Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York, 14853, USA.
Department of Entomology, Cornell University, Ithaca, New York, 14853, USA.
Am J Bot. 2018 Apr;105(4):677-686. doi: 10.1002/ajb2.1057. Epub 2018 Apr 23.
Pachypodium (Apocynaceae) is a genus of iconic stem-succulent and poisonous plants endemic to Madagascar and southern Africa. We tested hypotheses about the mode of action and macroevolution of toxicity in this group. We further hypothesized that while monarch butterflies are highly resistant to cardenolide toxins (a type of cardiac glycoside) from American Asclepias, they may be negatively affected by Pachypodium defenses, which evolved independently.
We grew 16 of 21 known Pachypodium spp. and quantified putative cardenolides by HPLC and also by inhibition of animal Na /K -ATPase (the physiological target of cardiac glycosides) using an in vitro assay. Pachypodium extracts were tested against monarch caterpillars in a feeding bioassay. We also tested four Asclepias spp. and five Pachypodium spp. extracts, contrasting inhibition of the cardenolide-sensitive porcine Na /K -ATPase to the monarch's resistant form.
We found evidence for low cardenolides by HPLC, but substantial toxicity when extracts were assayed on Na /K -ATPases. Toxicity showed phylogenetic signal, and taller species showed greater toxicity (this was marginal after phylogenetic correction). Application of Pachypodium extracts to milkweed leaves reduced monarch growth, and this was predicted by inhibition of the sensitive Na /K -ATPase in phylogenetic analyses. Asclepias extracts were 100-fold less potent against the monarch compared to the porcine Na /K -ATPase, but this difference was absent for Pachypodium extracts.
Pachypodium contains potent toxicity capable of inhibiting sensitive and cardenolide-adapted Na /K -ATPases. Given the monarch's sensitivity to Pachypodium, we suggest that these plants contain novel cardiac glycosides or other compounds that facilitate toxicity by binding to Na /K -ATPases.
帕奇潘迪乌姆（夹竹桃科）是一个标志性的肉质茎植物属，原产于马达加斯加和南非。我们测试了关于该组毒性作用方式和宏观进化的假设。我们进一步假设，虽然帝王蝶对来自美洲马利筋的强心苷毒素（一种心脏糖苷）具有高度抗性，但它们可能会受到独立进化的帕奇潘迪乌姆防御的负面影响。
我们种植了 21 种已知的帕奇潘迪乌姆属中的 16 种，并通过 HPLC 和体外测定动物 Na+/K+-ATP 酶（心脏糖苷的生理靶标）的抑制作用来定量测定推定的强心苷。在喂养生物测定中，我们还测试了帕奇潘迪乌姆提取物对帝王蝶幼虫的作用。我们还测试了四种马利筋属和五种帕奇潘迪乌姆属提取物，对比了对心脏糖苷敏感的猪 Na+/K+-ATP 酶的抑制作用与帝王蝶抗性形式的抑制作用。
我们通过 HPLC 发现了低浓度的强心苷，但在 Na+/K+-ATP 酶测定中发现了大量的毒性。毒性表现出系统发育信号，并且更高的物种显示出更大的毒性（在系统发育校正后，这是边缘的）。帕奇潘迪乌姆提取物在马利筋叶片上的应用降低了帝王蝶的生长，这在系统发育分析中通过抑制敏感的 Na+/K+-ATP 酶来预测。与猪 Na+/K+-ATP 酶相比，马利筋提取物对帝王蝶的效力低 100 倍，但对于帕奇潘迪乌姆提取物，这种差异不存在。
帕奇潘迪乌姆含有能够抑制敏感和强心苷适应的 Na+/K+-ATP 酶的强烈毒性。鉴于帝王蝶对帕奇潘迪乌姆的敏感性，我们认为这些植物含有新的心脏糖苷或其他化合物，通过与 Na+/K+-ATP 酶结合促进毒性。
