Gene conversion limits the cost of asexuality in immortal worms

Abstract

AbstractMost multicellular organisms reproduce sexually despite the costs associated with sexuality1. 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 extinction2. Nonetheless, successful asexual life histories persist among multicellular organisms, and explanatory mechanisms which may help limit the cost of asexuality remain enigmatic3–6. In search of these mechanisms, we looked at the molecular evolutionary changes in sexual and obligate asexual strains of the planarian flatworm,Schmidtea mediterranea. We find that the accumulation of deleterious mutations 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 fit alleles and the purification of deleterious mutations. Our analyses provide a potential mechanism for the maintenance of asexuality in a metazoan. Taken together, these findings suggest that gene conversion may be used by asexual lineages to unlink loci, counteracting some of the key challenges facing obligate asexual species.Significance statementSexual reproduction is the most common reproductive strategy for multicellular organisms, while obligate asexuality is very rare. This rarity of asexuality is driven by a reduced ability to adapt to new challenges and an accumulation of deleterious mutations associated with the lack of sexual recombination.Here we find that obligate asexual planarians are able to avoid the accumulation of deleterious mutations associated with asexuality by exploiting a form of recombination within the stem cells of the worm to remove deleterious mutations.This study reinvigorates the long-standing debate on how some asexual lineages persist, by asking this question in obligate fissiparous animals and showing that asexual recombination in somatic cells followed by selection can help limit the cost of asexuality.