Original data.

We investigate the contribution of a candidate gene, fiz (fezzik), to complex polygenic adaptation to juvenile malnutrition in Drosophila melanogaster. Experimental populations maintained for >250 generations of experimental evolution to a nutritionally poor larval diet (Selected populations) evolved several-fold lower fiz expression compared to unselected Control populations. Here we show that this divergence in fiz expression is mediated by a cis-regulatory polymorphism. This polymorphism, originally sampled from a natural population in Switzerland, is distinct from a second cis-regulatory SNP previously identified in non-African D. melanogaster populations, implying that two independent cis-regulatory variants promoting high fiz expression segregate in non-African populations. Enzymatic analyses of Fiz protein expressed in E. coli demonstrate that it has ecdysone oxidase activity acting on both ecdysone and 20-hydroxyecdysone. Four of five fiz paralogs annotated to ecdysteroid metabolism also show reduced expression in Selected larvae, implying that malnutrition-driven selection favored general downregulation of ecdysone oxidases. Finally, as an independent test of the role of fiz in poor diet adaptation, we show that fiz knockdown by RNAi results in faster larval growth on the poor diet, but at the cost of greatly reduced survival. These results imply that downregulation of fiz in Selected populations was favored by selection on the nutritionally poor diet because of its role in suppressing growth in response to nutrient shortage. However, they suggest that fiz downregulation is only adaptive in combination with other changes evolved by Selected populations, which ensure that the organism can sustain the faster growth promoted by fiz downregulation.

本研究旨在探讨候选基因fiz（fezzik）在黑腹果蝇（Drosophila melanogaster）应对幼年期营养不良的复杂多基因适应过程中的作用。本研究中，经250余代实验演化并饲养于低营养幼虫饲料的选育种群，其fiz基因的表达量较未选育的对照种群低数倍。本研究表明，fiz表达的这种差异由顺式调控多态性（cis-regulatory polymorphism）介导。该多态性最初采自瑞士的自然种群，与此前在非非洲黑腹果蝇种群中发现的第二种顺式调控单核苷酸多态性（SNP）存在差异，这表明非非洲种群中存在两种独立的顺式调控变异体，二者均可促进fiz高表达，并以多态形式在种群中分离。对在大肠杆菌（E. coli）中表达的Fiz蛋白进行酶学分析，结果显示其具备蜕皮激素氧化酶（ecdysone oxidase）活性，可同时作用于蜕皮激素（ecdysone）与20-羟基蜕皮激素（20-hydroxyecdysone）。注释为参与蜕皮类固醇代谢的5个fiz旁系同源基因（paralogs）中，有4个在选育种群的幼虫中同样出现表达量下调，这提示营养不良驱动的选择倾向于蜕皮激素氧化酶的整体下调。最后，为独立验证fiz在低营养饲料适应中的作用，本研究通过RNA干扰（RNAi）敲低fiz基因，结果显示幼虫在低营养饲料上的生长速率加快，但存活率大幅下降，此为该操作的代价。上述结果表明，选育种群中fiz基因的下调之所以受到低营养饲料下的选择青睐，是因为其在响应营养匮乏时能够抑制生长。但本研究同时提示，fiz下调仅在与选育种群演化出的其他遗传改变相结合时才具备适应性——这些改变可确保生物体能够维持由fiz下调所促进的更快生长速率。