Data from: Desiccation resistance and pigmentation variation reflects bioclimatic differences in the Drosophila americana species complex

Background: Disentangling the selective factors shaping adaptive trait variation is an important but challenging task. Many studies—especially in Drosophila—have documented trait variation along latitudinal or altitudinal clines, but frequently lack resolution about specific environmental gradients that could be causal selective agents, and often do not investigate covariation between traits simultaneously. Here we examined variation in multiple macroecological factors across geographic space and their associations with variation in three physiological traits (desiccation resistance, UV resistance, and pigmentation) at both population and species scales, to address the role of abiotic environment in shaping trait variation. Results: Using environmental data from collection locations of three North American Drosophila species—D. americana americana, D. americana texana and D. novamexicana—we identified two primary axes of macroecological variation; these differentiated species habitats and were strongly loaded for precipitation and moisture variables. In nine focal populations (three per species) assayed for each trait, we detected significant species-level variation for both desiccation resistance and pigmentation, but not for UV resistance. Species-level trait variation was consistent with differential natural selection imposed by variation in habitat water availability, although patterns of variation differed between desiccation resistance and pigmentation, and we found little evidence for pleiotropy between traits. Conclusions: Our multi-faceted approach enabled us to identify potential agents of natural selection and examine how they might influence the evolution of multiple traits at different evolutionary scales. Our findings highlight that environmental factors influence functional trait variation in ways that can be complex, and point to the importance of studies that examine these relationships at both population- and species-levels.

研究背景：解析塑造适应性性状变异的选择因子，是一项兼具重要性与挑战性的科研课题。诸多研究——尤以果蝇（Drosophila）类群为甚——已记录到沿纬度或海拔渐变群分布的性状变异，但此类研究往往无法明确可作为因果选择因子的具体环境梯度，且通常未同时探究性状间的协同变异关系。本研究在种群与物种两个尺度下，探究了地理空间中多种宏观生态因子的变异，以及它们与三种生理性状——抗干燥性、抗紫外线（UV）性与色素沉着——变异的关联，以此解析非生物环境对性状变异的塑造作用。
研究结果：基于3个北美果蝇物种——美洲果蝇美洲亚种（Drosophila americana americana）、美洲果蝇德克萨斯亚种（Drosophila americana texana）以及新墨西哥果蝇（Drosophila novamexicana）——的采集点环境数据，我们识别出宏观生态变异的两个主成分轴；这两个轴可有效区分不同物种的生境，且在降水与湿度变量上具有极高载荷。在针对每个性状检测的9个目标种群（每个物种3个种群）中，我们发现抗干燥性与色素沉着均存在显著的物种水平变异，但抗紫外线性未检测到此类变异。物种水平的性状变异与生境水分可利用性变异带来的差异化自然选择相契合，尽管抗干燥性与色素沉着的变异模式存在差异，且我们未发现性状间存在多效性的显著证据。
研究结论：本研究采用的多维度研究方法，使我们得以识别潜在的自然选择因子，并探究这些因子在不同进化尺度下如何影响多性状的演化。本研究结果表明，环境因子对功能性状变异的影响机制可能极为复杂，同时也凸显了同时在种群与物种尺度下探究此类关联的研究价值。