Data from: Large-scale patterns of diversification in the widespread legume genus Senna and the evolutionary role of extrafloral nectaries

Unraveling the diversification history of old, species-rich and widespread clades is difficult because of extinction, undersampling and taxonomic uncertainty. In the context of these challenges, we investigated the timing and mode of lineage diversification in Senna (Leguminosae) to gain insights into the evolutionary role of extrafloral nectaries (EFNs). EFNs secrete nectar, attracting ants and forming ecologically important ant-plant mutualisms. In Senna, EFNs characterize one large clade ("EFN clade"), including 80% of its 350 species. Taxonomic accounts make Senna the largest caesalpinioid genus, but quantitative comparisons to other taxa require inferences about rates. Molecular dating analyses suggest that Senna originated in the early Eocene, and its major lineages appeared during early/mid Eocene to early Oligocene. EFNs evolved in the late Eocene, after the main radiation of ants. The EFN clade diversified faster, becoming significantly more species-rich than non-EFN clades. The shift in diversification rates associated with EFN evolution supports the hypothesis that EFNs represent a (relatively old) key innovation in Senna. EFNs may have promoted the colonization of new habitats appearing with the early uplift of the Andes. This would explain the distinctive geographic concentration of the EFN clade in South America.

鉴于灭绝事件、采样不足及分类学不确定性等因素的干扰，阐明古老、物种丰富且分布广泛的支系的多样化演化历程历来极具挑战。针对上述挑战，我们以豆科（Leguminosae）决明属（Senna）为研究对象，探究其支系分化的时间与模式，以期深入解析花外蜜腺（extrafloral nectaries, EFNs）的演化功能。花外蜜腺可分泌花蜜，吸引蚂蚁并构建具有重要生态意义的蚁植共生体系。在决明属中，花外蜜腺为一个大型支系（下称‘花外蜜腺支系’）的典型特征，该支系涵盖决明属350个已知物种中的80%。分类学研究表明决明属是最大的云实类属群，但要实现与其他类群的定量比较，还需基于分化速率的相关推断。分子定年分析结果显示，决明属起源于始新世早期，其主要支系出现于始新世早中期至渐新世早期。花外蜜腺于始新世晚期，即在蚂蚁类群完成主要辐射演化之后演化形成。花外蜜腺支系的分化速率更快，其物种丰富度显著高于非花外蜜腺支系。与花外蜜腺演化相关的分化速率转变现象，支持了‘花外蜜腺是决明属中一项相对古老的关键创新性状’这一假说。花外蜜腺或推动了随安第斯山脉早期隆升而出现的新生境的定植过程，这一解释也能够说明花外蜜腺支系在南美洲呈现出的独特地理聚集分布格局。