Land-Bridge Calibration of Molecular Clocks and the Post-Glacial Colonization of Scandinavia by the Eurasian Field Vole <i>Microtus agrestis</i>

Phylogeography interprets molecular genetic variation in a spatial and temporal context. Molecular clocks are frequently used to calibrate phylogeographic analyses, however there is mounting evidence that molecular rates decay over the relevant timescales. It is therefore essential that an appropriate rate is determined, consistent with the temporal scale of the specific analysis. This can be achieved by using temporally spaced data such as ancient DNA or by relating the divergence of lineages directly to contemporaneous external events of known time. Here we calibrate a Eurasian field vole (Microtus agrestis) mitochondrial genealogy from the well-established series of post-glacial geophysical changes that led to the formation of the Baltic Sea and the separation of the Scandinavian peninsula from the central European mainland. The field vole exhibits the common phylogeographic pattern of Scandinavian colonization from both the north and the south, however the southernmost of the two relevant lineages appears to have originated in situ on the Scandinavian peninsula, or possibly in the adjacent island of Zealand, around the close of the Younger Dryas. The mitochondrial substitution rate and the timescale for the genealogy are closely consistent with those obtained with a previous calibration, based on the separation of the British Isles from mainland Europe. However the result here is arguably more certain, given the level of confidence that can be placed in one of the central assumptions of the calibration, that field voles could not survive the last glaciation of the southern part of the Scandinavian peninsula. Furthermore, the similarity between the molecular clock rate estimated here and those obtained by sampling heterochronous (ancient) DNA (including that of a congeneric species) suggest that there is little disparity between the measured genetic divergence and the population divergence that is implicit in our land-bridge calibration.系统地理学（phylogeography）是在时空框架内解析分子遗传变异的学科。分子钟（molecular clock）常被用于校准系统地理学分析，但越来越多的实证表明，分子进化速率在相关时间尺度下会发生衰减。因此，结合特定分析的时间尺度确定适配的进化速率至关重要。这一目标可通过两种途径实现：一是采用时序间隔样本（如古代DNA（ancient DNA）），二是将谱系分化直接与已知时间的同期外部事件相关联。本研究依托公认的冰后地球物理变化序列——该序列促成了波罗的海的形成以及斯堪的纳维亚半岛与欧洲大陆中部的分离——对欧亚普通田鼠（Microtus agrestis）的线粒体基因谱系进行了校准。普通田鼠呈现出从南北两侧殖民斯堪的纳维亚半岛的典型系统地理学模式，但两个相关谱系中最南端的一支似乎起源于斯堪的纳维亚半岛本地，或是邻近的西兰岛，时间大致处于新仙女木期（Younger Dryas）末期。本研究测得的线粒体替换速率与基因谱系时间尺度，与此前基于不列颠群岛与欧洲大陆分离事件的校准结果高度吻合。不过，鉴于我们对校准核心假设之一——普通田鼠无法在斯堪的纳维亚半岛南部的末次冰期存活——的置信度，本研究的结果可以说更为确凿。此外，本研究估算的分子钟速率，与通过异时样本（即古代DNA，包含同属物种的古代DNA）采样得到的速率高度相似，这表明我们测得的遗传分化与陆桥校准中隐含的种群分化之间几乎不存在差异。