DILP8 serves as a mature follicle sensor to prevent excessive accumulation of maturefollicles in Drosophila ovaries and oocyte aging.

Excessive mature follicle accumulation in ovaries harms oocyte health and offspring viability. As such, the number of mature follicles in ovaries is tightly controlled. In Drosophila, each ovary is comprised of [~]16 ovarioles, each containing 1-2 mature follicles, regardless of the females mating status. The mechanism by which the female flies count the number of mature follicles to coordinate egg release and oogenesis remains a mystery. Previous work, along with our RNAseq and antibody analysis, demonstrated that Drosophila insulin-like peptide 8 (DILP8) is expressed in somatic follicle cells of mature follicles but not in younger follicles. Contrary to previous findings, we found that DILP8 is not essential for mating-induced ovulation/egg laying. In contrast, global depletion or follicle-cell-specific knockdown of dilp8 leads to defective ovulation/egg laying and a significant increase of mature follicles in virgin females. In addition, we found that excessive accumulation of mature follicles in dilp8-knockdown females leads to poor oocyte quality. Furthermore, knockdown of Lgr3 (Leucine-rich repeat-containing G protein-coupled receptor 3), encoding a previously identified DILP8 receptor, showed similar ovulation/egg laying defects, accumulation of mature follicles, and poor oocyte quality in virgin females. Therefore, DILP8 functions as a mature follicle sensor to prevent excessive accumulation of mature follicles and maintain oocyte quality through the Lgr3 receptor in virgin females. Because mating is not always available in the wild and the DILP8/Lgr3 pathway is highly conserved across multiple species, our findings suggest that DILP8/Lgr3 is likely critical for maintaining the optimal reproductive fitness of virgin females and for species survival in the wild.