Viability of cytochrome C genotypes depends on cytoplasmic backgrounds in Tigriopus californicus

Because of their extensive functional interaction, mitochondrial DNA (mtDNA ) and nuclear genes may evolve to form coadapted complexes within reproduct ively isolated populations. As a consequence of coadaptation, the fitness o f particular nuclear alleles may depend on mtDNA genotype. Among population s of the copepod Tigriopus californicus, there are high levels of amino aci d substitutions in both the mtDNA genes encoding subunits of cytochrome c o xidase (COX) and the nuclear gene encoding cytochrome c (CYC), the substrat e for COX. Because of the functional interaction between enzyme and substra te proteins, we hypothesized that the fitness of CYC genotypes would depend on mtDNA genotype. To test this hypothesis. segregation ratios for CYC and a second nuclear marker (histone HI) unrelated to mitochondrial function w ere scored in F, progeny of several reciprocal interpopulation crosses. Gen otypic ratios at the CYC locus (but not the HI locus) differed between reci procal crosses and differed from expected Mendelian ratios. suggesting that CYC genotypic fitnesses were strongly influenced by cytoplasmic (including mtDNA) background. However, in most cases the nature of the deviations fro m Mendelian ratios and differences between reciprocal crosses are not consi stent with simple coevolution between CYC and mtDNA background. In a cross in which both newly hatched larvae and adults were sampled, only the adult sample showed deviations from Mendelian ratios, indicating that genotypic v iabilities differed. In two of six crosses, large genotypic ratio differenc es for CYC were observed between the sexes. These results suggest that sign ificant variation in nuclear-mtDNA coadaptation may exist between T. califo rnicus populations and that the relative viability of specific cytonuclear allelic combinations is somehow affected by sex.