Section of Integrative Biology, The University of Texas at Austin Austin, Texas 78712, USA.
Evolution. 2012 Feb;66(2):363-74. doi: 10.1111/j.1558-5646.2011.01433.x. Epub 2011 Sep 20.
The gain in fitness during adaptation depends on the supply of beneficial mutations. Despite a good theoretical understanding of how evolution proceeds for a defined set of mutations, there is little understanding of constraints on net fitness-whether fitness will reach a limit despite ongoing selection and mutation, and if there is a limit, what determines it. Here, the dsDNA bacteriophage SP6, a virus of Salmonella, was adapted to Escherichia coli K-12. From an isolate capable of modest growth on E. coli, four lines were adapted for rapid growth by protocols differing in use of mutagen, propagation method, and duration, but using the same media, temperature, and a continual excess of the novel host. Nucleotide changes underlying those adaptations differed greatly in number and identity, but the four lines achieved similar absolute fitness at the end, an increase of more than 4000-fold phage descendants per hour. Thus, the fitness landscape allows multiple genetic paths to the same approximate fitness limit. The existence and causes of fitness limits have ramifications to genome engineering, vaccine design, and "lethal mutagenesis" treatments to cure viral infections.
在适应过程中，适应性的提高取决于有益突变的供应。尽管我们对一组特定突变的进化过程有很好的理论理解，但对净适应性的限制却知之甚少——无论在持续的选择和突变下，适应性是否会达到极限，如果存在极限，是什么决定了它。在这里，dsDNA 噬菌体 SP6，一种沙门氏菌的病毒，被适应于大肠杆菌 K-12。从一个在大肠杆菌上生长能力适中的分离株中，通过使用不同诱变剂、繁殖方法和持续时间的方案，四条线被适应于快速生长，但使用相同的培养基、温度和持续过量的新型宿主。这些适应所依据的核苷酸变化在数量和身份上有很大的不同，但四条线最终达到了相似的绝对适应性，每小时产生的噬菌体后代增加了 4000 多倍。因此，适应性景观允许多条遗传途径达到相同的近似适应性极限。适应性极限的存在和原因对基因组工程、疫苗设计和治疗病毒感染的“致死性诱变”治疗有影响。
