Gene conversion limits the cost of asexuality in immortal worms

Most multicellular organisms reproduce sexually despite the costs associated with sexuality. This has been explained as the result of selection favouring the ability to recombine the genome. The lack of recombination in asexual species constrains their adaptability and leads to the accumulation of deleterious mutations, ultimately increasing their risk of extinction. Nonetheless, successful asexual life histories persist among multicellular organisms, and explanatory mechanisms which may help limit the cost of asexuality remain enigmatic. In search of these mechanisms, we looked at that the molecular evolutionary changes in the sexual and obligately asexual strains of the planarian flatworm, Schmidtea mediterranea. We find that the accumulation of deleterious mutations in highly conserved genes is largely avoided in the asexual strain. We find evidence that this is achieved by somatic gene conversion in stem cells allowing for the restoration of beneficial alleles and purification of deleterious mutations. Taken together, our analysis identifies gene conversion as a mechanism which may contribute to the maintenance of asexuality in an obligately fissiparous metazoan.