Mitochondrial complementation: a possible neglected factor behind early eukaryotic sex

Sex is ancestral in eukaryotes. Meiotic sex differs from bacterial ways of exchanging genetic material by involving the fusion of two cells. We examine the hypothesis that fusion evolved in early eukaryotes because it was directly beneficial, rather than a passive side-effect of meiotic sex. We assume that the uptake of (proto)mitochondria into eukaryotes preceded the evolution of cell fusion, and that Muller’s ratchet operating within symbiont lineages led to the accumulation of lineage-specific sets of mutations in asexual host cells. We examine if cell fusion, and the consequent biparental inheritance of symbionts, helps to mitigate the effects of this mutational meltdown of mitochondria. In our model, host cell fitness improves when two independently-evolved mitochondrial strains co-inhabit a single cytoplasm, mirroring mitochondrial complementation found in modern eukaryotes. If fusion incurs no cost, we find that an allele coding for fusion can invade a population of non-fusers. I...