Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, 14853, USA.
Department of Entomology, Cornell University, Ithaca, New York, 14853, USA.
Ecology. 2019 Dec;100(12):e02857. doi: 10.1002/ecy.2857. Epub 2019 Jul 31.
Inducible defense is a common form of phenotypic plasticity, and inducibility (change in defense after herbivore attack) has long been predicted to trade off with constitutive (or baseline) defense to manage resource allocation. Although such trade-offs likely constrain evolution within species, the extent to which they influence divergence among species is unresolved. We studied cardenolide toxins among genetic families in eight North American Asclepias species, spanning the full range of defense in the genus. Using common environment experiments and chemical assays, we report a consistent trade-off (negative genetic correlation) between concentrations of constitutive cardenolides and their inducibility within each species. However, no trade-off was found in a phylogenetic analysis across species. To investigate factors driving differences in defense allocation among species we used latitude as a proxy for growing season and herbivore pressure and found that divergence into lower latitudes resulted in evolution of higher cardenolides overall. Next we used an enzymatic assay of the cellular target of cardenolides (sodium-potassium ATPase) and confirm that higher cardenolides resulted in stronger toxicity to a sensitive species, but not to specialized monarch butterflies. Thus, plant speciation into biogeographic regions with alternative resources or pest pressure resulted in the macroevolution of cardenolide defense, especially against unspecialized herbivores. Nonetheless, trade-offs persist in the extent to which this defense is allocated constitutively or is inducible among genotypes within each species.
诱导防御是表型可塑性的一种常见形式，长期以来，诱导防御（食草动物攻击后防御的变化）被预测与组成型（或基线）防御之间存在权衡，以管理资源分配。尽管这种权衡可能限制了物种内部的进化，但它们在多大程度上影响了物种之间的分化还没有解决。我们研究了北美 8 种马兜铃属植物中遗传家族的卡地醇毒素，涵盖了该属防御的全部范围。通过共同环境实验和化学分析，我们报告了在每个物种中组成型卡地醇毒素的浓度与其诱导性之间存在一致的权衡（负遗传相关）。然而，在跨物种的系统发育分析中没有发现权衡。为了研究驱动物种间防御分配差异的因素，我们使用纬度作为生长季节和食草动物压力的替代物，并发现向低纬度的分化导致了整体上更高的卡地醇毒素的进化。接下来，我们使用卡地醇毒素的细胞靶标（钠钾 ATP 酶）的酶分析证实，更高的卡地醇毒素导致对敏感物种的毒性更强，但对专门的帝王蝶没有影响。因此，植物物种形成到具有替代资源或害虫压力的生物地理区域导致了卡地醇防御的宏观进化，特别是针对非特化的食草动物。尽管如此，在每个物种中，这种防御是组成型分配还是诱导型分配的程度仍然存在权衡。
