Abstract
In many diploid species the sex chromosomes play a special role in mediating reproductive isolation. In haplodiploids (i.e., females are diploid and males haploid), the whole genome behaves similar to the X/Z chromosomes of diploids, and thus haplodiploid systems can serve as a model for the role of sex chromosomes in speciation and hybridization. A previously described population of Finnish Formica wood ants displays genome-wide signs of ploidally and sexually antagonistic selection resulting from hybridization. Here, hybrid diploid females have increased survivorship but hybrid haploid males are inviable. In order to understand how this unusual natural population may sustain this antagonistic selection for hybrid status, we developed a mathematical model with hybrid incompatibility, female heterozygote advantage, recombination, and assortative mating. The rugged fitness landscape resulting from the conflict between heterozygote advantage and hybrid incompatibility results in sexual conflict in haplodiploids, which is absent in diploids. Thus, whereas heterozygote advantage always promotes longterm polymorphism in diploids, we find various outcomes in haplodiploids in which the conflict can be resolved either in favor of males, females, or via maximizing the number of introgressed individuals. We fit our model to data from the Finnish wood ant population in order to discuss its potential long-term fate. We highlight the general implications of our results for speciation and hybridization in haplodiploids versus diploids, and how such fitness conflicts could contribute to the outstanding role of sex chromosomes as hotspots of sexual conflict and genes involved in speciation.