Environmental clines in organismal defensive traits are usually attributed to stronger selection by enemies at lower latitudes or near the host's range center. Nonetheless, little functional evidence has supported this hypothesis, especially for coevolving plants and herbivores. We quantified cardenolide toxins in seeds of 24 populations of common milkweed (Asclepias syriaca) across 13 degrees of latitude, revealing a pattern of increasing cardenolide concentrations toward the host's range center. The unusual nitrogen-containing cardenolide labriformin was an exception and peaked at higher latitudes. Milkweed seeds are eaten by specialist lygaeid bugs that are even more tolerant of cardenolides than the monarch butterfly, concentrating most cardenolides (but not labriformin) from seeds into their bodies. Accordingly, whether cardenolides defend seeds against these specialist bugs is unclear. We demonstrate that Oncopeltus fasciatus (Lygaeidae) metabolized two major compounds (glycosylated aspecioside and labriformin) into distinct products that were sequestered without impairing growth. We next tested several isolated cardenolides in vitro on the physiological target of cardenolides (Na+/K+-ATPase); there was little variation among compounds in inhibition of an unadapted Na+/K+-ATPase, but tremendous variation in impacts on that of monarchs and Oncopeltus. Labriformin was the most inhibitive compound tested for both insects, but Oncopeltus had the greater advantage over monarchs in tolerating labriformin compared to other compounds. Three metabolized (and stored) cardenolides were less toxic than their parent compounds found in seeds. Our results suggest that a potent plant defense is evolving by natural selection along a geographical cline and targets specialist herbivores, but is met by insect tolerance, detoxification, and sequestration.

中文翻译：

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功能证据支持沿地理斜坡的适应性植物化学防御。

生物防御特征的环境变化通常归因于低纬度或宿主范围中心附近的敌人的更强选择。尽管如此，几乎没有功能性证据支持这一假设，尤其是对于共同进化的植物和食草动物。我们对跨越 13 个纬度的 24 个普通马利筋 (Asclepias syriaca) 种群的种子中的强心内酯毒素进行了量化，揭示了强心内酯浓度向宿主范围中心增加的模式。不寻常的含氮强心内酯 labriformin 是个例外，它在较高纬度地区达到峰值。乳草种子被专门的 lygaeid 虫子吃掉，这些虫子对强心内酯的耐受性比帝王蝶还要高，它们将种子中的大部分强心内酯（但不是 labriformin）浓缩到它们的体内。因此，cardenolides 是否能保护种子免受这些专业错误的侵害尚不清楚。我们证明 Oncopeltus fasciatus (Lygaeidae) 将两种主要化合物（糖基化的 aspecioside 和 labriformin）代谢成不同的产物，这些产物被隔离而不损害生长。接下来，我们在体外对强心内酯的生理靶点（Na+/K+-ATPase）测试了几种分离的强心内酯；化合物在抑制未适应的 Na+/K+-ATPase 方面几乎没有差异，但对君主和 Oncopeltus 的影响却存在巨大差异。Labriformin 是对这两种昆虫测试的最具抑制作用的化合物，但与其他化合物相比，Oncopeltus 在耐受 labriformin 方面比君主具有更大的优势。三种代谢（和储存）的强心内酯的毒性低于其在种子中发现的母体化合物。