Is facultative sex is the best of both worlds in the parasitoid wasp Lysiphlebus fabarum?

The prevalence of sexual reproduction has long puzzled evolutionary biologists. This is because asexual reproduction is a far more efficient way to transmit copies of one’s own genome to the next generation, and, all-else-being-equal, should predominate over sex. Asexual reproduction is not without its disadvantages though, the lack of genetic recombination can render parthenogenetic lineages vulnerable to extinction under environmental change, or compromise fitness due to the buildup of deleterious recessive mutations. Facultative sex, where individuals retain the ability to reproduce sexually and asexually, has been touted as ‘the best of both worlds’, providing the long-term genetic benefits of sex without the short-term costs. In this study I tested whether sex is facultative in the aphid parasitoid Lysiphlebus fabarum , and if so whether facultative sex is ‘the best of both worlds’. I found that females from asexual populations mate with males produced by females from an obligately sexual population, and produced offspring with paternal genetic contributions, confirming facultative sex in this species. I then assayed the fitness of these facultatively sexual wasps and found, contrary to predictions that they actually had significantly lower fitness than asexual or obligately sexual females, as their daughters were up to 40% more likely to fail to reproduce entirely. In contrast, obligately sexual females were not at a disadvantage compared to asexual females in this species; they had increased fecundity compared to asexual females which compensated for the costs of sex. In L. fabarum it seems that all is not equal; the fecundity advantage that obligate sex provides, along with the costs of occasional facultative sex by asexuals may balance the scales and allow stable coexistence of the different reproductive modes in nature.