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.
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Common milkweed seeds exhibit latitudinal clines more consistent with adaptation to growing season length than temperature
Overwintering monarch (Danaus plexippus) populations have declined since the 1990s. In response, restoration of milkweeds, including Asclepias syriaca (common milkweed), an important host plant in their breeding grounds, has become increasingly common. However, latitudinal variation in milkweed populations suggests the possibility of regional adaptation and the potential for seed provenance to affect restoration success. Using seeds from 20 populations throughout the range of A. syriaca, we tested whether seed mass, germination success, and germination time in the greenhouse demonstrate geographic clines consistent with available evidence for this species from other studies. In addition, we tested for patterns in germination traits consistent with adaptation to spring thermal conditions by planting seeds from 10 populations in growth chambers simulating Minnesota and Kentucky spring temperatures. Even after accounting for seed mass, seeds from higher latitudes germinated faster on average under all conditions. Elevated temperatures accelerated germination time and leaf development time; however, we did not detect geographic patterns in leaf development time, indicating that the processes underlying the latitudinal cline in germination time may be unique to the germination stage. In the thermal adaptation study, high-latitude populations produced larger seeds and seeds that germinated at a higher rate; however, neither latitudinal trend was observed in the geographic clines study, even though individual seed mass predicted germination success. High-latitude populations express more favorable germination traits in every setting measured, perhaps due to reduced dormancy. Consequently, we conclude that latitudinal clines are more consistent with adaptation to growing season length than to spring temperatures.
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- Award ID(s):
- 1936621
- NSF-PAR ID:
- 10399664
- Date Published:
- Journal Name:
- Restoration Ecology
- ISSN:
- 1061-2971
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Abstract As a general rule, plants defend against herbivores with multiple traits. The defense synergy hypothesis posits that some traits are more effective when co‐expressed with others compared to their independent efficacy. However, this hypothesis has rarely been tested outside of phytochemical mixtures, and seldom under field conditions. We tested for synergies between multiple defense traits of common milkweed (
Asclepias syriaca ) by assaying the performance of two specialist chewing herbivores on plants in natural populations. We employed regression and a novel application of random forests to identify synergies and antagonisms between defense traits. We found the first direct empirical evidence for two previously hypothesized defense synergies in milkweed (latex by secondary metabolites, latex by trichomes) and identified numerous other potential synergies and antagonisms. Our strongest evidence for a defense synergy was between leaf mass per area and low nitrogen content; given that these “leaf economic” traits typically covary in milkweed, a defense synergy could reinforce their co‐expression. We report that each of the plant defense traits showed context‐dependent effects on herbivores, and increased trait expression could well be beneficial to herbivores for some ranges of observed expression. The novel methods and findings presented here complement more mechanistic approaches to the study of plant defense diversity and provide some of the best evidence to date that multiple classes of plant defense synergize in their impact on insects. Plant defense synergies against highly specialized herbivores, as shown here, are consistent with ongoing reciprocal evolution between these antagonists. -
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Premise Leaf economic spectrum (LES) theory has historically been employed to inform vegetation models of ecosystem processes, but largely neglects intraspecific variation and biotic interactions. We attempt to integrate across environment–plant trait–herbivore interactions within a species at a range‐wide scale.
Methods We measured traits in 53 populations spanning the range of common milkweed (
Asclepias syriaca ) and used a common garden to determine the role of environment in driving patterns of intraspecific variation. We used a feeding trial to determine the role of plant traits in monarch (Danaus plexippus ) larval development.Results Trait–trait relationships largely followed interspecific patterns in LES theory and persisted in a common garden when individual traits change. Common milkweed showed intraspecific variation and biogeographic clines in traits. Clines did not persist in a common garden. Larvae ate more and grew larger when fed plants with more nitrogen. A longitudinal environmental gradient in precipitation corresponded to a resource gradient in plant nitrogen, which produces a gradient in larval performance.
Conclusions Biogeographic patterns in common milkweed traits can sometimes be predicted from LES, are largely driven by environmental conditions, and have consequences for monarch larval performance. Changes to nutrient dynamics of landscapes with common milkweed could potentially influence monarch population dynamics. We show how biogeographic trends in intraspecific variation can influence key ecological interactions, especially in common species with large distributions.
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Abstract Urbanization is altering landscapes globally at an unprecedented rate. While ecological differences between urban and rural environments often promote phenotypic divergence among populations, it is unclear to what degree these trait differences arise from genetic divergence as opposed to phenotypic plasticity. Furthermore, little is known about how specific landscape elements, such as green corridors, impact genetic divergence in urban environments. We tested the hypotheses that: (1) urbanization, and (2) proximity to an urban green corridor influence genetic divergence in common milkweed (
Asclepias syriaca ) populations for phenotypic traits. Using seeds from 52 populations along three urban-to-rural subtransects in the Greater Toronto Area, Canada, one of which followed a green corridor, we grew ~ 1000 plants in a common garden setup and measured > 20 ecologically-important traits associated with plant defense/damage, reproduction, and growth over four years. We found significant heritable variation for nine traits within common milkweed populations and weak phenotypic divergence among populations. However, neither urbanization nor an urban green corridor influenced genetic divergence in individual traits or multivariate phenotype. These findings contrast with the expanding literature demonstrating that urbanization promotes rapid evolutionary change and offer preliminary insights into the eco-evolutionary role of green corridors in urban environments.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1111/rec.13878
