Abstract
The evolution of reproductive isolation lies at the heart of understanding the process of speciation. Of particular interest is the relationship between pre‐ and postzygotic 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 prezygotic reproductive isolation, with female L. equestris hybridizing with male L. simulans, but with no hybridization in the reciprocal direction. We have recently described a mutant pale color form of L. simulans, that inherits as a single Mendelian locus and is pleiotropic for a number of other life history and behavioral traits. Here, we tested whether this locus also influences pre‐ and postzygotic reproductive isolation. Two sets of experimental crosses revealed that behavioral 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 postzygotic 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 supergene.
Highlights
The evolution of reproductive isolation is central to the process of speciation (Coyne & Orr, 2004; Nosil, 2012; Butlin & the Marie Curie SPECIATION Network, 2012; but see Harvey et al, 2019)
Our results show that the pale mutation in Lygaeus simulans influences prezygotic isolation between L. simulans and its sister species L. equestris
Pale male L. simulans were less likely to mate with female L. equestris than wild-type L. simulans males
Summary
The evolution of reproductive isolation is central to the process of speciation (Coyne & Orr, 2004; Nosil, 2012; Butlin & the Marie Curie SPECIATION Network, 2012; but see Harvey et al, 2019). The focus of much contemporary work on reproductive isolation and speciation has been the underlying genetic changes that either lead to, or are otherwise associated with, divergence between populations (Butlin & the Marie Curie SPECIATION Network, 2012; Nosil & Schluter, 2011) This has included the search for genes directly shaping pre- or postzygotic reproductive isolation, and understanding broader genomic changes linked to speciation and its consequences. The mutation does not appear to be sex-linked nor sex-limited and is presumably autosomal Because of this rather widespread pleiotropy, here we asked whether the mutant influenced the nature and extent of hybridization between these two species and whether having a pale L. simulans allele instead of a wild-type allele at this locus influenced the fitness of the hybrid offspring. We predicted that there would be variation in fitness (egg to nymph viability) between hybrids from crosses of pale L. simulans with L. equestris and crosses of wild-type L. simulans with L. equestris
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