Seminal Fluid Protein Acp29AB Shifts Egg-Laying Timing in Drosophila Without Detectable Effects on Female Fitness.

Seminal fluid proteins (SFPs) can alter female physiology and behavior, with consequences that may align with or oppose female interests. Many SFPs are known to shape aspects of the female post-mating response, yet evidence for direct effects on female fitness is scarce. In Drosophila melanogaster, the SFP Acp29AB is firmly linked to sperm defense (P1) and sperm storage, but its impacts on females are poorly understood. Using a GAL4-UAS RNAi system with perfectly matched genetic backgrounds, we manipulated Acp29AB gene expression either ubiquitously or specifically in the male accessory gland and then asked how knockdown alters male sperm competition and, in female mates, oviposition dynamics and competitive fitness. Both knockdowns impaired sperm defense, consistent with past work on Acp29AB. We uncovered a previously undetected effect on oviposition timing: across 14 days after a single mating, females mated to control males laid more eggs early in life than females mated to knockdown males, advancing the timing of reproduction. With accessory gland-specific knockdown, this shift occurred without a change in total egg output, which may explain why earlier work focused on totals alone did not detect an effect. Despite accelerated oviposition in mates of control males, female competitive fitness in a five-day, early-life assay that included direct (female-female) and indirect (larval) competition did not differ between treatments. Thus, Acp29AB enhances male sperm defense and promotes earlier egg laying in mates, but under our experimental conditions has no detectable net effect on female fitness. These results broaden the functional scope of Acp29AB to include female reproductive schedule and suggest that some SFPs may evolve rapidly while either being effectively neutral or having context-dependent effects on female fitness.