Epistasis and maternal effects in experimental adaptation to chronic nutritional stress in Drosophila

Based on ecological and metabolic arguments some authors predict that adaptation to novel, harsh environments should involve alleles showing negative (diminishing return) epistasis and/or that it should be mediated in part by evolution of maternal effects. While the first prediction has been supported in microbes, there has been little experimental support for either prediction in multicellular eukaryotes. Here we use a line-cross design to study the genetic architecture of adaptation to chronic larval malnutrition in a population of Drosophila melanogaster which evolved on an extremely nutrient-poor larval food for 84 generations. We assayed three fitness-related traits (developmental rate, adult female weight and egg-to-adult viability) under the malnutrition conditions in 14 crosses between this selected population and a non-adapted control population originally derived from the same base population. All traits showed a pattern of negative epistasis between alleles improving performance under malnutrition. Furthermore, evolutionary changes in maternal traits accounted for half of the 68 % increase in viability and for the whole of 8 % reduction in adult female body weight in the selected population (relative to unselected controls). These results thus support both of the above predictions and point to the importance of non-additive effects in adaptive microevolution.

基于生态学与代谢学相关论证，部分学者提出：生物对全新严苛环境的适应性演化，应涉及携带负（收益递减）上位性（epistasis）的等位基因（allele），且该适应性演化可部分通过母体效应（maternal effects）的演化介导实现。尽管该第一项预测在微生物研究中已得到验证，但在多细胞真核生物中，两项预测均鲜有实验证据支持。本研究采用系间杂交设计（line-cross design），针对在极端营养匮乏幼虫培养基上历经84代演化的黑腹果蝇（Drosophila melanogaster）种群，探究其对长期幼虫营养不良的适应性演化遗传架构（genetic architecture）。我们将该选育种群与源自同一基础种群的未适应对照种群进行14组杂交，并在营养不良条件下测定了3个适合度相关性状（fitness-related traits）：发育速率、成年雌蝇体重以及卵到成虫存活率。所有性状均呈现出在营养不良条件下提升生存表现的等位基因之间存在负上位性模式。此外，相较于未选育对照组，选育种群中母体性状的演化贡献了68%存活率提升中的一半，以及成年雌蝇体重8%下降的全部幅度。因此本研究结果同时支持上述两项预测，并凸显了非加性效应在适应性微演化中的重要性。