Table_1_The Cycad Genus Cycas May Have Diversified From Indochina and Occupied Its Current Ranges Through Vicariance and Dispersal Events.DOCX

Biogeographically, cycads were once widely distributed but the extant cycads are restricted to tropical and subtropical regions. They originated ∼ 300 Ma and re-diversified recently around 12 Ma, with the genus Cycas being the most rapidly diversified and largely distributed lineage. However, the forces that shaped the diversification and biogeography of the genus remain to be fully understood. Here, we first retrieved and used DNA sequences from GenBank (nuclear: PHYP, RPB1, HZP, AC3, F3H, SAMS, and GTP; chloroplasts: plant barcode trnH-psbA, trnL-trnF, trnS-trnG, and psbM-trnD) to assemble a complete dated phylogeny of Cycas. Then, we employed the Bayesian Binary Method to reconstruct the historical biogeography of the extant Cycas and finally, using the Bayesian approach for diversification analysis, we explored the evolutionary events that might have shaped the rapid diversification and large distribution of Cycas across the Pacific Islands. Our analysis pointed to Indo-China as the origin of the genus, which may have dispersed firstly across the Pacific Islands during the late Miocene aided by multiple excursions of sea levels and the development of a key innovation, i.e., a spongy endocarp particularly in the seeds of the subsection Rumphiae. The colonization of South China, which was thought to be the origin of the genus, may have occurred more recently aided by both dispersal and vicariance events. However, no significant shifts in the evolutionary events (speciation, extinction, mass extinction) that shaped the diversity of the genus were observed. Overall, our study re-clarifies the historical biogeography and the evolutionary forces that shaped the current diversity of the genus Cycas.

在生物地理学上，苏铁类植物曾广泛分布，但现存的苏铁类植物仅限于热带和亚热带地区。它们起源于约3亿年前，并在最近约1200万年前重新多样化。其中，属Cycas的物种多样化速度最快，分布范围最广。然而，塑造该属多样化与生物地理学的力量尚未完全明了。本研究首先从GenBank（核基因：PHYP、RPB1、HZP、AC3、F3H、SAMS和GTP；叶绿体基因：植物条形码trnH-psbA、trnL-trnF、trnS-trnG和psbM-trnD）中检索并使用DNA序列，构建了Cycas的完整定年系统发育树。随后，我们采用贝叶斯二元法重建了现存Cycas的历史生物地理学，并最终利用贝叶斯方法进行多样化分析，探讨了可能塑造Cycas在太平洋岛屿快速多样化和广泛分布的进化事件。我们的分析指出，印度支那可能是该属的起源地，该属可能最早在中新世晚期通过多次海平面波动和关键创新（即，在Rumphiae亚属种子中的海绵状内果皮）的发育，扩散至太平洋岛屿。而南中国的殖民化，曾被认为是该属的起源地，可能发生在较晚的时期，得益于扩散和歧变事件。然而，在塑造该属多样性的进化事件（物种形成、灭绝、大规模灭绝）中，并未观察到显著的变化。总体而言，本研究重新明确了Cycas属的历史生物地理学及其塑造当前多样性的进化力量。