Abstract
BackgroundFor wide-ranging species, intraspecific variation can occur as a result of reproductive isolation from local adaptive differences or from physical barriers to movement. Cutthroat trout (Oncorhynchus clarkii), a widely distributed fish species from North America, has been divided into numerous putative subspecies largely based on its isolation in different watersheds. In this study, we examined mtDNA sequence variation of cutthroat trout to determine the major phylogenetic lineages of this polytypic species. We use these data as a means of testing whether geographic isolation by watershed boundaries can be a primary factor organizing intraspecific diversification.ResultsWe collected cutthroat trout from locations spanning almost the entire geographic range of this species and included samples from all major subspecies of cutthroat trout. Based on our analyses, we reveal eight major lineages of cutthroat trout, six of which correspond to subspecific taxonomy commonly used to describe intraspecific variation in this species. The Bonneville cutthroat trout (O. c. utah) and Yellowstone cutthroat trout (O. c. bouvieri) did not form separate monophyletic lineages, but instead formed an intermixed clade. We also document the geographic distribution of a Great Basin lineage of cutthroat trout; a group typically defined as Bonneville cutthroat trout, but it appears more closely related to the Colorado River lineage of cutthroat trout.ConclusionOur study indicates that watershed boundaries can be an organizing factor isolating genetic diversity in fishes; however, historical connections between watersheds can also influence the template of isolation. Widely distributed species, like cutthroat trout, offer an opportunity to assess where historic watershed connections may have existed, and help explain the current distribution of biological diversity across a landscape.
Highlights
For wide-ranging species, intraspecific variation can occur as a result of reproductive isolation from local adaptive differences or from physical barriers to movement
Study sites We sampled extant populations of cutthroat trout from 311 locations representing the majority of their geographic range (Figure 1, Additional file 1: Table S1) to compare mitochondrial sequence divergence at the NADH dehydrogenase subunit 2 gene (ND2) and determine major phylogenetic branching among groups
Our data indicate that watershed boundaries do organize primary phylogenetic divisions in cutthroat trout, but historical connections that are no longer evident appear to have influenced the pattern of diversification
Summary
For wide-ranging species, intraspecific variation can occur as a result of reproductive isolation from local adaptive differences or from physical barriers to movement. We examined mtDNA sequence variation of cutthroat trout to determine the major phylogenetic lineages of this polytypic species. We use these data as a means of testing whether geographic isolation by watershed boundaries can be a primary factor organizing intraspecific diversification. Species with wide geographic ranges often exhibit significant morphological, behavioral, or genetic variability across their range [1] Such differences have been attributed to local adaptation and genetic drift as a result of isolation of populations by limited gene flow [2]. The height of land that separates watershed boundaries and the relatively common occurrence of movement barriers within watersheds often isolate fish populations allowing for significant genetic differentiation [12,13,14,15]
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