Mitochondrial introgression is restricted relative to nuclear markers in a water strider (Hemiptera: Gerridae) hybrid zone

Although cytonuclear incompatibilities between species have been implicated in a variety of theoretical and experimental studies, the influence of such fitness interactions on gene introgression has been demonstrated in very few hybrid zones. We examined patterns of introgression between two species of Limnoporus Stal, 1868 water striders from 10 populations transecting a hybrid zone in Alberta and British Columbia. DNA-sequence variation in mitochondrial locus COI was compared with two nuclear loci, EF1-alpha and ITS 1. The spatial distribution of haplotype lineages showed strong barriers to gene exchange for mtDNA. Constraints on introgression were weaker for the nuclear markers, particularly ITS 1. The mtDNA from Limnoporus dissortis Drake and Harris, 1930 was associated with nuclear genes from Limnoporus notabilis Drake and Hottes, 1925 less often than expected by random mating, indicating that some cytoplasmic and nuclear combinations are favored over others and that cytonuclear incompatibilities play a role in structuring this hybrid zone. Mitochondrial mobility across the hybrid zone is likely to be restrained by the unusual hybrid inviability of these species. In a reversal of the pattern expected on the basis of Haldane's rule, females are the XX sex and yet hybrid females are much more inviable than males. Differential removal of hybrid females, which carry mtDNA, can explain disproportionately low mitochondrial introgression across the hybrid zone.