Abstract
The adaptation of monarch butterflies (Danaus plexippus) to milkweed plants and their ability to sequester toxic cardenolides is a model system for plant-herbivore coevolution. However, the physiological mechanisms underlying cardenolide sequestration and its temporal dynamics remain largely unknown. Here, we show that the polar cardenolide ouabain passes through the isolated midgut epithelium of D. plexippus in vitro and is also absorbed into the body cavity of monarch caterpillars. Remarkably, the same pattern was observed in caterpillars of the related, but non-sequestering milkweed butterfly Euploea core, and even in the non-adapted Solanaceae specialist Manduca sexta, although uptake across gut epithelia occurred at a lower rate. Furthermore, we demonstrated that cardenolides begin to cross the epithelium in the anterior part of the intestine and can be detected in body tissues as soon as one minute after ingestion. Finally, we show that not all cardenolides are translocated into butterfly tissues during metamorphosis, and that the most apolar cardenolides are removed with the last caterpillar exuviae. As a result, adult butterflies contain no cardenolides less polar than the milkweed cardenolide calactin. We conclude that uptake by the intestinal epithelium is a very rapid process and that quantitative differences in cardenolide sequestration among lepidopteran caterpillars are only partially mediated by the gut epithelium, likely involving additional mechanisms such as metabolism or excretion. In addition, the translocation of cardenolides from the caterpillar is a selective process which may be due to the limited mobility of highly apolar cardenolides.