Hybrid zones provide natural experiments in recombination within and between genomes that may have strong effects on organismal fitness. On the East Coast of North America, two distinct lineages of the European green crab (Carcinus maenas) have been introduced in the last two centuries. These two lineages with putatively different adaptive properties have hybridized along the coast of the eastern Gulf of Maine, producing new nuclear and mitochondrial combinations that show clinal variation correlated with water temperature. To test the hypothesis that mitochondrial or nuclear genes have effects on thermal tolerance, we first measured the response to cold stress in crabs collected throughout the hybrid zone, then sequenced the mitochondrial CO1 gene and two nuclear SNPs representative of nuclear genetic lineage. Mitochondrial haplotype had a strong association with the ability of crabs to right themselves at 4.5 &[deg]C that was sex specific: haplotypes originally from northern Europe gave male crabs an advantage while there was no haplotype effect on righting in female crabs. By contrast, the two nuclear SNPs that were significant outliers in a comparison between northern and southern C. maenas populations had no effect on righting response at low temperature. These results add C. maenas to the short list of ectotherms in which mitochondrial variation has been shown to affect thermal tolerance, and suggests that natural selection is shaping the structure of the hybrid zone across the Gulf of Maine. Our limited genomic sampling does not eliminate the strong possibility that mitonuclear coadaptation may play a role in the differences in thermal phenotypes documented here. Linkage between mitochondrial genotype and thermal tolerance suggests a role for local adaptation in promoting the spread of invasive populations of C. maenas around the world
