Department of Ecology and Evolution, University of Lausanne-Biophore, Switzerland.
Am Nat. 2011 Jun;177(6):728-37. doi: 10.1086/659948.
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.
专业化在昆虫食草动物的大多数谱系中很常见，而昆虫食草动物是地球上最多样化的生物群体之一。为了解决专业化如何以及为何在进化过程中得以维持的问题，我们假设植物防御和潜在宿主植物的其他生态特征将预测专食性根食性草食动物（红色乳草象甲，Tetraopes tetraophthalmus）的表现。使用比较系统发育和功能性状方法，我们评估了 18 种 Asclepias 物种中昆虫宿主范围的决定因素。幼虫存活率随与真正宿主 Asclepias syriaca 的系统发育距离的增加而降低，这表明对植物特征的适应导致了专业化。在所测量的几种根性状中，只有强心苷（有毒防御化学物质）与幼虫存活率相关，并且在系统发育控制的多元回归分析中，强心苷也解释了系统发育距离效应。此外，与其他 Tetraopes 象甲有已知关联的乳草物种比缺乏 Tetraopes 食草动物的物种更好的宿主，并且与 A. syriaca 相似的比具有发散值的物种具有特定叶面积值（与叶功能和栖息地归属相关的性状）更好的宿主。因此，我们得出结论，系统发育距离是 Asclepias 物种表型和生态特征的综合指标，尤其是防御性强心苷，可用于解释专业化和宿主在进化过程中转变的限制。
