The arduous path to obligate asexuality in Daphnia

Evolutionary transitions to asexuality are often studied by comparing asexuals found in nature with closely related sexuals. Yet, asexuals sampled in nature may represent only the most successful lineages and therefore provide limited information about the average properties of newly emerging asexuals. These properties are crucial in determining the success of asexual lineages and hence the frequency with which transitions to asexual life cycles evolve. Here, we generate new asexual lineages of Daphnia pulex through contagious asexuality, in which rare males from obligate asexual lineages transmit asexuality to new lineages by mating with sexual females. Unlike asexuals found in nature, most new lineages were capable of both sexual and asexual reproduction (facultative asexuality) and produced non-identical (i.e. non-clonal) daughters when reproducing asexually. They also showed reduced fertility compared to natural asexuals, especially when reproducing asexually, which is expected to reduce the success rate of sex-asex transitions in nature. Taken together, these results suggest that creating new asexual lineages by contagion is more complex than previously thought and may result in diverse, non-clonal offspring, on which subsequent selection can act. If general, such initial fitness valleys associated with sex-asex transitions may be key to resolving the evolutionary 'paradox of sex'.