Data from: Evidence for the adaptive significance of secondary compounds in vertebrate-dispersed fruits

Although the primary function of fleshy fruits is to attract seed dispersers, many ripe fruits contain toxic secondary compounds. A number of hypotheses have been proposed to explain this evolutionary paradox, most of which describe the potential adaptive role secondary compounds may play in seed dispersal. However, some authors have argued that fruit secondary compounds may be non-adaptive, and instead explain their occurrence as a pleiotropic consequence of selection for defense of leaves and other tissues. We address these alternative evolutionary hypotheses through a comparative examination of iridoid glycosides in leaves, unripe fruits, and ripe fruits of Lonicera x bella (Belle's bush honeysuckle), combined with an examination of fruit damage and removal in natural populations. We provide several lines of evidence that fruit secondary compounds cannot be explained solely as a consequence of foliar defense, including higher concentrations and more individual compounds in fruits compared to leaves and a negative relationship between iridoid glycoside concentration and fruit damage. However, we also show that the composition and concentrations of secondary compounds in leaves and fruits are not entirely independent, emphasizing that selection in different plant parts is intrinsically linked. We conclude that the adaptive significance of chemical traits is best considered in a whole-plant context that includes fruit-frugivore interactions.

尽管肉质果实的核心功能是吸引种子传播者，但多数成熟果实中却含有有毒的次生代谢物（secondary compounds）。学界已提出多种假说以解释这一进化悖论，其中多数假说阐述了次生代谢物在种子传播过程中可能发挥的适应性作用。然而，部分学者认为果实中的次生代谢物并非适应性演化的结果，而是叶片及其他组织防御机制选择所带来的多效性副产物。本研究通过对贝氏忍冬（Lonicera x bella，Belle's bush honeysuckle）的叶片、未成熟果实及成熟果实中的环烯醚萜苷（iridoid glycosides）进行比较分析，并结合对自然种群中果实受损与被取食情况的调查，对这两种对立的进化假说展开验证。本研究提供多维度证据表明，果实中的次生代谢物不能仅以叶片防御的副产物来解释：相较于叶片，果实中的次生代谢物浓度更高、单体种类更多，且环烯醚萜苷浓度与果实受损程度呈负相关关系。然而本研究同时发现，叶片与果实中的次生代谢物组成及浓度并非完全独立，这说明不同植物器官所受的选择压力存在内在关联。综上，我们认为在探讨化学性状的适应性意义时，应将其置于包含果实-食果动物互作在内的全植株演化框架中进行考量。