Natural variation in copper tolerance in Drosophila melanogaster is shaped by transcriptional and physiological changes in the gut

Increases in industrialisation and anthropogenic activity have resulted in an increase in pollutants released into the environment. Of these pollutants, heavy metals such as copper are particularly concerning due to their bio-accumulative nature. Due to its highly heterogeneous distribution and its dual nature as both an essential micronutrient and toxic element, the genetic basis of copper tolerance is likely shaped by a complex interplay of physiological and environmental factors. Drosophila melanogaster, a long-standing sentinel of environmental toxins, is uniquely suited for the study of copper tolerance in arthropods and other more diverse species. In this study, we utilized the natural variation present in multiple populations of D. melanogaster collected across Europe to screen for variation in copper tolerance, which we found to be highly variable both within and between locations. While these collection locations covered a wide range of atmospheric and soil pollution levels, the degree of urbanization at the collection sites, rather than any other combination of environmental factors, was linked to copper tolerance. Moreover, differential expression analysis revealed that metabolism, reproduction, and protease induction contribute to copper response in tolerant and sensitive lines to different degrees. Additionally, the greatest transcriptomic and physiological responses to copper toxicity were seen in the midgut; where preservation of gut acidity is strongly linked to greater tolerance. Overall, our study provides a unique perspective on the genetic and environmental factors that shape copper tolerance in natural D. melanogaster populations and identifies new genes and physiological traits involved in this complex phenotype.