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 Lygaeus equestris and L. simulans show a classic pattern of asymmetric pre-zygotic reproductive isolation, with female L. equestris hybridising with male L. simulans, but with no hybridisation in the reciprocal direction. We have recently described a mutant pale colour form of L. simulans, 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 L. simulans, with the pale form less successful at mating with female L. equestris. 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 L. simulans and a male L. equestris. 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.
This data was collected through the laboritory experiments carried out in the Shuker Lab at the University of St Andrews as described in detail in the methods section of the manuscript. In short, two species, Lygaeus simulans (of which there were two different colour morphs) and L. equestris were paired up in different treatments combinations (species, colour morph) and data was collected on whether pairs copulated, copulation duration, egg production, offspring production and hybridisation. Data was then analysed using R statistical software.
READ ME files have been included with the data.