A single pleiotropic locus influences the rate of hybridisation between two sibling species of Lygaeus bugs

The evolution of reproductive isolation lies at the heart of understanding the process of speciation. Of particular interest is the relationship between pre- and post-zygotic reproductive isolation, and the genetic architecture of traits that contribute to one or both forms of reproductive isolation. The sibling species of seed bug <i>Lygaeus equestris</i> and <i>L. simulans</i> show a classic pattern of asymmetric pre-zygotic reproductive isolation, with female <i>L. equestris</i> hybridising with male <i>L. simulans</i>, but with no hybridisation in the reciprocal direction. We have recently described a mutant pale colour form of <i>L. simulans</i>, that inherits as a single Mendelian locus and is pleiotropic for a number of other life history and behavioural traits. Here we tested whether this locus also influences pre- and post-zygotic reproductive isolation. Two sets of experimental crosses revealed that behavioural isolation varied with mutant versus wild-type phenotype for male <i>L. simulans</i>, with the pale form less successful at mating with female <i>L. equestris</i>. In terms of trying to assess post-zygotic isolation, levels of hybrid offspring production were uniformly low across the experiments. However, we did obtain, for the first time, hybrid offspring from a pairing between a female <i>L. simulans</i> and a male <i>L. equestris</i>. In this instance, the female was of the pale mutant genotype. Together with evidence for heterozygote advantage in terms of nymph survival, we consider our results in terms of possible mechanisms of reproductive isolation between this species pair, the role of the pale mutation, and the possible genetic architectures underlying the mutation, from a single gene to a super-gene.