Chilling out: Cooler climates triggered divergence of Sabal (Arecaceae: Coryphoideae: Sabaleae) at the end of the mid-Miocene climatic optimum

In this study we explore the biogeographic patterns and processes underlying the diversification of Sabal, a genus of palm with a range that expands from the tropical rainforests of northern South America into the deciduous subtropical forests of the southeastern United States. We estimate divergence times among species of Sabal using two dated fossils as calibration points to inform ancestral reconstructions of the historical distribution of Sabal, and then use floral volatile composition data in select species of Sabal to integrate plant ecological interactions into our discussion of species distributions. Our results suggest extant Sabal originated within the last 21 million years with a divergence event ~14 Mya that effectively split the most recent common ancestor of all southeastern US and West Indian species from the ancestor of a clade of species with their current distribution in Mexico. This divergence event corresponds with the end of the mid-Miocene climatic optimum, which resulted in cooler climates across southeastern North America where extant cold-tolerant taxa S. minor and S. palmetto are currently distributed. Floral volatile data indicate a generalist pollination strategy involving day foraging insects. Among sampled species, floral scent profiles can mostly be characterized by quantitative differences in five main compounds: benzaldehyde, benzyl alcohol, 2-phenylethanol, (E)-β-ocimene, and hexanal. These profiles align with major clades recovered in our phylogenetic analysis of Sabal and are indicative of evolutionary trends in pollination ecology and reproductive isolation within the genus.

Methods
Gene alignment used to estimate divergence times: A supermatrix was generated from all gene alignments generated by Heyduk et al. 2015 (https://doi.org/10.5061/dryad.jm78g) by concatenating separate gene alignment files. We used PartitionFinder V. 2.1.1 (Lanfear et al. 2017) to estimate the optimal partitioning scheme among genes. The partitioned alignment contained 49 partitions with substitution models assigned as either GTR, GTR+G, and GTR+G+I.

本研究旨在探究扇叶棕榈属（Sabal）物种分化背后的生物地理学格局与过程。该棕榈属的分布范围从南美洲北部的热带雨林延伸至美国东南部的落叶亚热带森林。
本研究以两件定年化石作为校准点，估算扇叶棕榈属各物种间的分化时间，以辅助该属历史分布的祖先重建；随后选取部分扇叶棕榈属物种的花挥发物成分数据，将植物生态交互作用纳入物种分布相关讨论中。

研究结果显示，现存扇叶棕榈属类群起源于距今2100万年以内，约1400万年前发生的一次分化事件将所有美国东南部与西印度群岛物种的最近共同祖先，与当前分布于墨西哥的演化支物种祖先彻底分隔开来。该分化事件与中新世中期气候最适宜期的结束时间相吻合，此次气候适宜期结束后北美东南部地区气候转冷，而现存耐寒类群小扇叶棕榈（S. minor）与美国扇叶棕榈（S. palmetto）正分布于此区域。

花挥发物数据表明，扇叶棕榈属采用泛化传粉策略，其传粉者为日间觅食的昆虫。在所采样的物种中，花香谱主要可通过五种主要化合物的含量差异进行区分：苯甲醛、苯甲醇、2-苯乙醇、(E)-β-罗勒烯与己醛。此类花香谱与本研究扇叶棕榈属系统发育分析所鉴定得到的主要演化支相契合，同时也反映了该属传粉生态学与生殖隔离的演化趋势。

研究方法
用于估算分化时间的基因比对：本研究通过拼接Heyduk等人2015年（https://doi.org/10.5061/dryad.jm78g）所生成的所有基因比对文件，构建得到超级矩阵。我们使用分区优化软件PartitionFinder V. 2.1.1（Lanfear等人2017年）估算基因间的最优分区方案。该分区比对包含49个分区，所指定的替换模型分别为GTR、GTR+G以及GTR+G+I。