Mating allows asexual female wasps to produce offspring both sexually and asexually, but their sexual daughters have reduced fecundity

Sexual reproduction is widespread despite being less efficient than asexual reproduction. While its long-term evolutionary benefits are well recognized, it remains unclear why facultative sex, which should allow lineages to balance the advantages of both sexual and asexual reproduction, is not more common. One potential reason is that intermittent sex imposes novel costs compared to obligate sex, limiting the persistence of facultatively sexual lineages and constraining introgression. We test this possibility using the sexually polymorphic aphid parasitoid wasp Lysiphlebus fabarum. We reared offspring from females who reproduced asexually or by facultative sex for three generations (without sex after the founding generation). We genotyped F1 females to confirm facultative sex and used antennomere number, a phenotypic marker associated with reproductive mode, to test whether mixed sexual-asexual genotypes could be tracked across generations. We found multiple barriers to facultative sex. Mating did not always result in offspring with sexual-asexual genotypes, and when sex did occur, broods were usually comprised of asexually and sexually produced offspring. Additionally, lineages founded by females produced through facultative sex showed reduced fecundity compared to asexually produced broods. We saw declining frequencies of the sexual antenna phenotype in the generations after facultative sex, suggesting that genotypes generated by facultative sex are selected against. Overall, our results provide empirical support for the idea that facultative sex can impose costs that persist beyond the initial sexual event. These costs, together with low realised rates of sex, may limit long-term introgression and explain why facultative sex appears rare compared to obligate sex.