Historical Studies on Cutthroat Trout Taxonomy Based on Morphological-Meristic Characters, Allozymes, and Chromosomes
Gary H. Thorgaard, Kevin R. Bestgen, Patrick Trotter, Eric J. Loudenslager, and Paul A. Wheeler
Abstract.—There has been considerable interest in the systematics and classification of Cutthroat Trout since the 1800s. Cutthroat Trout native to western North America (currently classified as Oncorhynchus clarkii) have historically been grouped or separated using many different classification schemes. Since the 1960s, Robert Behnke has been a leader in these efforts. Introductions of nonnative trout (other forms of Cutthroat Trout, and Rainbow Trout O. mykiss) have obscured some historical patterns of distribution and differentiation. Morphological and meristic analyses have often grouped the various forms of Cutthroat Trout together based on the shared presence of the “cutthroat mark,” high scale counts along the lateral line, and the presence of basibranchial teeth. Spotting patterns and counts of gill rakers and pyloric caeca have in some cases been helpful in differentiation of groups (e.g., Coastal Cutthroat Trout O. c. clarkii, Lahontan Cutthroat Trout O. c. henshawi, and Westslope Cutthroat Trout O. c. lewisi) currently classified as subspecies. The historical genetic methods of allozyme genotyping through protein electrophoresis and chromosome analyses were often helpful in differentiating the various subspecies of Cutthroat Trout. Allozyme genotyping allowed four major groups to be readily recognized (Coastal Cutthroat Trout, Westslope Cutthroat Trout, the Lahontan Cutthroat Trout subspecies complex, and Yellowstone Cutthroat Trout O. c. bouvieri subspecies complex) while chromosome analyses showed similarity between the Lahontan and Yellowstone Cutthroat trout subspecies complex trout (possibly reflecting shared ancestral type) and differentiated the Coastal and Westslope Cutthroat trouts from each other and those two groups. DNA results may yield higher resolution of evolutionary relationships of Cutthroat Trout and allow incorporation of ancient museum samples. Accurate resolution of taxonomic differences among various Cutthroat Trout lineages, and hybridization assessments, requires several approaches and will aid in conservation of these charismatic and increasingly rare native fishes.