Why selection for mitochondrial quality drives the evolution of sexes

The evolution of sexes is closely tied to uniparental inheritance (UPI) of mitochondrial DNA (mtDNA), where only females transmit mtDNA. Unlike nuclear DNA, mtDNA is highly polyploid and never evolved to be part of meiotic sex. Modelling shows that UPI increases mtDNA mutational variance, enhancing selection for high-quality mtDNA and promoting the emergence of sexes from mating types in unicellular eukaryotes. Paternal control of mitochondrial transfer favours some degree of mtDNA leakage, whereas maternal control favours strict UPI, leading to sexual conflict driving turnover in transmission mechanisms. In multicellular organisms, mitotic segregation of mtDNA increases variance in gametes, again facilitating selection. Surprisingly, germline evolution seems to reflect mtDNA mutation rates: plants and sessile metazoans have low rates and produce gametes from somatic cells, while bilaterians and ctenophores with higher rates sequester germlines with restricted cell division. High mtDNA ploidy in oocytes allows early embryonic cell division without replication, reducing mutational variance across tissues and enhancing somatic fitness. Germline mtDNA quality is maintained by mitotic over-proliferation of germ cells and the selective transfer of mtDNA into primordial oocytes linked with massive apoptotic germ-cell atresia. Overall, selection for mtDNA quality elucidates the evolution of sexes and the architecture of the female germline. This article is part of the theme issue 'Evolutionary genetics of mitochondria: on diverse and common evolutionary constraints across eukarya'.