Evolution of specialists in an experimental microcosm
Evolution of specialists in an experimental microcosm
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DOI：
10.1534/genetics.103.025205
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发表时间：
2004-08-01
期刊：
影响因子：
3.3
通讯作者：
Dean, AM
中科院分区：
文献类型：
--
作者：
Dykhuizen, DE;Dean, AM
以大肠杆菌的乳糖操纵子为模型系统，探讨了适应对平衡多态性持久性的影响。当使用两种不同的天然分离的lac操纵子（TD2和TD10）时，趋化剂对两种可替代资源甲基半乳糖苷和乳果糖的竞争产生稳定的频率依赖选择。这种平衡多态性的命运是通过监测fhuA（-）的频率来跟踪进化时间的，fhuA（-）是一种相关的中性遗传市场，赋予对噬菌体T5的抗性。在9个趋化因子中，有4个的lac多态性在实验终止时持续了400-600代。在其他5个趋化因子中，fhuA多态性以及lac操纵子多态性在86 ~ 219代之间丢失。对lac操纵子单态的13个趋化培养中，有4个在450 ~ 550代中保持了中性的fhuA多态性，直到它们被终止；其余的在63 ~ 303代之间变成单态。每个半乳糖苷上的专一分子都是从维持fhuA多态性的趋化因子中分离出来的，无论是在lac操纵子上的多态性还是单态。从四种保留lac多态性的趋化因子中分离出的三种菌株改变了它们对半乳糖苷的偏好。大多数fhuA多态性缺失的趋化因子也含有专门性。这些结果表明，lac的初始多态性对长期适应进化的结果影响不大。相反，进化菌株的适应性是由基因组中其他地方产生的突变决定的，这一事实得到了证实，从进化背景中分离出来的操纵子无法单独解释这两种专家的存在。我们的研究结果表明，一旦稳定下来，生态专门化阻止了整个种群的选择性扫荡，从而促进了相关中性多态性的维持。
The impact of adaptation on the persistence of a balanced polymorphism was explored using the lactose operon of Escherichia coli as a model system. Competition in chemostats for two substitutable resources, methylgalactoside and lactulose, generates stabilizing frequency-dependent selection when two different naturally isolated lac operons (TD2 and TD10) are used. The fate of this balanced polymorphism was tracked over evolutionary time by monitoring the frequency of fhuA(-), a linked neutral genetic market that confers resistance to the bacteriophage T5. In four out of nine chemostats the lac polymorphism persisted for 400-600 generations when the experiments were terminated. In the other five chemostats the fhuA polymorphism, and consequently the lac operon polymorphism, was lost between 86 and 219 generations. Four of 13 chemostat cultures monomorphic for the lac operon retained the neutral fhuA polymorphism for 450-550 generations until they were terminated; the remainder became monomorphic at fhuA between 63 and 303 generations. Specialists on each galactoside were isolated from chemostats that maintained the fhuA polymorphism, whether polymorphic or monomorphic at the lac operon. Strains isolated from three of four chemostats in which the lac polymorphism was preserved had switched their galactoside preference. Most of the chemostats where the fhuA polymorphism was lost also contained specialists. These results demonstrate that the initial polymorphism at lac was of little consequence to the outcome of long-term adaptive evolution. Instead, the fitnesses of evolved strains were dominated by mutations arising elsewhere in the genome, a fact confirmed by showing that operons isolated from their evolved backgrounds were alone unable to explain the presence of both specialists. Our results suggest that, once stabilized, ecological specialization prevented selective sweeps through the entire population, thereby promoting the maintenance of linked neutral polymorphism.