Genetic Diversity and Population Structure in the Kuskokwim River Drainage Support the Recurrent Evolution Hypothesis for Sockeye Salmon Life Histories

Riverine sockeye salmon Oncorhynchus nerka (those that do not use nursery lakes for juvenile rearing) tend to be characterized by greater genetic diversity and weaker population structure than lake-type populations. These findings have led to the recurrent evolution hypothesis for sockeye salmon life histories, which proposes that riverine sockeye salmon are the principal colonizing form and play the dominant role in the long-term persistence of sockeye salmon in a dynamic landscape. However, previous studies in Kamchatka (Russia) and Alaska have suggested that the general patterns of riverine sockeye salmon do not apply in all parts of the species' range. In this study, we examined genetic diversity and population structure in lake-type and riverine sockeye salmon from the Kwethluk and Holitna rivers (Kuskokwim drainage, western Alaska), near the northern edge of the species' range in North America. As expected, lake-type sockeye salmon exhibited lower genetic diversity and stronger genetic structure over short distances than did neighboring riverine populations, which is consistent with increased straying of the riverine form. In the Kwethluk River, allele identity and genetic evidence of population bottlenecks in lake-type samples suggested that lake-type sockeye salmon were derived from riverine ancestors. These findings further support the recurrent evolution hypothesis and are consistent with the dynamic nature of the floodplain habitats that support these riverine sockeye salmon.