Department of Biology, Graduate School of Science, Chiba University, Chiba, Japan.
Herbario Isidro Palacios, Instituto de Investigaciones de Zonas Desérticas, Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico.
Ann Bot. 2018 Mar 5;121(3):535-548. doi: 10.1093/aob/mcx165.
Biogeographic transition zones are promising areas to study processes of biogeographic evolution and its influence on biological groups. The Mexican transition zone originated due to the overlap of Nearctic and Neotropical biota, which promoted great biological diversification. However, since most previous studies in this area were focused on revealing the phylogeography of Nearctic plants, how historical biogeographic configuration influenced the expansion and diversification of the Neotropical flora remains almost unknown. Using the cycad genus Dioon (Zamiaceae), this study aimed to test whether the biogeographic provinciality of the Mexican transition zone reflects the history of diversification of Neotropical plants.
Two chloroplast DNA (cpDNA) regions were analysed from 101 specimens of 15 Dioon species to reveal the distribution of haplogroups. In addition, genome-wide single nucleotide polymorphisms (SNPs) from 84 specimens were used to test the concordance between phylogenetic clusters and the biogeographic provinces. An ultrametric tree was constructed from the sequences containing SNPs to reconstruct the biogeographic events of vicariance and dispersal of Dioon across the Neotropical biogeographic provinces.
Four Dioon lineages with strong phylogeographic structures were recognized using both cpDNA and SNP data. The lineages correspond to two clades that originated from a common ancestor in Eastern Mexico. One clade expanded and diversified in South-east Mexico and Central America. Another clade diversified into three lineages that dispersed to North-east, South and North-west Mexico. Each lineage was biogeographically delimitated. Biogeographic provinces might have provided disparate ecological conditions that facilitated speciation in Dioon since the Miocene.
The current genetic structure and species diversity of Dioon depict the history of expansion and diversification of the northernmost Neotropical provinces. Past biogeographic connectivities were favoured by elevated topographies, since mountain systems served as corridors for the migration of Dioon and as refugia of tropical communities that diversified during the formation of modern Neotropical forests.
生物地理过渡带是研究生物地理进化过程及其对生物群影响的有前途的区域。墨西哥过渡带是由于新北界和新热带生物群的重叠而形成的，这促进了巨大的生物多样性。然而，由于该地区以前的大多数研究都集中在揭示新北界植物的系统地理学，历史生物地理结构如何影响新热带植物区系的扩展和多样化仍然知之甚少。本研究以苏铁科的迪翁属（Dioon）植物为例，旨在检验墨西哥过渡带的生物地理区域性是否反映了新热带植物多样化的历史。
从 15 种迪翁属的 101 个标本中分析了两个叶绿体 DNA（cpDNA）区域，以揭示单倍型群的分布。此外，还利用 84 个标本的全基因组单核苷酸多态性（SNP）来检验系统发育聚类与生物地理区系之间的一致性。从包含 SNP 的序列构建了一个超树，以重建迪翁属通过新热带生物地理区系的分歧和扩散的生物地理事件。
利用 cpDNA 和 SNP 数据，识别出具有强烈系统地理学结构的 4 个迪翁属谱系。这些谱系对应于起源于墨西哥东部共同祖先的两个分支。一个分支在墨西哥东南部和中美洲扩展和多样化。另一个分支分化为三个谱系，分别扩散到墨西哥东北部、南部和西北部。每个谱系都具有生物地理边界。自中新世以来，生物地理区系可能为迪翁属的物种形成提供了不同的生态条件。
迪翁属目前的遗传结构和物种多样性描绘了新热带最北部省份的扩展和多样化历史。过去的生物地理连通性受到抬升地形的影响，因为山脉系统不仅为迪翁属的迁移提供了走廊，还为热带社区的多样化提供了避难所，这些社区在现代新热带森林形成过程中多样化。
