Mitochondrial Variation and Biogeographic History of Chinook Salmon

The genetic variation of many species in the Northern Hemisphere has been influenced by climatic changes during the Pleistocene Epoch. Phylogeographic studies can help determine intraspecific relationships and postglacial recolonization routes for many of these species, potentially leading to a more complete understanding of how flora and fauna respond to dramatic climate change. We analyzed the variation in the mitochondrial DNA sequences of Chinook salmon Oncorhynchus tshawytscha from California to the Kamchatka Peninsula, Russia, to gain insight into the role of Pleistocene glaciation in the genetic structuring of this species. The geographic distribution of haplotypes revealed high levels of genetic diversity in the Columbia River drainage. Nested clade analysis suggests northward expansion from this area during the late Pleistocene, a hypothesis that is further supported by evidence of demographic growth and population expansion in the northern portion of the species' range. Estimates of the divergence time between two phylogenetic clades are correlated with interglacial periods of the Pleistocene. The data suggest the absence of Chinook salmon from the current northern part of its distribution until the late Pleistocene, when northward expansion from a southern refugium occurred (during an interglacial period), followed by the recent genetic divergence of these northern populations.