Ebersbach J, Schnitzler J, Favre A, Muellner-Riehl A N
Department of Molecular Evolution and Plant Systematics & Herbarium (LZ), Institute of Biology, Leipzig University, Johannisallee 21-23, D-04103, Leipzig, Germany.
German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Deutscher Platz 5e, D-04103, Leipzig, Germany.
BMC Evol Biol. 2017 May 25;17(1):119. doi: 10.1186/s12862-017-0967-2.
A large number of taxa have undergone evolutionary radiations in mountainous areas, rendering alpine systems particularly suitable to study the extrinsic and intrinsic factors that have shaped diversification patterns in plants. The species-rich genus Saxifraga L. is widely distributed throughout the Northern Hemisphere, with high species numbers in the regions adjacent to the Qinghai-Tibet Plateau (QTP) in particular the Hengduan Mountains and the Himalayas. Using a dataset of 297 taxa (representing at least 60% of extant Saxifraga species), we explored the variation of infrageneric diversification rates. In addition, we used state-dependent speciation and extinction models to test the effects of geographic distribution in the Hengduan Mountains and the entire QTP region as well as of two morphological traits (cushion habit and specialized lime-secreting glands, so-called hydathodes) on the diversification of this genus.
We detected two to three rate shifts across the Saxifraga phylogeny and two of these shifts led to radiations within two large subclades of Saxifraga, sect. Ciliatae Haworth subsect. Hirculoideae Engl. & Irmsch. and sect. Porphyrion Tausch subsect. Kabschia Engl. GEOSSE analyses showed that presence in the Hengduan Mountains had a positive effect on diversification across Saxifraga. Influence of these mountains was strongest in Saxifraga sect. Ciliatae subsect. Hirculoideae given its pronounced distribution there, and thus the radiation in this group can be classified at least partially as geographic. In contrast, the evolution of the cushion life form and lime-secreting hydathodes had positive effects on diversification only in selected Saxifraga sections, including sect. Porphyrion subsect. Kabschia. We therefore argue that radiation in this group was likely adaptive.
Our study underlines the complexity of processes and factors underpinning plant radiations: Even in closely related lineages occupying the same life zone, shifts in diversification are not necessarily governed by similar factors. In conclusion, alpine plant radiations result from a complex interaction among geographical settings and/or climatic modifications providing key opportunities for diversification as well as the evolution of key innovations.
大量分类群在山区经历了进化辐射，这使得高山系统特别适合研究塑造植物多样化模式的外在和内在因素。物种丰富的虎耳草属广泛分布于北半球，特别是在青藏高原（QTP）周边地区，即横断山脉和喜马拉雅山脉，物种数量众多。我们使用了一个包含297个分类群的数据集（代表现存虎耳草属物种的至少60%），探索了属下多样化速率的变化。此外，我们使用了依赖状态的物种形成和灭绝模型，以测试横断山脉和整个QTP地区的地理分布以及两个形态特征（垫状习性和专门的排石灰腺体，即所谓的排水器）对该属多样化的影响。
我们在虎耳草系统发育过程中检测到两到三次速率变化，其中两次变化导致了虎耳草属两个大亚支内的辐射，即睫毛虎耳草组（Ciliatae Haworth）下的小耳状亚组（Hirculoideae Engl. & Irmsch.）和紫花虎耳草组（Porphyrion Tausch）下的卡布希亚亚组（Kabschia Engl.）。地理物种丰富度和特有性分析（GEOSSE）表明，横断山脉的存在对虎耳草属的多样化有积极影响。鉴于该山脉在睫毛虎耳草组下的小耳状亚组中分布显著，其影响最为强烈，因此该类群中的辐射至少部分可归类为地理辐射。相比之下，垫状生活型和排石灰排水器的进化仅在虎耳草属的特定组中对多样化有积极影响，包括紫花虎耳草组下的卡布希亚亚组。因此，我们认为该类群中的辐射可能是适应性的。
我们的研究强调了支撑植物辐射的过程和因素的复杂性：即使在占据相同生活区域的密切相关谱系中，多样化的变化也不一定受相似因素的支配。总之，高山植物辐射是地理环境和/或气候改变之间复杂相互作用的结果，这些因素为多样化以及关键创新的进化提供了关键机会。
