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

Sexual reproduction persists despite being less efficient than asexual reproduction. Researchers investigated why facultative sex, which combines aspects of both modes, is not more common in nature. Facultative sex could provide a balance between the advantages of sexual and asexual reproduction. To test this, the researchers studied the aphid parasitoid wasp Lysiphlebus fabarum, which exhibits both asexual and facultative sexual reproduction.

The scientists raised offspring from females that reproduced either exclusively asexually or through facultative sex for three generations. They confirmed the presence of facultative sex and tracked the mixed sexual-asexual genotypes across generations using antennomere number, a trait linked to reproductive mode. Their findings revealed several barriers to maintaining facultative sex.

When mating occurred, offspring were often a mix of sexually and asexually produced, but not always. Moreover, lineages founded by facultatively sexual females showed reduced fecundity compared to those produced asexually. The researchers also observed a decline in the sexual antenna phenotype in subsequent generations, indicating that genotypes resulting from facultative sex are selected against.

These results support the notion that facultative sex can impose costs that persist beyond the initial sexual event. The cumulative impact of these costs, combined with low rates of actual sex, may limit long-term introgression and explain why facultative sex is rare compared to obligate sex.