Comparative phylogeography of codistributed aquatic insects in western North America: insights into dispersal and regional patterns of genetic structure

Phylogeographic studies provide insights about complex systems at different evolutionary scales. In addition to providing evidence about ecological processes at the organismal level, a synthesis of studies across taxa can illuminate broad phylogeographic patterns at the landscape scale. We compared the phylogeographic patterns of two codistributed species of aquatic insects, Pteronarcella badia (Plecoptera: Pteronarcyidae) and Drunella grandis (Ephemeroptera: Ephemerellidae), across much of their range in western North America. We conducted population genetic analyses of P.badia and D.grandis individuals that co-occurred in 22 western North American localities. Patterns of genetic structure and gene flow were compared to test for differential dispersal ability and to determine whether broad-scale phylogeographic trends are consistent with patterns seen in terrestrial taxa across a similar geographic area. We also evaluated whether patterns observed in these species fit previously proposed models of genetic structure in aquatic organisms. Pteronarcella badia showed greater genetic structure and lower rates of dispersal than D.grandis. Both taxa showed several shared geographic regions of genetic isolation, including the Bitterroot Valley in Montana, the Pacific Northwest, the southern Rocky Mountains and the Colorado Plateau. These broad-scale phylogeographic patterns are largely consistent with trends observed in several terrestrial taxa across the sample area. D.grandis showed patterns of gene flow consistent with an isolation-by-distance model of genetic structure. Patterns observed in P.badia reflect elements of multiple models and may highlight the value of the dendritic network approach to understanding genetic patterns of aquatic taxa at broad geographic scales.