroat trout subspecies. We also compare morphological distance measures with that of distance measures based on mtDNA sequence divergence. DNA sequence divergence was positively correlated with morphological distance measures, indicating that morphologically more similar subspecies have lower sequence divergence relative to morphologically more distant subspecies. Given these results, integrating both approaches to describing intraspecific variation in cutthroat trout may be important in providing comprehensive conservation plans for wide-ranging species.