Search for: All records

Multiple hypotheses have been put forth to understand why defense chemistry in individual plants is so diverse. A major challenge has been teasing apart the importance of concentration vs. composition of defense compounds and resolving the mechanisms of diversity effects that determine plant resistance against herbivores. Accordingly, we first outline nonexclusive mechanisms by which phytochemical diversity may increase toxicity of a mixture compared to the average effect of each compound alone. We then leveraged independent in vitro, in vivo transgenic, and organismal experiments to test the effect of equimolar concentrations of purified milkweed toxins in isolation vs. mixtures on the specialist and sequestering monarch butterfly. We show that cardenolide toxin mixtures from milkweed plants enhance resistance against this herbivore compared to equal concentrations of single compounds. In mixtures, highly potent toxins dominated the inhibition of the monarch’s target enzyme (Na⁺/K⁺-ATPase) in vitro, revealing toxin-specific affinity for the adapted enzyme in the absence of other physiological adaptations of the monarch. Mixtures also caused increased mortality in CRISPR-edited adultDrosophila melanogasterwith the monarch enzyme in vivo, whereas wild-type flies showed lower survival regardless of mixture type. Finally, although experimentally administered mixtures were not more toxic to monarch caterpillars than single compounds overall, increasing caterpillar sequestration from mixtures resulted in an increasing burden for growth compared to single compounds. Phytochemical diversity likely provides an economical plant defense by acting on multiple aspects of herbivore physiology and may be particularly effective against sequestering specialist herbivores. 

In coevolution between plants and insects, reciprocal selection often leads to phenotype matching between chemical defense and herbivore offense. Nonetheless, it is not well understood whether distinct plant parts are differentially defended and how herbivores adapted to those parts cope with tissue-specific defense. Milkweed plants produce a diversity of cardenolide toxins and specialist herbivores have substitutions in their target enzyme (Na + /K + –ATPase), each playing a central role in milkweed–insect coevolution. The four-eyed milkweed beetle ( Tetraopes tetrophthalmus ) is an abundant toxin-sequestering herbivore that feeds exclusively on milkweed roots as larvae and less so on milkweed leaves as adults. Accordingly, we tested the tolerance of this beetle’s Na + /K + –ATPase to cardenolide extracts from roots versus leaves of its main host ( Asclepias syriaca ), along with sequestered cardenolides from beetle tissues. We additionally purified and tested the inhibitory activity of dominant cardenolides from roots (syrioside) and leaves (glycosylated aspecioside). Tetraopes’ enzyme was threefold more tolerant of root extracts and syrioside than leaf cardenolides. Nonetheless, beetle-sequestered cardenolides were more potent than those in roots, suggesting selective uptake or dependence on compartmentalization of toxins away from the beetle’s enzymatic target. Because Tetraopes has two functionally validated amino acid substitutions in its Na + /K + –ATPase compared to the ancestral form in other insects, we compared its cardenolide tolerance to that of wild-type Drosophila and CRISPR-edited Drosophila with Tetraopes ’ Na + /K + –ATPase genotype. Those two amino acid substitutions accounted for >50% of Tetraopes’ enhanced enzymatic tolerance of cardenolides. Thus, milkweed’s tissue-specific expression of root toxins is matched by physiological adaptations in its specialist root herbivore. 

Plant secondary metabolites that defend leaves from herbivores also occur in floral nectar. While specialist herbivores often have adaptations providing resistance to these compounds in leaves, many social insect pollinators are generalists, and therefore are not expected to be as resistant to such compounds. The milkweeds, Asclepias spp., contain toxic cardenolides in all tissues including floral nectar. We compared the concentrations and identities of cardenolides between tissues of the North American common milkweed Asclepias syriaca, and then studied the effect of the predominant cardenolide in nectar, glycosylated aspecioside, on an abundant pollinator. We show that a generalist bumblebee, Bombus impatiens, a common pollinator in eastern North America, consumes less nectar with experimental addition of ouabain (a standard cardenolide derived from Apocynacid plants native to east Africa) but not with addition of glycosylated aspecioside from milkweeds. At a concentration matching that of the maximum in the natural range, both cardenolides reduced activity levels of bees after four days of consumption, demonstrating toxicity despite variation in behavioral deterrence (i.e., consumption). In vitro enzymatic assays of Na+/K+-ATPase, the target site of cardenolides, showed lower toxicity of the milkweed cardenolide than ouabain for B. impatiens, indicating that the lower deterrence may be due to greater tolerance to glycosylated aspecioside. In contrast, there was no difference between the two cardenolides in toxicity to the Na+/K+-ATPase from a control insect, the fruit fly Drosophila melanogaster. Accordingly, this work reveals that even generalist pollinators such as B. impatiens may have adaptations to reduce the toxicity of specific plant secondary metabolites that occur in nectar, despite visiting flowers from a wide variety of plants over the colony’s lifespan. 

Abstract Herbivores that sequester toxins are thought to have cracked the code of plant defences. Nonetheless, coevolutionary theory predicts that plants should evolve toxic variants that also negatively impact specialists. We propose and test the selective sequestration hypothesis, that specialists preferentially sequester compounds that are less toxic to themselves while maintaining toxicity to enemies. Using chemically distinct plants, we show that monarch butterflies sequester only a subset of cardenolides from milkweed leaves that are less potent against their target enzyme (Na⁺/K⁺‐ATPase) compared to several dominant cardenolides from leaves. However, sequestered compounds remain highly potent against sensitive Na⁺/K⁺‐ATPases found in most predators. We confirmed this differential toxicity with mixtures of purified cardenolides from leaves and butterflies. The genetic basis of monarch adaptation to sequestered cardenolides was also confirmed with transgenicDrosophilathat were CRISPR‐edited with the monarch's Na⁺/K⁺‐ATPase. Thus, the monarch's selective sequestration appears to reduce self‐harm while maintaining protection from enemies. 

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 Oncopeltu s. 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. 

Dormancy has repeatedly evolved in plants, animals, and microbes and is hypothesized to facilitate persistence in the face of environmental change. Yet previous experiments have not tracked demography and trait evolution spanning a full successional cycle to ask whether early bouts of natural selection are later reinforced or erased during periods of population dormancy. In addition, it is unclear how well short-term measures of fitness predict long-term genotypic success for species with dormancy. Here, we address these issues using experimental field populations of the plantOenothera biennis, which evolved over five generations in plots exposed to or protected from insect herbivory. While populations existed above ground, there was rapid evolution of defensive and life-history traits, but populations lost genetic diversity and crashed as succession proceeded. After >5 y of seed dormancy, we triggered germination from the seedbank and genotyped >3,000 colonizers. Resurrected populations showed restored genetic diversity that reduced earlier responses to selection and pushed population phenotypes toward the starting conditions of a decade earlier. Nonetheless, four defense and life-history traits remained differentiated in populations with insect suppression compared with controls. These findings capture key missing elements of evolution during ecological cycles and demonstrate the impact of dormancy on future evolutionary responses to environmental change.