Date Published: May 31, 2019
Publisher: Public Library of Science
Author(s): Katherine Fiocca, Meghan Barrett, Edward A. Waddell, Jennifer Viveiros, Cheyenne McNair, Sean O’Donnell, Daniel R. Marenda, Olena Riabinina.
Mannitol, a sugar alcohol used in commercial food products, has been previously shown to induce sex-biased mortality in female Drosophila melanogaster when ingested at a single concentration (1 M). We hypothesized that sex differences in energy needs, related to reproductive costs, contributed to the increased mortality we observed in females compared to males. To test this, we compared the longevity of actively mating and non-mating flies fed increasing concentrations of mannitol. We also asked whether mannitol-induced mortality was concentration-dependent for both males and females, and if mannitol’s sex-biased effects were consistent across concentrations. Females and males both showed concentration-dependent increases in mortality, but female mortality was consistently higher at concentrations of 0.75 M and above. Additionally, fly longevity decreased further for both sexes when housed in mixed sex vials as compared to single sex vials. This suggests that the increased energetic demands of mating and reproduction for both sexes increased the ingestion of mannitol. Finally, larvae raised on mannitol produced expected adult sex ratios, suggesting that sex-biased mortality due to the ingestion of mannitol occurs only in adults. We conclude that sex and reproductive status differences in mannitol ingestion drive sex-biased differences in adult fly mortality.
D-mannitol (henceforth mannitol) is a 6-carbon polyol produced via microbial fermentation, particularly by yeasts, and is the most common naturally-occurring polyol in plants and fungi [1–4]. Mannitol is commonly used as a sweet additive in consumer products as it is only partially absorbed in the human small intestine without increasing insulin secretion or blood glucose [1,5].
This study aimed to determine if mannitol’s effects were consistently sex-biased in adults and larvae across increasing concentrations of mannitol, and if mating status or culturing condition affected mortality. Mortality induced by mannitol was concentration-dependent for both sexes. Female adult flies ingesting mannitol showed significant decreases in longevity to 21 days compared to controls from 0.5 M-2 M mannitol, while males raised on 0.75 M-2 M mannitol showed significant decreases in longevity compared to controls. Females had an LC50 of 0.76 M at 21 days, while males reached a maximum mortality of 30.2% as a result of 1 M mannitol treatment. These decreases in longevity to 21 days were concentration-dependent at concentrations below 1 M mannitol for both sexes. Survival was somewhat rescued at the highest concentrations of mannitol tested (1.5 M and 2 M) for both males and females. A potential explanation for this rescue could be that flies detect the presence of mannitol at these extreme concentrations, which may decrease palatability. Alternatively, media was observed to become crystalline at these higher concentrations, which may deter normal consumption and reduce mortality. It is important to note that flies were still able to consume and excrete media at these concentrations given the presence of blue dye in their crops (at 24 hours) and fecal matter (throughout the experiment).
Mannitol caused concentration-dependent decreases in longevity to 21 days in both male and female fruit flies at concentrations of 0.75 M (males) or 0.5 M (females) and above. Female longevity was more significantly decreased compared to that of males at concentrations of 0.75M and above. Actively mating males and females had decreased longevity compared to virgin males and females at concentrations of 1 M and above. Mannitol fed to larvae did not alter adult sex ratios, suggesting that sex-biased mortality due to mannitol occurs only in adults. Overall, our results support our hypothesis that sex differences in energy needs, related to the nutritional and behavioral demands of mating and reproduction, contribute to decreased longevity in females compared to males. We further conclude that both males and females have mannitol-induced decreases in longevity when mated, as compared to virgins, due to the increased costs of reproduction for both sexes.