Hoffman Eric A, Schueler Frederick W, Blouin Michael S
3029 Cordley Hall, Oregon State University, Department of Zoology, Corvallis, Oregon 97331, USA.
Evolution. 2004 Nov;58(11):2536-45. doi: 10.1111/j.0014-3820.2004.tb00882.x.
Although studies of population genetic structure are very common, whether genetic structure is stable over time has been assessed for very few taxa. The question of stability over time is particularly interesting for frogs because it is not clear to what extent frogs exist in dynamic metapopulations with frequent extinction and recolonization, or in stable patches at equilibrium between drift and gene flow. In this study we collected tissue samples from the same five populations of leopard frogs, Rana pipiens, over a 22-30 year time interval (11-15 generations). Genetic structure among the populations was very stable, suggesting that these populations were not undergoing frequent extinction and colonization. We also estimated the effective size of each population from the change in allele frequencies over time. There exist few estimates of effective size for frog populations, but the data available suggest that ranid frogs may have much larger ratios of effective size (Ne) to census size (Nc) than toads (bufonidae). Our results indicate that R. pipiens populations have effective sizes on the order of hundreds to at most a few thousand frogs, and Ne/Nc ratios in the range of 0.1-1.0. These estimates of Ne/Nc are consistent with those estimated for other Rana species. Finally, we compared the results of three temporal methods for estimating Ne. Moment and pseudolikelihood methods that assume a closed population gave the most similar point estimates, although the moment estimates were consistently two to four times larger. Wang and Whitlock's new method that jointly estimates Ne and the rate of immigration into a population (m) gave much smaller estimates of Ne and implausibly large estimates of m. This method requires knowing allele frequencies in the source of immigrants, but was thought to be insensitive to inexact estimates. In our case the method may have failed because we did not know the true source of immigrants for each population. The method may be more sensitive to choice of source frequencies than was previously appreciated, and so should be used with caution if the most likely source of immigrants cannot be identified clearly.
虽然对种群遗传结构的研究非常普遍，但对于很少的分类群评估了遗传结构是否随时间稳定。随着时间推移的稳定性问题对于蛙类来说尤其有趣，因为尚不清楚蛙类在频繁灭绝和重新定殖的动态集合种群中存在的程度，或者在漂变和基因流平衡的稳定斑块中存在的程度。在本研究中，我们在22 - 30年的时间间隔（11 - 15代）内从豹蛙（林蛙）的相同五个种群中收集了组织样本。种群间的遗传结构非常稳定，这表明这些种群没有经历频繁的灭绝和定殖。我们还根据等位基因频率随时间的变化估计了每个种群的有效大小。对蛙类种群有效大小的估计很少，但现有数据表明蛙科蛙类的有效大小（Ne）与普查大小（Nc）的比率可能比蟾蜍（蟾蜍科）大得多。我们的结果表明，林蛙种群的有效大小在数百到至多几千只蛙的数量级，Ne/Nc比率在0.1 - 1.0的范围内。这些Ne/Nc的估计与对其他林蛙物种的估计一致。最后，我们比较了三种估计Ne的时间方法的结果。假设种群封闭的矩估计法和伪似然法给出了最相似的点估计，尽管矩估计始终大两到四倍。联合估计Ne和种群迁入率（m）的Wang和Whitlock的新方法给出的Ne估计值要小得多，而m的估计值却大得不合理。该方法需要知道移民源中的等位基因频率，但被认为对不精确估计不敏感。在我们的案例中，该方法可能失败了，因为我们不知道每个种群移民的真正来源。该方法可能比之前认为的对源频率的选择更敏感，因此如果不能清楚地识别最可能的移民源，应谨慎使用。
