Does river corridor affect chromosome forms within the black fly Simulium arcticum complex (Diptera:Simuliidae)?

Abstract

Sex-linked chromosomal inversions in black flies (Diptera:Simuliidae) are important in the process of species differentiation. These inversions occur early in the differentiation process before DNA monophyly of sibling species. Nine sibling species and an additional 22 cytotypes have been described for the Simulium arcticum complex. An unresolved question is whether these chromosome types spread throughout a river corridor or from one corridor to another. We determined the sex-linked chromosomal configuration of ∼7300 male larvae of the S. arcticum complex at 46 collection sites in 14 different drainages and asked whether flies in the same river corridor are chromosomally more similar than are flies of different corridors. This research could identify factors important in divergence. Flies within the same river corridor were chromosomally more similar than were flies between different corridors. Comparisons between sites suggest that river distance and Euclidian distance are associated negatively with chromosomal type similarity, but river distance showed the highest similarity comparisons for any given distance. The chromosomes of flies at sites ≤50 km apart were more similar than were the chromosomes of flies at sites >50 km apart. Flies among different river corridors were less similar chromosomally even when these sites were closer in Euclidian distance than in river distance. Flies were chromosomally most similar in streams of equivalent size, particularly if those streams were <25 km apart in river distance. Most sibling species had considerable chromosomal similarities in large, medium, and small streams as measured by proportional abundance. However, some cytotypes appeared to be restricted to large rivers, whereas other cytotypes were restricted to medium-sized streams.