Patterns of population structure in two closely related, partially sympatric caddisflies in Eastern Europe: historic introgression, limited dispersal, and cryptic diversity1

We used 2 caddisflies, Drusus discolor and Drusus romanicus, to test explicitly whether closely related species that occupy similar niches and occur in partial sympatry maintain comparable population structure and share a similar population history. We used mitochondrial sequence data to analyze and compare the population structure and the phylogeography of 105 specimens of D. discolor and 74 individuals of D. romanicus collected in southeastern Europe. We examined the relationship between both species with phylogenetic inference and coalescent modeling and used the results to assign larvae to species. We were able to unambiguously assign larvae to species level based on clearly defined association criteria within a phylogenetic analysis of all specimens. The species were closely related and not reciprocally monophyletic in our haplotype phylogeny. One D. romanicus haplotype from the Bucegi Mountains was nested within D. discolor, a result that suggests isolation with migration, introgression, or incomplete lineage sorting between the 2 species. For each species, we examined population genetic structure with median joining networks, analysis of molecular variance (AMOVA), exact tests of population differentiation, and Mantel tests of isolation by distance. We used tests for selective neutrality (Tajima's D, Fu's F) to infer potential population growth and expansion. Species differed in their genetic population structure. Drusus discolor had haplotype overlap among several mountain ranges in the study region. No D. romanicus haplotypes were shared among any regions examined, and levels of divergence between haplotype clades exceeded those of D. discolor by a factor of up to 2.1. The different degree of population differentiation and divergence of both species probably reflects different Pliocene/Pleistocene population histories and might be related to differences in dispersal capabilities or competitive exclusion of D. romanicus by D. discolor in the mountain ranges north and west of the Western Carpathians. Based on our results, we discuss the importance of the Carpathian Mountains and Bulgarian highlands as Pliocene/Pleistocene refugia and centers of diversification.