School of Biological Sciences and Swire Institute of Marine Science, Faculty of Science, The University of Hong Kong, Hong Kong, SAR, People's Republic of China.
School of Natural Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, Australia.
Mol Biol Evol. 2024 May 3;41(5). doi: 10.1093/molbev/msae091.
Studying range expansions is central for understanding genetic variation through space and time as well as for identifying refugia and biological invasions. Range expansions are characterized by serial founder events causing clines of decreasing genetic diversity away from the center of origin and asymmetries in the two-dimensional allele frequency spectra. These asymmetries, summarized by the directionality index (ψ), are sensitive to range expansions and persist for longer than clines in genetic diversity. In continuous and finite meta-populations, genetic drift tends to be stronger at the edges of the species distribution in equilibrium populations and populations undergoing range expansions alike. Such boundary effects are expected to affect geographic patterns in genetic diversity and ψ. Here we demonstrate that boundary effects cause high false positive rates in equilibrium meta-populations when testing for range expansions. In the simulations, the absolute value of ψ (|ψ|) in equilibrium data sets was proportional to the fixation index (FST). By fitting signatures of range expansions as a function of ɛ |ψ|/FST and geographic clines in ψ, strong evidence for range expansions could be detected in data from a recent rapid invasion of the cane toad, Rhinella marina, in Australia, but not in 28 previously published empirical data sets from Australian scincid lizards that were significant for the standard range expansion tests. Thus, while clinal variation in ψ is still the most sensitive statistic to range expansions, to detect true signatures of range expansions in natural populations, its magnitude needs to be considered in relation to the overall levels of genetic structuring in the data.
研究范围扩张对于理解遗传多样性在空间和时间上的变化，以及识别避难所和生物入侵至关重要。范围扩张的特征是一系列创始事件，导致遗传多样性从起源中心逐渐减少的渐变，并导致二维等位基因频率谱的不对称。这些不对称性由方向指数（ψ）总结，对范围扩张敏感，并且比遗传多样性的渐变持续时间更长。在连续和有限的元种群中，遗传漂变在平衡种群和经历范围扩张的种群的分布边缘往往更强。这种边界效应预计会影响遗传多样性和ψ的地理模式。在这里，我们证明了当测试范围扩张时，边界效应会导致平衡元种群中的高假阳性率。在模拟中，平衡数据集的ψ绝对值（|ψ|）与固定指数（FST）成正比。通过拟合范围扩张的特征作为ɛ|ψ|/FST 和 ψ 的地理渐变的函数，我们可以在最近澳大利亚甘蔗蟾蜍（Rhinella marina）快速入侵的澳大利亚数据中检测到范围扩张的有力证据，但在 28 个先前发表的澳大利亚蜥蜴的经验数据集中没有检测到，这些数据对标准范围扩张测试很重要。因此，虽然 ψ 的渐变仍然是范围扩张最敏感的统计量，但为了在自然种群中检测到真正的范围扩张特征，需要考虑其幅度与数据中遗传结构的总体水平的关系。
