Nazaire Mare, Wang Xiao-Quan, Hufford Larry
School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236 USA.
Am J Bot. 2014 Jan;101(1):104-18. doi: 10.3732/ajb.1300320. Epub 2014 Jan 1.
Numerous molecular phylogenetic studies have used new biogeographic tools to explain species distributions. However, questions remain about origins, timing, direction of movement, and relationships between range expansion and diversification. We investigated geographic origins and temporal and spatial diversification of Mertensia, giving particular attention to divergence between Asian and North American lineages and radiation of western North American clades.
Divergence time estimation and biogeographic analyses were based on phylogeny reconstruction inferred from nuclear ribosomal ITS and 12 plastid DNA sequence regions and a broad sampling of Mertensia, Boraginaceae, and core eudicots.
Mertensia split from Asperugo in the late Oligocene to mid Miocene (26.83-12.22 million years ago [Ma]), followed by the first divergence in the crown group in the late Miocene (10.36-5.19 Ma). The ancestral area is inferred to have been Asia or a widespread distribution across Asia, Beringia, and circumboreal locales. Initial range expansion of North American Mertensia occurred in Beringia and the Pacific Northwest (7.70-4.22 Ma), followed by diversification of three clades (Pacific Northwest, southern Rocky Mountains, central Rocky Mountains).
The crown divergence of extant Mertensia coincides with the onset of extreme cooling and fragmentation of a once extensive mixed mesophytic forest that was subsequently replaced by a boreal coniferous forest. Early diversification likely occurred when Beringia was connected and available for floristic exchange. The north-south orientation of the Rocky Mountain Range and Pleistocene glacial-interglacial cycles appear to have been important in the North American diversification of Mertensia.
众多分子系统发育研究已使用新的生物地理学工具来解释物种分布。然而，关于起源、时间、迁移方向以及分布范围扩张与物种多样化之间的关系等问题依然存在。我们研究了滨紫草属的地理起源以及时间和空间上的多样化，特别关注亚洲和北美谱系之间的分歧以及北美西部分支的辐射情况。
分歧时间估计和生物地理学分析基于从核糖体ITS和12个质体DNA序列区域推断出的系统发育重建，以及对滨紫草属、紫草科和核心真双子叶植物的广泛采样。
滨紫草属在渐新世晚期至中新世中期（2683 - 1222万年前[Ma]）与糙草属分化，随后在中新世晚期（1036 - 519 Ma）冠群出现首次分化。推测其祖先区域为亚洲或广泛分布于亚洲、白令陆桥和环北方地区。北美滨紫草属最初的分布范围扩张发生在白令陆桥和太平洋西北部（770 - 422 Ma），随后三个分支（太平洋西北部、落基山脉南部、落基山脉中部）实现了多样化。
现存滨紫草属的冠群分化与曾经广泛的中生混交林的极端冷却和破碎化开始时间一致，该混交林随后被北方针叶林所取代。早期多样化可能发生在白令陆桥相连且可供植物区系交流之时。落基山脉的南北走向以及更新世冰期 - 间冰期循环似乎在北美滨紫草属的多样化过程中起到了重要作用。
