Fitzpatrick Benjamin M, Johnson Jarrett R, Kump D Kevin, Shaffer H Bradley, Smith Jeramiah J, Voss S Randal
Ecology & Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA.
BMC Evol Biol. 2009 Jul 24;9:176. doi: 10.1186/1471-2148-9-176.
Hybrid zones represent valuable opportunities to observe evolution in systems that are unusually dynamic and where the potential for the origin of novelty and rapid adaptation co-occur with the potential for dysfunction. Recently initiated hybrid zones are particularly exciting evolutionary experiments because ongoing natural selection on novel genetic combinations can be studied in ecological time. Moreover, when hybrid zones involve native and introduced species, complex genetic patterns present important challenges for conservation policy. To assess variation of admixture dynamics, we scored a large panel of markers in five wild hybrid populations formed when Barred Tiger Salamanders were introduced into the range of California Tiger Salamanders.
At three of 64 markers, introduced alleles have largely displaced native alleles within the hybrid populations. Another marker (GNAT1) showed consistent heterozygote deficits in the wild, and this marker was associated with embryonic mortality in laboratory F2's. Other deviations from equilibrium expectations were idiosyncratic among breeding ponds, consistent with highly stochastic demographic effects.
While most markers retain native and introduced alleles in expected proportions, strong selection appears to be eliminating native alleles at a smaller set of loci. Such rapid fixation of alleles is detectable only in recently formed hybrid zones, though it might be representative of dynamics that frequently occur in nature. These results underscore the variable and mosaic nature of hybrid genomes and illustrate the potency of recombination and selection in promoting variable, and often unpredictable genetic outcomes. Introgression of a few, strongly selected introduced alleles should not necessarily affect the conservation status of California Tiger Salamanders, but suggests that genetically pure populations of this endangered species will be difficult to maintain.
杂交区域为观察进化提供了宝贵机会，这些系统具有异常的动态性，新特性起源和快速适应的潜力与功能失调的可能性并存。最近形成的杂交区域是特别令人兴奋的进化实验，因为可以在生态时间尺度上研究对新遗传组合的持续自然选择。此外，当杂交区域涉及本地物种和引入物种时，复杂的遗传模式给保护政策带来了重大挑战。为了评估混合动态的变化，我们对五个野生杂交种群中的大量标记进行了评分，这些种群是在条纹虎螈被引入加州虎螈分布范围时形成的。
在64个标记中的3个标记上，引入的等位基因在杂交种群中已基本取代了本地等位基因。另一个标记（GNAT1）在野外显示出一致的杂合子缺失，并且该标记与实验室F2代的胚胎死亡率相关。与平衡预期的其他偏差在繁殖池塘之间是特异的，这与高度随机的种群统计学效应一致。
虽然大多数标记以预期比例保留了本地和引入的等位基因，但强烈的选择似乎正在消除一小部分位点上的本地等位基因。这种等位基因的快速固定仅在最近形成的杂交区域中可检测到，尽管它可能代表了自然界中经常发生的动态。这些结果强调了杂交基因组的可变和镶嵌性质，并说明了重组和选择在促进可变且通常不可预测的遗传结果方面的效力。少数经过强烈选择的引入等位基因的渗入不一定会影响加州虎螈的保护状况，但表明难以维持这个濒危物种的基因纯合种群。
