Populations of a Connecticut River midge structured by geological history and downstream gene £ow

Connecticut River midges in the genus Axarus are constrained to semi-isolated populations associated with submerged clay exposures, and gene flow between populations is restricted. I sampled larval midges from fifteen spatially separated sites in the Connecticut River and determined karyotypes for 3111 larvae in two closely related species. In one of these species, an undescribed species referred to as Axarus species varvestris, 5 chromosomal rearrangements were present as polymorphisms. Four of these are paracentric inversions, and the fifth is a terminal insertion or deletion. I used chromosomal rearrangement frequencies to calculate Nei’s genetic distance measures for all pairwise population comparisons and I constructed a neighbor-joining phenetic tree for the populations using these distances. Genetic distance between populations of A. sp. varvestris increases with geographic separation, and tree structure indicates that restricted gene flow only occurs in a downstream direction. The effect of unidirectional (downstream) gene flow on population genetic structure in riverine midges is described here for the first time. A genetic break is apparent between populations associated with 12 000-year-old sediments originating in different ancient lakes. The geological break between lakes also coincides with a break in the species composition of the Axarus populations.