Genomic signals of admixture and reinforcement between two closely related species of European sepsid flies

Interspecific gene flow by hybridization may weaken species barriers and adaptive divergence, but also initiate reinforcement of reproductive isolation trough natural and sexual selection. The extent and consequences of interspecific gene flow in natural systems remain poorly understood. To assess genome-wide patterns of gene flow between the two closely related European dung fly species Sepsis cynipsea and Sepsis neocynipsea (Diptera: Sepsidae), we analyzed whole-genome resequencing data from pooled males of laboratory and field populations by means of the ABBA-BABA test.

We contrasted genome-wide variation in DNA sequence differences between samples from sympatric populations of these two species in France and Switzerland with that of interspecific pairs of samples involving allopatric populations from Estonia and Italy. In the French Cevennes, we detected a relative excess of sequence identity suggesting interspecific gene flow in sympatry.

In contrast, at two sites in Switzerland we observed a relative depletion of sequence identity compatible with reinforcement of species boundaries in sympatry. Our results suggest that the species boundaries between S. cynipsea and S. neocynipsea in Europe are not fixed but depend on the eco-geographic context.