Cis-regulatory polymorphism atfizecdysone oxidase contributes to polygenic adaptation to malnutrition inDrosophila

Abstract

AbstractWe investigate the contribution of a candidate gene,fiz(fezzik), to complex polygenic adaptation to juvenile malnutrition inDrosophila melanogaster. We show that experimental populations adapted during >250 generations of experimental evolution to a nutritionally poor larval diet (Selected populations) evolved several-fold lowerfizexpression compared to unselected Control populations. This divergence infizexpression is mediated by a cis-regulatory polymorphism. This polymorphism, which was originally present in a sample from a natural population in Switzerland, is distinct from a second cis-regulatory SNP previously identified in non-AfricanD. melanogasterpopulations, implying that two independent cis-regulatory variants promoting highfizexpression segregate in non-African populations. Enzymatic analyses of Fiz protein expressed inE. colidemonstrate that it has ecdysone oxidase activity acting on both ecdysone and 20-hydroxyecdysone. Four of fivefizparalogs annotated to ecdysteroid metabolism also show reduced expression in Selected larvae, suggesting that malnutrition-driven selection favored general downregulation of ecdysone oxidases. Finally, as an independent test of the role offizin poor diet adaptation, we show thatfizknockdown by RNAi results in faster larval growth on the poor diet, but at the cost of greatly reduced survival. These results imply that downregulation offizin Selected populations was favored because of its role in suppressing growth in response to nutrient shortage. However,fizdownregulation is only adaptive in combination with other changes evolved by Selected populations, such as in nutrient acquisition and metabolism, which ensure that the organism can actually sustain the faster growth promoted byfizdownregulation.