Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14850, USA.
Ann Bot. 2013 Jun;111(6):1251-61. doi: 10.1093/aob/mcs225. Epub 2012 Oct 18.
Understanding the evolutionary patterns of ecologically relevant traits is a central goal in plant biology. However, for most important traits, we lack the comprehensive understanding of their taxonomic distribution needed to evaluate their evolutionary mode and tempo across the tree of life. Here we evaluate the broad phylogenetic patterns of a common plant-defence trait found across vascular plants: extrafloral nectaries (EFNs), plant glands that secrete nectar and are located outside the flower. EFNs typically defend plants indirectly by attracting invertebrate predators who reduce herbivory.
Records of EFNs published over the last 135 years were compiled. After accounting for changes in taxonomy, phylogenetic comparative methods were used to evaluate patterns of EFN evolution, using a phylogeny of over 55 000 species of vascular plants. Using comparisons of parametric and non-parametric models, the true number of species with EFNs likely to exist beyond the current list was estimated.
To date, EFNs have been reported in 3941 species representing 745 genera in 108 families, about 1-2 % of vascular plant species and approx. 21 % of families. They are found in 33 of 65 angiosperm orders. Foliar nectaries are known in four of 36 fern families. Extrafloral nectaries are unknown in early angiosperms, magnoliids and gymnosperms. They occur throughout monocotyledons, yet most EFNs are found within eudicots, with the bulk of species with EFNs being rosids. Phylogenetic analyses strongly support the repeated gain and loss of EFNs across plant clades, especially in more derived dicot families, and suggest that EFNs are found in a minimum of 457 independent lineages. However, model selection methods estimate that the number of unreported cases of EFNs may be as high as the number of species already reported.
EFNs are widespread and evolutionarily labile traits that have repeatedly evolved a remarkable number of times in vascular plants. Our current understanding of the phylogenetic patterns of EFNs makes them powerful candidates for future work exploring the drivers of their evolutionary origins, shifts, and losses.
了解生态相关特征的进化模式是植物生物学的核心目标。然而，对于大多数重要特征，我们缺乏评估其在生命之树中进化模式和速度所需的全面了解。在这里，我们评估了一种常见的植物防御特征在维管植物中的广泛系统发育模式：即额外的花外蜜腺（EFN），这是一种分泌花蜜的植物腺体，位于花外。EFN 通常通过吸引无脊椎动物捕食者来间接保护植物，这些捕食者可以减少草食性。
我们编译了过去 135 年来发表的 EFN 记录。在考虑到分类学变化后，使用了超过 55000 种维管植物的系统发育，使用系统发育比较方法来评估 EFN 进化的模式。使用参数和非参数模型的比较，估计了可能存在于当前列表之外的具有 EFN 的物种的真实数量。
迄今为止，EFN 已在 3941 种代表 108 科 745 属的植物中被报道，约占维管植物物种的 1-2%和约 21%的科。它们在 65 个被子植物目中的 33 个目中被发现。在 36 个蕨类植物科中，已知有 4 个科具有叶外蜜腺。在早期被子植物、木兰类和裸子植物中没有发现花外蜜腺。它们存在于单子叶植物中，但大多数 EFN 都在真双子叶植物中发现，具有 EFN 的大多数物种是蔷薇目。系统发育分析强烈支持 EFN 在植物进化枝中的反复获得和丧失，尤其是在更衍生的双子叶科中，并表明 EFN 存在于至少 457 个独立谱系中。然而，模型选择方法估计，未报告的 EFN 案例数量可能与已报告的物种数量一样高。
EFN 是广泛存在且进化不稳定的特征，在维管植物中已经多次进化。我们目前对 EFN 系统发育模式的理解使它们成为未来探索其进化起源、转移和丧失的驱动力的有力候选者。
