Levin Bruce R, Cornejo Omar E
Department of Biology, Emory University, Atlanta, GA, USA.
PLoS Genet. 2009 Aug;5(8):e1000601. doi: 10.1371/journal.pgen.1000601. Epub 2009 Aug 14.
In bacteria, recombination is a rare event, not a part of the reproductive process. Nevertheless, recombination -- broadly defined to include the acquisition of genes from external sources, i.e., horizontal gene transfer (HGT) -- plays a central role as a source of variation for adaptive evolution in many species of bacteria. Much of niche expansion, resistance to antibiotics and other environmental stresses, virulence, and other characteristics that make bacteria interesting and problematic, is achieved through the expression of genes and genetic elements obtained from other populations of bacteria of the same and different species, as well as from eukaryotes and archaea. While recombination of homologous genes among members of the same species has played a central role in the development of the genetics and molecular biology of bacteria, the contribution of homologous gene recombination (HGR) to bacterial evolution is not at all clear. Also, not so clear are the selective pressures responsible for the evolution and maintenance of transformation, the only bacteria-encoded form of HGR. Using a semi-stochastic simulation of mutation, recombination, and selection within bacterial populations and competition between populations, we explore (1) the contribution of HGR to the rate of adaptive evolution in these populations and (2) the conditions under which HGR will provide a bacterial population a selective advantage over non-recombining or more slowly recombining populations. The results of our simulation indicate that, under broad conditions: (1) HGR occurring at rates in the range anticipated for bacteria like Streptococcus pneumoniae, Escherichia coli, Haemophilus influenzae, and Bacillus subtilis will accelerate the rate at which a population adapts to environmental conditions; (2) once established in a population, selection for this capacity to increase rates of adaptive evolution can maintain bacteria-encoded mechanisms of recombination and prevent invasion of non-recombining populations, even when recombination engenders a modest fitness cost; and (3) because of the density- and frequency-dependent nature of HGR in bacteria, this capacity to increase rates of adaptive evolution is not sufficient as a selective force to provide a recombining population a selective advantage when it is rare. Under realistic conditions, homologous gene recombination will increase the rate of adaptive evolution in bacterial populations and, once established, selection for higher rates of evolution will promote the maintenance of bacteria-encoded mechanisms for HGR. On the other hand, increasing rates of adaptive evolution by HGR is unlikely to be the sole or even a dominant selective pressure responsible for the original evolution of transformation.
在细菌中，重组是一个罕见事件，并非繁殖过程的一部分。然而，重组——广义上定义为包括从外部来源获取基因，即水平基因转移（HGT）——作为许多细菌物种适应性进化的变异来源发挥着核心作用。许多生态位扩展、对抗生素和其他环境压力的抗性、毒力以及使细菌既有趣又成问题的其他特征，都是通过表达从相同和不同物种的其他细菌群体以及真核生物和古细菌中获得的基因和遗传元件来实现的。虽然同一物种成员之间同源基因的重组在细菌遗传学和分子生物学发展中发挥了核心作用，但同源基因重组（HGR）对细菌进化的贡献尚不完全清楚。同样，对于负责转化（HGR唯一由细菌编码的形式）进化和维持的选择压力也不那么清楚。通过对细菌群体内突变、重组和选择以及群体间竞争进行半随机模拟，我们探究了：（1）HGR对这些群体适应性进化速率的贡献；（2）在何种条件下HGR会使细菌群体相对于非重组或重组较慢的群体具有选择优势。我们模拟的结果表明，在广泛的条件下：（1）像肺炎链球菌、大肠杆菌、流感嗜血杆菌和枯草芽孢杆菌等细菌预期速率范围内发生的HGR将加速群体适应环境条件的速率；（2）一旦在群体中确立，对这种增加适应性进化速率能力的选择可以维持细菌编码的重组机制，并防止非重组群体的入侵，即使重组会带来适度适合度代价；（3）由于细菌中HGR的密度和频率依赖性，当重组群体稀少时，这种增加适应性进化速率的能力作为一种选择力并不足以使其具有选择优势。在现实条件下，同源基因重组将增加细菌群体适应性进化的速率，并且一旦确立，对更高进化速率的选择将促进细菌编码的HGR机制的维持。另一方面，通过HGR提高适应性进化速率不太可能是导致转化最初进化的唯一甚至主导选择压力。
