Egg-load as a physiological factor driving shifts in olfactory preference between protein food and oviposition sites in a synovigenic insect

Synovigenic insects, such as the fruit fly Bactrocera dorsalis (Hendel), alternate between seeking protein-rich food for reproductive development and oviposition sites to maintain optimal fitness. We investigated the factors influencing this shift in olfactory preference using two odor choices: protein (torula yeast, TY) and oviposition site (guava juice, GJ). The experiment consisted of three sequential steps: (1) gravid flies made an initial odor choice between TY and GJ odors, (2) flies that chose GJ in the first choice were allowed to oviposit in response to GJ odor and then underwent a second two-choice assay; (3) following the second two-choice assay, flies were dissected to quantify egg-load. Among the flies that initially preferred the GJ odor, individuals that laid fewer eggs tended to exhibit a continued preference for GJ odor during the second choice, while those that laid more eggs shifted their preference to TY odor. Dissections following the second choice revealed that flies with fewer mature eggs remaining in their ovaries post-oviposition exhibited a stronger preference for TY, while those retaining more mature eggs continued to prefer GJ. These findings indicate that the mature egg-load at a given moment drives olfactory preference shifts in synovigenic insects. In contrast, no consistent relationship was observed between immature egg-load and odor preference. Furthermore, electrophysiological analyses demonstrated heightened sensitivity to protein odors in gravid flies with fewer mature egg-load, and an opposite trend in those with a higher mature egg-load, underscoring the critical role of mature egg-load in modulating olfactory preference in B. dorsalis females.