Is facultative sex 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 parthenogenesis is a more efficient mode of reproduction, 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 owing 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 found that parthenogenetic females of the sexually polymorphic aphid parasitoid Lysiphlebus fabarum readily engage in facultative sex, but facultative sex was not the 'best of both worlds' owing to elevated rates of reproductive failure compared to obligate sex and parthenogenesis. By contrast, obligately sexual females had increased fecundity compared to asexual females. 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 must be factored in when attempting to understand why and how different reproductive modes coexist in nature.