Phylogeny of the Pacific Trouts and Salmons (Oncorhynchus) and Genera of the Family Salmonidae

Abstract

Seven genera—Brachymystax, Acantholingua, Salmothymus, Hucho, Salvelinus, Salmo, and Oncorhynchus—make up the living Salmoninae. Relationships of 33 extant and 4 fossil salmonid species and subspecies were studied on the basis of 119 characters analyzed by parsimony algorithms. Twelve equally parsimonious trees each requiring 253 steps were calculated. Monophyly of recognized genera is consistent with all 12 estimates. The earliest branch of the family Salmonidae is the subfamily Coregoninae. Its sister group is the clade including the Thymallinae and Salmoninae. Within the Salmoninae, Eosalmo, from the Eocene of British Columbia, is the sister group of all living genera, as previously shown by Mark Wilson. The living Asian species Brachymystax lenok is the sister species of all other living Salmoninae, as documented by Carroll Norden. Three species of archaic trouts from the Mediterranean area—Acantholingua ohridana, Salmothymus obtusirostris, and Salmothymus (Platysalmo) platycephalus—branch off after Brachymystax but before diversification of all other salmonines. Platysalmo platycephalus Behnke is the sister species of Salmothymus obtusirostris and is placed in Salmothymus. The clade beyond the archaic trouts includes four genera in two clades: (1) Hucho plus Salvelinus and (2) Salmo plus Oncorhynchus. The Atlantic trouts and salmons are a monophyletic group, Salmo, and the Pacific trouts and salmons are a monophyletic group, Oncorhynchus. The terms “trout” and “salmon” refer roughly to life history modes, not to phylogenetic relationships. Morphological and mitochondrial DNA data disagree regarding the relationship of Oncorhynchus clarki (cutthroat trout) to Oncorhynchus mykiss (rainbow, California golden, and redband trouts) and the relationship of Oncorhynchus gorbuscha (pink salmon) to Oncorhynchus keta (chum salmon). Parsimony analysis suggests that Oncorhynchus mykiss is the sister of the Pacific salmon, not of O. clarki, though O. clarki and O. mykiss hybridize, Oncorhynchus gorbuscha is the sister species of O. nerka (sockeye salmon), not of keta, though O. gorbuscha and O. keta hybridize. Mitochondrial DNA is interpreted as having been transferred by introgression between O. gorbuscha and O. keta, providing misleading evidence of relationship. Fossil species of Oncorhynchus document a minimum age of 6 million years for the modern species of Pacific trouts and salmons.