Balancing selection is frequently invoked as a mechanism that maintains variation within and across populations. However, there are few examples of balancing selection operating on loci underpinning complex traits, which frequently display high levels of variation. We investigated mechanisms that may maintain variation in a focal polymorphism—leaf chemical profiles of a perennial wildflower (Boechera stricta, Brassicaceae)—explicitly interrogating multiple ecological and genetic processes including spatial variation in selection, antagonistic pleiotropy and frequency-dependent selection. A suite of common garden and greenhouse experiments showed that the alleles underlying variation in chemical profile have contrasting fitness effects across environments, implicating two ecological drivers of selection on chemical profile: herbivory and drought. Phenotype–environment associations and molecular genetic analyses revealed additional evidence of past selection by these drivers. Together, these data are consistent with balancing selection on chemical profile, probably caused by pleiotropic effects of secondary chemical biosynthesis genes on herbivore defence and drought response.

平衡选择经常被用作维持群体内部和群体之间差异的机制。然而，很少有对支持复杂性状的基因座进行平衡选择的例子，这些基因座经常表现出高水平的变异。我们研究了可能维持多年生野花（ Boechera stricta ，十字花科）叶子化学特征的焦点多态性变异的机制，明确询问了多种生态和遗传过程，包括选择的空间变异、拮抗性多效性和频率依赖性选择。一系列常见的花园和温室实验表明，化学谱变化背后的等位基因在不同环境中具有截然不同的适应性效应，这暗示了化学谱选择的两个生态驱动因素：食草和干旱。表型-环境关联和分子遗传学分析揭示了这些驱动因素过去选择的更多证据。总之，这些数据与化学谱的平衡选择一致，这可能是由次级化学生物合成基因对草食动物防御和干旱反应的多效性影响引起的。