Data from: Pollinator adaptation and the evolution of floral nectar sugar composition

A longstanding debate concerns whether nectar sugar composition evolves as an adaptation to pollinator dietary requirements or whether it is 'phylogenetically constrained'. Here we use a modeling approach to evaluate the hypothesis that nectar sucrose proportion (NSP) is an adaptation to pollinators. We analyze ~2,100 species of asterids, spanning several plant families and pollinator groups (PGs), and show that the hypothesis of adaptation cannot be rejected: NSP evolves toward two optimal values, high NSP for specialist-pollinated and low NSP for generalist-pollinated plants. However, the inferred adaptive process is weak, suggesting that adaptation to PG only provides a partial explanation for how nectar evolves. Additional factors are therefore needed to fully explain nectar evolution and we suggest that future studies might incorporate floral shape and size and the abiotic environment into the analytical framework. Further, we show that NSP and PG evolution are correlated – in a manner dictated by pollinator behavior. This contrasts with the view that a plant necessarily has to adapt its nectar composition to ensure pollination but rather suggests that pollinators adapt their foraging behavior or dietary requirements to the nectar sugar composition presented by the plants. Finally, we document unexpectedly sucrose-poor nectar in some specialized nectarivorous bird-pollinated plants from the Old World, which might represent an overlooked form of pollinator deception. Thus, our broad study provides several new insights into how nectar evolves and we conclude by discussing why maintaining the conceptual dichotomy between adaptation and constraint might be unhelpful for advancing this field.

长期以来学界存在一项核心争论：花蜜糖组成的演化究竟是作为对传粉者饮食需求的适应性响应，还是仅受系统发育约束（phylogenetically constrained）。本研究通过建模方法，评估「花蜜蔗糖比例（NSP）的演化是对传粉者的适应性响应」这一假说。我们分析了覆盖多个植物科与传粉者类群（PGs）的约2100种菊超目（asterids）植物，结果显示无法推翻适应性演化假说：花蜜蔗糖比例（NSP）会朝着两个最优值演化——专性传粉植物对应高比例NSP，泛化传粉植物对应低比例NSP。但本次研究推断出的适应性演化过程强度较弱，表明针对传粉者类群的适应性仅能部分解释花蜜的演化机制。因此需引入额外因素方可完整阐释花蜜的演化规律，我们建议未来研究可将花的形态、大小及非生物环境纳入分析框架。此外，我们发现NSP与传粉者类群的演化存在相关性，且这种相关性由传粉者的行为所决定。这与「植物必须调整花蜜组成以保障传粉顺利」的传统观点相悖，反而表明传粉者会根据植物提供的花蜜糖组成，调整自身的觅食行为或饮食需求。最后，我们在部分来自旧大陆的专性食蜜鸟类传粉植物中，发现了蔗糖含量极低的花蜜，这或许代表了一种此前被忽视的传粉者欺骗策略。综上，这项覆盖范围广泛的研究为花蜜演化机制提供了多项新见解；最后我们还讨论了，为何保留「适应性演化与约束」这一概念二分法，或许不利于该领域的后续发展。