Patterns of reproductive isolation within and between two Lygaeus species characterized by sexual conflicts over mating

Theory suggests that, under some circumstances, sexual conflict over mating can lead to divergent sexually antagonistic coevolution among populations for traits associated with mating, and that this can promote reproductive isolation and hence speciation. However, sexual conflict over mating may also select for traits (e.g. male willingness to mate) that enhance gene flow between populations, limiting population divergence. In the present study, we compare pre- and post-mating isolation within and between two species characterized by male-female conflict over mating rate. We quantify sexual isolation among five populations of the seed bug Lygaeus equestris collected from Italy and Sweden, and two replicates of a population of the sister-species Lygaeus simulans, also collected from Italy. We find no evidence of reproductive isolation amongst populations of L. equestris, suggesting that sexual conflict over mating has not led to population divergence in relevant mating traits in L. equestris. However, there was strong asymmetric pre-mating isolation between L. equestris and L. simulans: male L. simulans were able to mate successfully with female L. equestris, whereas male L. equestris were largely unable to mate with female L. simulans. We found little evidence for strong post-mating isolation between the two species, however, with hybrid F2 offspring being produced. Our results suggest that sexual conflict over mating has not led to population divergence, and indeed perhaps supports the contrary theoretical prediction that male willingness to mate may retard speciation by promoting gene flow.