Data from: Evolution of specialization: a phylogenetic study of host range in the red milkweed beetle (Tetraopes tetraophthalmus)

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, A. 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 similar specific leaf area values (a trait related to leaf function and habitat affiliation) to 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.

植食性昆虫（insect herbivores）是地球上物种多样性最高的生物类群之一，宿主特化现象在其绝大多数演化支中极为普遍。为探究宿主特化在演化时间尺度上得以维持的机制与成因，本研究提出假说：潜在宿主植物的防御特性与其他生态属性，可预测专性取食根部的植食性昆虫——红乳草甲虫（red milkweed beetle, *Tetraopes tetraophthalmus*）的生存表现。本研究采用比较系统发育与功能性状分析方法，对18种马利筋属（*Asclepias*）植物的昆虫宿主范围决定因子展开评估。结果显示，幼虫存活率随与真正宿主叙利亚马利筋（*A. syriaca*）的系统发育距离增加而下降，这表明针对植物性状的适应性演化是驱动宿主特化形成的核心因素。在测定的多项根部性状中，仅强心苷（cardenolides，有毒防御化学物质）与幼虫存活率呈显著相关；且在系统发育控制的多元回归分析中，强心苷同样解释了系统发育距离的效应机制。此外，与其他红乳草甲虫存在已知取食关联的马利筋物种，其作为宿主的质量显著高于未被该属甲虫取食的物种；而比叶面积（specific leaf area，与叶片功能及生境归属相关的功能性状）与叙利亚马利筋相近的马利筋物种，其宿主质量也显著高于比叶面积差异较大的物种。综上，本研究认为系统发育距离可作为马利筋属物种表型与生态属性的综合衡量指标，尤其是其防御性强心苷组分，该指标可用于解释演化时间尺度上的宿主特化现象以及宿主转移所受的演化限制。