Female oviposition decisions are influenced by the microbial environment

In ovipositing animals, egg placement decisions can be key determinants of offspring survival. One oviposition strategy reported across taxa is egg clustering, whereby a female lays multiple eggs next to one another or next to the eggs of other females. The fitness benefits of egg clustering, especially in mixed maternity clusters, are unknown. In some species, mothers provision eggs with diffusible defence compounds, such as antimicrobials, raising the possibility of public good benefits arising from egg clustering. We conducted 4 experiments (EXP1A, EXP1B, EXP2, EXP3) to test the hypothesis that the microbial environment affects fecundity and egg placement in groups of D. melanogaster females. In each experiment, we collected data on the number of eggs laid, the proportion of those eggs laid in clusters, egg cluster size and number, and egg to adult viability. We also tested D. melanogaster and Ceratitis capitata egg wash for antimicrobial activity against E. coli, Saccharomyces cerevisiae, Alcaligenes faecalis and Micrococcus luteus, and whole eggs against E. coli.