Genetic incompatibilities in reciprocal hybrids between populations of Tigriopus californicus with low to moderate mitochondrial sequence divergence.

All mitochondrial-encoded proteins and RNAs function through interactions with nuclear-encoded proteins, which are critical for mitochondrial performance and eukaryotic fitness. Coevolution maintains inter-genomic (i.e., mitonuclear) compatibility within a taxon, but hybridization can disrupt coevolved interactions, resulting in hybrid breakdown. Thus, mitonuclear incompatibilities may be important mechanisms underlying reproductive isolation and, potentially, speciation. Here we utilize Pool-seq to assess the effects of mitochondrial genotype on nuclear allele frequencies in fast- and slow-developing reciprocal inter-population F2 hybrids between relatively low-divergence populations of the intertidal copepod Tigriopus californicus. We show that mitonuclear interactions lead to elevated frequencies of coevolved (i.e., maternal) nuclear alleles on two chromosomes in crosses between populations with 1.5% or 9.6% fixed differences in mitochondrial DNA nucleotide sequence. However, we also find evidence of excess mismatched (i.e., non-coevolved) alleles on three or four chromosomes per cross, respectively, and of allele frequency differences consistent with effects involving only nuclear loci (i.e., unaffected by mitochondrial genotype). Thus, our results for low-divergence crosses suggest an underlying role for mitonuclear interactions in variation in hybrid developmental rate, but despite substantial effects of mitonuclear coevolution on individual chromosomes, no clear bias favouring coevolved interactions overall.