The complex migratory traits of northern California's Chinook salmon - which have led some to regard the early- versus late-migrating fish as different species - result from a single, small gene region, researchers report. They say their findings, a new way to think about salmon "ecotypes," may help facilitate conservation and restoration of the fish, particularly the prized early-migrating salmon. The seasonal migrations observed in some salmon species are used to characterize them into ecotypes. The differences between them are often assumed to be complex and influenced by many genes, each with small effect. Chinook salmon, an economically and culturally important iconic species, exhibit several of these ecotypes. Winter- and spring-run ecotypes return to rivers and streams early in the year. Those that arrive later include fall- and late-fall-run ecotypes. These ecotypes are often considered "distinct" groups of animals, with hallmark traits and presumed genetic divergence due to a lack of interbreeding. However, the genetic architecture underlying the phenotypic differences observed in Chinook migratory timing is poorly understood. In collaboration with indigenous fisheries, Neal Thompson and colleagues sequenced whole genomes of 160 Chinook salmon from the Klamath and Sacramento river basins - a sample that included fish from all early- and late-migrating ecotypes. The findings show that the complex migratory phenotype is controlled by a single, small genomic region that was nearly perfectly associated with run timing, but not with other traits presumed to be central to Chinook ecotypes, such as maturity or body fat. What's more, the authors demonstrate that differences in migration timing do not prevent interbreeding between ecotypes. Thompson et al. conclude that the complex migratory phenotype results from a simple Mendelian polymorphism and that run timing and other associated traits are due to the migration environment, rather than complex genetics. In a related Perspective, Garrett McKinney discusses the study in greater detail.