Parallel evolution of the summer steelhead ecotype in multiple populations from Oregon and Northern California

Parallel adaptive divergence of migratory and reproductive behavior can occur in multiple populations when similar selection is acting on these traits. Timing of migration, sexual maturity, and reproduction can have major impacts on the dynamics and viability of a population. Life-history variation in steelhead, Oncorhynchus mykiss, including variation in anadromous run timing, reproductive maturity, and spawn timing, represents an important aspect of their biology and adaptation to local habitats. Here we present a genetic analysis of naturally spawning steelhead to evaluate the genetic relationships and ancestry of summer- and winter-run reproductive ecotypes from multiple river basins in Oregon and Northern California. We infer the phylogeographic relationships among populations of both summer- and winter-run steelhead ecotypes using 12 microsatellite loci and 90 single nucleotide polymorphisms. Phylogenetic trees and analysis of molecular genetic variance revealed that pairs of phenotypically and genetically distinct reproductive ecotypes within rivers were each other's closest relatives. Isolation by distance was also observed, confirming that genetic relatedness was strongly associated with geographic distance, and indicating limited gene flow among river basins. These patterns support the hypothesis that the summer-run steelhead ecotype has repeatedly evolved through parallel evolution in multiple river basins. Our results, together with further investigations of the underlying molecular basis for the divergence of winter- and summer-run steelhead life-history traits, inform management efforts for these ecotypes and improve our understanding of the role of adaptive genetic variation in conservation.