Experimental introgression inDrosophila: asymmetric postzygotic isolation associated with chromosomal inversions and an incompatibility locus on the X chromosome

Abstract

AbstractInterspecific gene flow (introgression) is an important source of new genetic variation, but selection against it can reinforce reproductive barriers between interbreeding species. We used an experimental approach to trace the role of chromosomal inversions and incompatibility genes in preventing introgression between two partly sympatricDrosophila virilisgroup species,D. flavomontanaandD. montana. We backcrossed F1hybrid females from a cross betweenD. flavomontanafemale andD. montanamale with the males of the parental species for two generations and sequenced pools of parental strains and their reciprocal 2ndgeneration backcross (BC2mon and BC2fla) females. Contrasting the observed amount of introgression (mean hybrid index, HI) in BC2female pools along the genome to simulations under different scenarios allowed us to identify chromosomal regions of restricted and increased introgression. We found no deviation from the HI expected under a neutral null model for any chromosome for the BC2mon pool, suggesting no evidence for genetic incompatibilities in backcrosses towardsD. montana. In contrast, the BC2fla pool showed high variation in the observed HI between different chromosomes, and massive reduction of introgression on the X chromosome (large X-effect). This observation is compatible with reduced recombination combined with at least one dominant incompatibility locus residing within the X inversion(s). Overall, our study suggests that genetic incompatibilities arising within chromosomal inversions can play an important role in speciation.