Evolution of Specialization: A Phylogenetic Study of Host Range in the Red Milkweed Beetle (Tetraopes tetraophthalmus)

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Specialization is common in most lineages of insect herbivores, one of the most diverse groups of organisms on earth. To address how and why specialization is maintained over evolutionary time, we hypothesized that plant defense and other ecological attributes of potential host plants would predict the performance of a specialist root-feeding herbivore (the red milkweed beetle, Tetraopes tetraophthalmus). Using a comparative phylogenetic and functional trait approach, we assessed the determinants of insect host range across 18 species of Asclepias. Larval survivorship decreased with increasing phylogenetic distance from the true host, Asclepias syriaca, suggesting that adaptation to plant traits drives specialization. Among several root traits measured, only cardenolides (toxic defense chemicals) correlated with larval survival, and cardenolides also explained the phylogenetic distance effect in phylogenetically controlled multiple regression analyses. Additionally, milkweed species having a known association with other Tetraopes beetles were better hosts than species lacking Tetraopes herbivores, and milkweeds with specific leaf area values (a trait related to leaf function and habitat affiliation) similar to those of A. syriaca were better hosts than species having divergent values. We thus conclude that phylogenetic distance is an integrated measure of phenotypic and ecological attributes of Asclepias species, especially defensive cardenolides, which can be used to explain specialization and constraints on host shifts over evolutionary time.

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The American Naturalist

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Chemical analyses were conducted in the Cornell Chemical Ecology Core Facility, with support from P. Feeny, R. Halitschke, A. Kessler, New Life Sciences Initiative, College of Agriculture and Life Sciences, Center for a Sustainable Future, Boyce Thompson Institute, and Departments of Ecology and Evolutionary Biology, Neurobiology and Behavior, Entomology, Plant Biology, and Horticulture. Our research and laboratory (http:// are supported by National Science Foundation grants DEB-0447550 and DEB-0950231 to A.A.A. and a postdoctoral fellowship from the Swiss National Science Foundation (PA0033-121483 to S.R.).

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University of Chicago Press


chemical ecology; host range evolution; phylogenetic GLS; insect herbivore; plant defense; root herbivory


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Rasmann, S., & Agrawal, A. A. (2011). Evolution of Specialization: A Phylogenetic Study of Host Range in the Red Milkweed Beetle (Tetraopes tetraophthalmus). The American Naturalist, 177(6), 728–737.

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