Historical biology of Oncorhynchus.

Abstract

The Salmoninae comprise eight extant genera and at least 27 extant species of northern hemisphere freshwater and diadromous fishes. A parsimony analysis of the phyletic relationships of 31 extant and four fossil salmonid taxa, using 122 characters, reveals a phylogenetic history that enables reconstruction of the evolution of the remarkable life history and behavior adaptations of trout and salmon during the Cenozoic in North America and Eurasia. The phylogenetic analysis demonstrates that the problematic extant archaic trouts Brachymystax, Acantholingua, Salmothymus, and Platysalmo are cladistically intermediate between grayling, Thymallus, and the advanced salmonines Hucho, Salvelinus, Salmo and Oncorhynchus. The fossil Eosalmo, the oldest salmonine, is one of the archaic trouts and in fact is the sister group to all other salmonines. Advanced salmonines form two lineages: (1) the clade of chars, Salvelinus, and huchen, Hucho; and (2) the clade of Atlantic and Pacific salmons and trouts, Salmo and Oncorhynchus. In the context of this phylogeny, the fossil forms Rhabdofario and Smilodonichthys are determined to be members of the clade of Pacific salmons and trouts. Because the Pacific salmons and trouts form a coherent spatio-historical lineage, they are all referred here to the genus Oncorhynchus. Primitive salmonine sister-groups (Brachymystax, Acantholingua, Salmothymus, Platysalmo) are strictly freshwater; derived salmonines are diadromous. Within the two clades Salmo and Oncorhynchus, full-scale anadromy has independently developed out of amphidromy. Salmonid spawning behaviours similarly form a stepped series, demonstrating greater and greater levels of nesting and territorial behaviors. These culminate in high post-spawning mortality in anadromous members of Salmo and Oncorhynchus. Migratory behaviors of Pacific salmons also demonstrate a stepped sequence when mapped on the salmonid phylogeny, indicating historical shifts in timing of downstream migration and smolt transformation. Oncorhynchus has been present in North America since the Mid-Miocene. Most of the speciation in this lineage occurred by Late Miocene time. Early freshwater clades, including cutthroat trout, Oncorhynchus clarki, reached the Lahontan basin while connections to the Pacific were open. Derived clades occupy coastal California and Pacific Northwest drainages.