Rare sexual reproduction by females of “obligate parthenogenetic” lineages of <i>Daphnia</i> cf. <i>pulex</i>

Rare sexual reproduction in otherwise asexual lineages is predicted to strongly alleviate the long-term costs of asexuality while avoiding most of the cost of sex. Here, we test for rare sexual reproduction in females of obligate parthenogenetic (OP) lineages of Daphnia cf. pulex. We find that a small proportion of ~0.5% of offspring contain paternal alleles when they are produced in the presence of males from a so-called cyclical parthenogenetic lineage of the species, thus demonstrating sexual reproduction. Rare sexual reproduction occurred in two independent tests, using females from two different OP lineages, suggesting that it may be a regular feature in OP lineages of the species. Even though it could not be established whether the sexual events involved fertilization of haploid or diploid eggs, these results disclose a reproductive trait entirely new to Daphnia biology, aligning with recent results in other organisms. Indeed, the occurrence of rare or cryptic events of sexual reproduction may be a common feature in lineages previously characterized as obligately parthenogenetic, with significant implications for the evolution of obligate asexuality, its long-term costs, and its genomic consequences, including the evolution of polyploidy.