Reconstruction of a beech population bottleneck using archival demographic information and Bayesian analysis of genetic data

Range expansion and contraction has occurred in the history of most species and can seriously impact patterns of genetic diversity. Historical data about range change are rare and generally appropriate for studies at large scales, whereas the individual pollen and seed dispersal events that form the basis of geneflow and colonization generally occur at a local scale. In this study we investigated range change in Fagus sylvatica on Mont Ventoux, France, using historical data from 1838 to the present and Approximate Bayesian Computation (ABC) analyses of genetic data. From the historical data we identified a population minimum in 1845 and located remnant populations at least 200 years old. The ABC analysis selected a demographic scenario with three populations, corresponding to two remnant populations and one area of recent expansion. It also identified expansion from a smaller ancestral population but did not find that this expansion followed a population bottleneck, as suggested by the historical data. Despite a strong support to the selected scenario for our dataset, the ABC approach showed a low power to discriminate among scenarios on average and a low ability to accurately estimate effective population sizes and divergence dates, probably due to the temporal scale of the study. This study provides an unusual opportunity to test ABC analysis in a system with a well documented demographic history and identify discrepancies between the results of historical, classical population genetic and ABC analyses. The results also provide valuable insights into genetic processes at work at a fine spatial and temporal scale in range change and colonization.

多数物种的演化历史中均发生过分布范围的扩张与收缩，这一过程会对其遗传多样性格局产生显著影响。有关分布范围变化的历史数据较为匮乏，且通常仅适用于大尺度研究；而构成基因流与种群拓殖基础的单个花粉、种子扩散事件，一般仅发生在局域尺度范围内。本研究以法国旺图山（Mont Ventoux）的欧洲山毛榉（Fagus sylvatica）为研究对象，整合1838年至今的历史数据与遗传数据的近似贝叶斯计算（Approximate Bayesian Computation, ABC）分析，探究其分布范围的变化过程。基于历史数据，本研究确定该种群在1845年达到种群数量最低点，并发现了至少有200年历史的残存种群。近似贝叶斯计算分析筛选出包含三个种群的种群历史动态情景，对应两个残存种群与一个近期扩张区域。该分析同时推断出种群由较小的祖先种群扩张而来，但并未如历史数据所暗示的那样，发现此次扩张伴随种群瓶颈事件。尽管本数据集对筛选出的情景具有较高支持度，但近似贝叶斯计算方法整体上对不同情景的区分效力较低，且难以准确估算有效种群大小与分化时间，这一缺陷可能源于本研究的时间尺度限制。本研究为在具有完备种群历史动态记录的研究系统中验证近似贝叶斯计算分析提供了难得的契机，同时也有助于揭示历史记录、经典种群遗传学分析与近似贝叶斯计算分析结果之间的不一致性。本研究结果还为解析分布范围变化与种群拓殖过程中精细时空尺度下的遗传作用机制提供了重要参考。