Multigenerational exposure to high-fat and high-sugar diets exacerbates reproductive distress in an insect model

In several insect models, high-fat diets and high-sugar diets have detrimental effects, but it is largely unknown if multigenerational exposure can curb this metabolic distress through adaptation. Our study aimed to investigate how high-fat and high-sugar diets affect the fitness of the common house fly Musca domestica, and if multigenerational experimental evolution with diet-induced selection pressure leads to compensatory metabolic changes. House fly larvae were reared on a high-fat, high-sugar, or control diet for 51 consecutive generations whereupon we measured larval, pupal, and adult life-history traits. To test direct effects of the diets on life-history traits, we switched the 50th generation of control diet flies onto the experimental diets. Initial exposure to the high-fat diet caused a significant decrease in pupal viability, larval dry weight, and adult-weight-to-length ratio, whereas relative larval fat content significantly increased. The high-sugar diet solely increased adult weight-to-length ratio. When comparing the effects of initial exposure to the effects after 51 generations of experimental evolution, the high-fat diet flies exhibited the same detrimental effects as previously observed. Additionally, after 51 generations, the high-fat-diet and high-sugar-diet flies experienced a drastic decrease in female fecundity. Direct comparison of the single-generational exposed and the multigenerational evolved lines confirmed that the diet-induced adverse metabolic effects expanded across generations. Our results suggest that house flies do not adapt to unbalanced diets and instead experience additional reproductive disruptions when selected on these diets for multiple generations.