Departamento de Botânica, Instituto de Biociências, Universidade de São Paulo, São Paulo, SP, Brazil.
J Evol Biol. 2012 Nov;25(11):2325-40. doi: 10.1111/j.1420-9101.2012.02615.x. Epub 2012 Sep 27.
Much effort has been devoted to understanding the function of extrafloral nectaries (EFNs) for ant-plant-herbivore interactions. However, the pattern of evolution of such structures throughout the history of plant lineages remains unexplored. In this study, we used empirical knowledge on plant defences mediated by ants as a theoretical framework to test specific hypotheses about the adaptive role of EFNs during plant evolution. Emphasis was given to different processes (neutral or adaptive) and factors (habitat change and trade-offs with new trichomes) that may have affected the evolution of ant-plant associations. We measured seven EFN quantitative traits in all 105 species included in a well-supported phylogeny of the tribe Bignonieae (Bignoniaceae) and collected field data on ant-EFN interactions in 32 species. We identified a positive association between ant visitation (a surrogate of ant guarding) and the abundance of EFNs in vegetative plant parts and rejected the hypothesis of phylogenetic conservatism of EFNs, with most traits presenting K-values < 1. Modelling the evolution of EFN traits using maximum likelihood approaches further suggested adaptive evolution, with static-optimum models showing a better fit than purely drift models. In addition, the abundance of EFNs was associated with habitat shifts (with a decrease in the abundance of EFNs from forest to savannas), and a potential trade-off was detected between the abundance of EFNs and estipitate glandular trichomes (i.e. trichomes with sticky secretion). These evolutionary associations suggest divergent selection between species as well as explains K-values < 1. Experimental studies with multiple lineages of forest and savanna taxa may improve our understanding of the role of nectaries in plants. Overall, our results suggest that the evolution of EFNs was likely associated with the adaptive process which probably played an important role in the diversification of this plant group.
人们投入了大量精力来理解植物外分泌蜜腺（EFN）在蚂蚁-植物-食草动物相互作用中的功能。然而，植物谱系历史上这些结构的进化模式仍未被探索。在这项研究中，我们使用了蚂蚁介导的植物防御的经验知识作为理论框架，以测试关于 EFN 在植物进化过程中的适应性作用的具体假设。重点放在可能影响蚂蚁-植物共生关系进化的不同过程（中性或适应性）和因素（栖息地变化和与新刚毛的权衡）上。我们在 Bignonieae 族（紫葳科）的一个支持良好的系统发育中测量了所有 105 种植物的 7 种 EFN 定量特征，并在 32 种植物中收集了有关蚂蚁-EFN 相互作用的实地数据。我们发现蚂蚁访问量（蚂蚁保护的替代物）与营养植物部分的 EFN 丰度之间存在正相关关系，并拒绝了 EFN 系统发育保守性的假设，大多数特征的 K 值<1。使用最大似然方法对 EFN 特征的进化进行建模进一步表明了适应性进化，静态最优模型比纯漂移模型具有更好的拟合度。此外，EFN 的丰度与栖息地变化（从森林到稀树草原的 EFN 丰度降低）相关，并且在 EFN 的丰度和有柄腺毛（即具有粘性分泌物的毛）之间检测到潜在的权衡。这些进化关联表明种间存在分歧选择，并且可以解释 K 值<1。对森林和稀树草原类群的多个谱系进行实验研究可能会提高我们对蜜腺在植物中的作用的理解。总的来说，我们的结果表明，EFN 的进化可能与适应性过程有关，这可能在这个植物群的多样化中发挥了重要作用。
