Resetting the rules: Sex chromosome turnover as an escape hatch for mitonuclear conflict

Rachel L Moran 

Mitochondrial and nuclear genomes must remain coadapted to sustain oxidative phosphorylation, yet their distinct inheritance often fosters conflict. Sex chromosomes are a key arena for these dynamics: by biasing co-transmission between nuclear-encoded mitochondrial (N-mt) genes and maternally inherited mtDNA, they can amplify or suppress mitonuclear incompatibilities. Existing syntheses emphasize stable XY (mammals) and ZW (birds) systems, where genomic context is conserved. In contrast, many fishes and amphibians undergo frequent shifts among XY, ZW, and polygenic or environmental sex determination, repeatedly resetting the linkage backdrop for mitonuclear interaction. I propose a comparative framework in which sex-chromosome turnover acts as an evolutionary “escape hatch” from Y- or W-linked deleterious N-mt alleles by entrapping N-mt loci into, or releasing them from, non-recombining sex-linked strata, thereby altering the sex bias and intensity of conflict through time. This hypothesis predicts cyclical pulses of hybrid dysfunction and genomic “scars’’ that record historical entrapment, offering testable predictions for when turnover entrenches versus relieves mitonuclear conflict. By integrating evidence from stable and labile vertebrate systems, this article outlines how sex-chromosome dynamics shape mitonuclear coadaptation, postzygotic isolation, and ultimately lineage diversification.