Tissue-specific metabolomic signatures for a doublesex model of reduced sexual dimorphism

Abstract

Sex has a major effect on the metabolome. However, we do not yet understand the degree to which these quantitative sex differences in metabolism are associated with anatomical dimorphism and modulated by sex-specific tissues. In the fruit fly, Drosophila melanogaster, knocking out the doublesex (dsx) gene gives rise to adults with intermediate sex characteristics. Here we sought to determine the degree to which this key node in sexual development leads to sex differences in the fly metabolome. We measured 91 metabolites across head, thorax and abdomen in Drosophila, comparing the differences between distinctly sex-dimorphic flies with those of reduced sexual dimorphism: dsx null flies. Notably, in the reduced dimorphism flies, we observed a sex difference in only 1 of 91 metabolites, kynurenate, whereas 51% of metabolites (46/91) were significantly different between wildtype XX and XY flies in at least one tissue, suggesting that dsx plays a major role in sex differences in fly metabolism. Kynurenate was consistently higher in XX flies in both the presence and absence of functioning dsx. We observed tissue-specific consequences of knocking out dsx. Metabolites affected by sex were significantly enriched in branched chain amino acid metabolism and the mTOR pathway. This highlights the importance of considering variation in genes that cause anatomical sexual dimorphism when analyzing sex differences in metabolic profiles and interpreting their biological significance.

Keywords: Drosophila melanogaster; metabolism; sex differences; sexual dimorphism.

Fig 1.. PCA of metabolome samples by tissue.

PCA for (A) head tissue, (B) thorax tissue and (C) abdomen tissue. Dsx null flies are colored in orange, wildtype flies are colored in brown. Triangles represent XX and squares represent XY fly samples.

Fig 2.. Density of sex effect sizes across metabolites in wildtype and dsx null sexes.

(A) Density plot across SD metabolites, those metabolites with a significant effect of sex in wildtype flies (N = 46 metabolites). Sex effect sizes across metabolites are significantly different between wildtype and dsx null groups: head (p = 0.002), thorax (p = 3E-04) and abdomen (p = 1E-06). (B) Density plot across all metabolites (N = 91 metabolites). Sex effect sizes across metabolites are significantly different between wildtype and dsx null groups: head (p = 0.007), thorax (p = 0.02), abdomen (p = 6E-04). Vertical lines mark the median (dashed) and mean (solid) effect size across metabolites. Effect sizes are plotted as absolute values on a log10 scale.

Fig 3.. Tissue specificity of SD metabolites.

(A) Venn diagram of number of SD metabolites that overlap across tissues. (B) Sex effect sizes for seven SD metabolites significantly different between wildtype sexes across all three tissues. Effect sizes are calculated from the TukeyHSD function in R. Kynurenate, in light green, is the one metabolite that maintains a significant sex difference in both wildtype and dsx null flies across all three tissues. Effect sizes are plotted on a log10 scale.