Small-scale genetic structure in a stream-dwelling caddisfly in eastern Canada

Abstract

We used mitochondrial DNA (cytochrome c oxidase subunit I) to examine the population genetic structure and phylogeography for 12 populations of a widely distributed predatory caddisfly (Rhyacophila minor) in small, unconnected streams along a 45-km stretch of North Mountain, Nova Scotia, Canada. We detected patterns of strong genetic subdivision (FST=0.312) with streams separated by >3.5km being significantly different whereas adjacent streams were not (no significant FST values), suggesting some, but very limited, current overland dispersal by adults. A significant phylogeographic break was found between eastern and western populations, corresponding to changes in bedrock and surficial geology. Genetic diversity within the streams was positively correlated with watershed size and the spatial variation in geology. We conclude that genetic divergence among populations of R. minor is a result of dispersal barriers and the accumulation of differences among the streams due to random genetic drift. Our study suggests that a better understanding of how dispersal interacts with landscape features at small spatial scales will improve our ability to link the movement of individuals to ecological and demographic processes.