Riverscape Genetics of Rainbow Darters, Etheostoma caeruleum, Reveals Lasting Impacts of Glacial History on Contemporary Populations
We conducted a riverscape analysis of Rainbow Darters, Etheostoma caeruleum, across their range. We aimed to evaluate population genetic structure and diversity and to identify the possible drivers, contemporary or historical, that underlie the genetic patterns. This study genotyped 508 Rainbow Darters, collected from 67 locations across 23 river basins, at ten DNA microsatellite loci. Genetic clusters were assessed using STRUCTURE, Akaike information criteria (AIC), and discriminant analysis of principal components (DAPC). The STRUCTURE analysis identified two genetic clusters, one comprised of fish collected from more southerly sites and one comprised of fish from more northerly sites. Interestingly, the two clusters predominantly corresponded to sites that were either unglaciated or glaciated during the last glacial maximum (LGM). The AIC and DAPC identified additional structure, including one genetically distinct population in the Meramec River basin and another in the “Driftless” area of the upper Mississippi. Additional genetic clusters also occurred within the glaciated region, which may reflect expansion from different refugia or limited gene flow across river basins since the LGM. Our findings demonstrate the lasting legacy of glacial history on the contemporary genetic structure and diversity of a North American riverine species.
Map of Rainbow Darter sample sites with assigned ancestry under STRUCTURE analysis. Numbers and region names correspond to USGS HUC2 watershed designations. Colored circles correspond to the proportion of individuals at the site assigned to each genetic cluster identified in the STRUCTURE analysis. Inset shows more detailed map of sampling sites in the Rock River and Illinois River basins.
STRUCTURE results for K = 2 and K = 14. Each bar represents an individual’s assigned ancestry to a cluster. Names above chart correspond to river basin (HUC6). Glaciated and unglaciated correspond to areas that were/were not covered in ice during the last glacial maximum, separated by vertical black bar.
Assigned ancestry plot using AIC criteria. Optimal K determined to be K = 6. Individuals are grouped by glacial region and by HUC6 basin.
Cluster assignments at sampling locations under the AIC method. K = 6. Pie graphs represent proportion of individuals at the site assigned to a given cluster. Individuals with less than 85% assigned ancestry in either cluster are shown in gray. Inset shows more detailed map of sampling sites in the Rock River and Illinois River basins.
Scatterplot representing DAPC clusters. Clusters were assigned to K = 9 genetic clusters without providing any a priori population assignment.
FST isolation by distance plots. Segregation of allele frequencies is shown between pairs of basins versus geographic (river/land) distance. (A, B) All basin pairs in this study with comparisons within regions shown in black and between regions in gray for overland distance (A) and river distance (B). (C, D) Within region site comparisons, grouped by regions for overland distance (C) and river distance (D). The Tennessee River and Arkansas-White-Red regions were omitted since only two and one basin were sampled in each region, respectively.
Contributor Notes
Associate Editors: M. P. Davis and W. L. Smith.