Department of Animal Ecology and Systematics, Justus Liebig University Giessen, Giessen, Germany.
PLoS One. 2011;6(10):e26307. doi: 10.1371/journal.pone.0026307. Epub 2011 Oct 20.
The Tibetan Plateau is not only the highest and largest plateau on earth; it is also home to numerous freshwater lakes potentially harbouring endemic faunal elements. As it remains largely unknown whether these lakes have continuously existed during the Last Glacial Maximum (LGM), questions arise as to whether taxa have been able to exist on the plateau since before the latest Pleistocene, from where and how often the plateau was colonized, and by which mechanisms organisms conquered remote high altitude lentic freshwater systems. In this study, species of the plateau-wide distributed freshwater gastropod genus Radix are used to answer these biogeographical questions.
METHODOLOGY/PRINCIPAL FINDINGS: Based on a broad spatial sampling of Radix spp. on the Tibetan Plateau, and phylogenetic analyses of mtDNA sequence data, three probably endemic and one widespread major Radix clade could be identified on the plateau. Two of the endemic clades show a remarkably high genetic diversity, indicating a relatively great phylogenetic age. Phylogeographical analyses of individuals belonging to the most widely distributed clade indicate that intra-plateau distribution cannot be explained by drainage-related dispersal alone.
CONCLUSIONS/SIGNIFICANCE: Our study reveals that Radix spp. persisted throughout the LGM on the Tibetan Plateau. Therefore, we assume the continuous existence of suitable water bodies during that time. The extant Radix diversity on the plateau might have been caused by multiple colonization events combined with a relatively long intra-plateau evolution. At least one colonization event has a Palaearctic origin. In contrast to freshwater fishes, passive dispersal, probably by water birds, might be an important mechanism for conquering remote areas on the plateau. Patterns found in Radix spp. are shared with some terrestrial plateau taxa, indicating that Radix may be a suitable model taxon for inferring general patterns of biotic origin, dispersal and survival on the Tibetan Plateau.
青藏高原不仅是地球上最高、最大的高原，也是众多可能蕴藏特有动物区系要素的淡水湖的所在地。由于这些湖泊在末次冰盛期（LGM）是否一直存在还不得而知，因此出现了一些问题，例如在更新世晚期之前，高原上的分类单元是否能够存在，高原是如何以及何时被殖民的，以及生物体是通过哪些机制征服了偏远的高海拔淡水湖泊系统。在这项研究中，使用分布广泛的淡水腹足纲贝类高原宽基贝属（Radix）的物种来回答这些生物地理学问题。
方法/主要发现：基于对青藏高原上广泛分布的 Radix 种的广泛空间采样，以及 mtDNA 序列数据分析的系统发育分析，可以在高原上确定三个可能的特有和一个广泛分布的主要 Radix 进化枝。两个特有进化枝显示出极高的遗传多样性，表明它们具有相对较长的进化年龄。属于分布最广的进化枝的个体的系统发育分析表明，高原内的分布不能仅用与水系有关的扩散来解释。
结论/意义：我们的研究表明，Radix 种在整个 LGM 期间都在青藏高原上存活下来。因此，我们假设在那个时期存在持续适合的水体。高原上现存的 Radix 多样性可能是由多次殖民事件和相对较长的高原内进化相结合造成的。至少有一次殖民事件起源于古北界。与淡水鱼类不同，被动扩散，可能是通过水禽进行的，可能是征服高原偏远地区的一个重要机制。在 Radix 种中发现的模式与一些陆地高原分类单元共享，这表明 Radix 可能是推断青藏高原生物起源、扩散和生存一般模式的合适模式分类单元。
