Functional evidence supports adaptive plant chemical defense along a geographical cline

Abstract

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.

Population-specific patterns of toxin sequestration in monarch butterflies from around the world

“…The most likely explanation for the Puerto Rican population's reduced sequestration on A. syriaca and A. speciosa is a lack of evolutionary history with these hosts (Figure 1B). Divergence times between Puerto Rican monarchs and their migratory North American ancestors are uncertain but likely occurred within the last 20,000 years Under this scenario, Puerto Rican monarchs may have never evolved mechanisms to efficiently sequester a subset of distinctive cardenolide compounds (e.g., labriformin) present in widespread temperate North American milkweeds, including A. syriaca, A. speciosa, and A. eriocarpa (Nelson et al 1981, Agrawal et al 2022). Further research with additional monarch populations from the Caribbean and South America and/or additional North American milkweed species could help to resolve this question.…”

“…Recent evidence suggests that A. syriaca has undergone significant demographic expansions coinciding with post-glacial expansion (5-12 thousand years ago) and agricultural land use changes in North America (100-250 years ago) (Boyle et al 2022). Under this scenario, Puerto Rican monarchs may have never evolved mechanisms to efficiently sequester a subset of distinctive cardenolide compounds (e.g., labriformin) present in widespread temperate North American milkweeds, including A. syriaca , A. speciosa , and A. eriocarpa (Nelson et al 1981, Agrawal et al 2022). Further research with additional monarch populations from the Caribbean and South America and/or additional North American milkweed species could help to resolve this question.…”

“…We note that some of these peaks are likely constituent fragments of larger, more intact cardenolides; for example, the widespread cardenolides calotropin, calotoxin, calactin, and uscharin share a common aglycone precursor (calotropogenin). In order to verify the identity of some major sequestered compounds, we tested authentic standards for the compounds calactin, calotropin, and frugoside—reported to be the three major compounds sequestered from A. curassavica (Agrawal et al 2021)—as well as aspecioside, reported to be a major sequestered compound from A. syriaca (Seiber et al 1986; Malcolm et al 1989; Agrawal et al 2022) (Table S3). Authentic standards were provided by A. Agrawal and are the same as those used in Agrawal et al (2021).…”

Profile of Anurag A. Agrawal

“…In Agrawal’s Inaugural Article ( 2 ), he explores the evolutionary interaction between common milkweeds and large milkweed bugs, using monarch butterflies as a point of comparison. He also explores geographical patterns in milkweed defense by sampling common milkweed populations across almost 13° of latitude, from Quebec City, Canada, to Bishop, North Carolina.…”

“…Agrawal, who was elected to the National Academy of Sciences in 2021, is currently the James Perkins Professor of Environmental Studies at Cornell University in Ithaca, New York. In his Inaugural Article ( 2 ), Agrawal explores the dynamics of an evolutionary arms race between common milkweeds ( Asclepias syriaca ) and large milkweed bugs ( Oncopeltus fasciatus ).…”