Strong GAL4 expression compromises Drosophila fat body function

The ability to direct tissue-specific overexpression of transgenic proteins in genetically tractable organisms like Drosophila melanogaster has facilitated innumerable biological discoveries. However, transgenic proteins can themselves impact cellular and physiological processes in ways that are often ignored or poorly defined. Here we discovered that the yolk-GAL4 transgene, which directs strong expression of the yeast GAL4 transcription factor in the Drosophila fat body, induces significant physiological defects in adult female flies. We found that yolk-GAL4 disrupts adipose tissue integrity and reduces fat body lipid stores, egg production, and resistance to systemic bacterial infections. Knocking down GAL4 expression in yolk-GAL4 heterozygotes using RNAi fully suppressed each of these defects, thus confirming that the GAL4 transgene product induces these phenotypes. Comparing a panel of additional fat body driver lines, we found that GAL4 expression levels directly correlate with infection susceptibility, but not with fat levels or egg production. To determine whether other transgenic proteins can impair fat body function, we constructed new fly lines in which the yolk enhancer directs expression of either cytoplasmic or nuclear-localized mCherry, or an alternative transactivator, LexA. We found that only nuclear-localized mCherry and LexA increased infection susceptibility similarly to GAL4, suggesting that intranuclear transgenic proteins in general can curtail the fat body's induced immune response in a manner highly sensitive to transgene expression strength. Additionally, these new lines can be valuable tools for future studies. More broadly, our findings highlight the potential for transgenes to substantially impact organismal biology and emphasize the importance of rigorously characterizing genetic tools to optimally leverage model systems like Drosophila.