A molecular phylogeny of aquatic gastropods provides a new perspective on biogeographic history of the Snake River Region

Mitochondrial DNA sequences of aquatic gastropods of the subgenus Pyrgulopsis (Natricola) were analyzed to test a commonly accepted hypothesis concerning the early history of the Snake River in the northwestern US. Distributions of Natricola and other regional biota were previously used to infer that the Snake River flowed to the Pacific through southeastern Oregon and northern California during the Neogene prior to its capture by the Columbia River in the late Pliocene (2 Ma). A molecular phylogeny based on partial sequences of COI and NDI (1149 bp) indicates that the Natricola clade is restricted to the modern Snake-Columbia River Basin and the Oregon Lakes region whereas northern California populations previously assigned to this subgenus belong to other lineages. The Natricola clade is not deeply subdivided into Oregon Lakes and Snake River Basin units consistent with late Pliocene fragmentation of the hypothesized paleodrainage, but instead is shallowly structured and contains multiple transitions among these two geographic areas. The strongly supported sister relationship between Natricola and a species from northwest Nevada (P. imperialis) is consistent with a recent proposal that the ancestral Snake River did not flow through southeast Oregon but instead flowed south to the Humboldt River. Within the context of this hypothesis, the multiple transitions between the Snake River Basin and the Oregon Lakes region that occurred within Natricola may be attributed to a late Pleistocene connection between these areas that was unrelated to the early course of the Snake River.