Amitosis confers benefits of sex in the absence of sex to Tetrahymena

Sex appears to be the most successful reproductive strategy in eukaryotes despite its many costs.  While a complete explanation for sex's success remains elusive, several evolutionary benefits of sex have been identified. It is predicted that, by forgoing these benefits, asexual lineages are evolutionary dead-ends. Consistent with this prediction, many asexual lineages show signs of accelerated accumulation of deleterious mutations compared to their sexual relatives.  Despite these low expectations, some asexual eukaryotic lineages appear to be successful, including the ciliate Tetrahymena.  Here, we show that the mechanism of somatic nuclear division in Tetrahymena, known as amitosis, provides benefits similar to sex, allowing for the long-term success of asexual lineages.  We found that, when compared to mitosis, amitosis with chromosome copy number control reduces mutation load deterministically, slows the accumulation of deleterious mutations under genetic drift, and accelerates adaptation.  These benefits arise because, like sex, amitosis can generate substantial genetic variation in fitness among (asexual) progeny.  Our results indicate that the ability of Tetrahymena to persist in the absence of sex may depend on non-sexual genetic mechanisms conferring benefits typically provided by sex, as has been found in other asexual lineages.