Mitonuclear effects on sex ratio persist across generations in interpopulation hybrids

Eukaryotic energy production requires tight coordination between gene products from both the nuclear and mitochondrial genomes. Because males and females often have different energetic strategies, this mitonuclear coordination can be expected to differentially impact the two sexes. Previous work found evidence for sex-specific mitonuclear effects in the copepod Tigriopus californicus by comparing two parental lines and their reciprocal F1 crosses. However, an alternative hypothesis is that the patterns could instead be driven by the parental source of nuclear alleles. Here we test this alternative hypothesis by extending the same cross to F2 hybrids, who receive both maternal and paternal nuclear alleles from F1 hybrids. Results confirm mitonuclear effects on sex ratio, with distorted ratios persisting from the F1 to F2 generations, despite reduced fitness in F2 hybrids. No sex by cross interactions were found for other phenotypic traits measured. Mitochondrial DNA content was shown to be higher in females, the more stress-tolerant sex. Both routine metabolic rate and oxidative DNA damage were found to be lower in F2 hybrids than in parentals. Confirmation of sex-biased mitonuclear effects in T. californicus is notable, given that the species lacks sex chromosomes, which can confound interpretations of sex-specific mitochondrial effects.