Quer J, Hershey C L, Domingo E, Holland J J, Novella I S
Department of Biology and Center for Molecular Genetics, University of California, San Diego, La Jolla, California 92093-0116, USA.
J Virol. 2001 Aug;75(16):7315-20. doi: 10.1128/JVI.75.16.7315-7320.2001.
The replicative fitness of a genetically marked (MARM-C) population of vesicular stomatitis virus was examined in competition assays in BHK-21 cells. In standard fitness assays involving up to eight competition passages of the mixed populations, MARM-C competes equally with the wild type (wt), but very prolonged competitions always led to the wt gaining dominance over MARM-C in a very slowed, nonlinear manner (J. Quer et al., J. Mol. Biol. 264:465-471, 1996). In the present study we show that a number of quite unrelated environmental perturbations, which decreased virus replication during competitions, all led to an accelerated dominance of the wt over MARM-C. These perturbations were (i) the presence of added (or endogenously generated) defective interfering particles, (ii) the presence of the chemical mutagen 5-fluorouracil (5-FU), or (iii) an increase in temperature to 40.5 degrees C. Thus, the "neutral fitness" of the MARM-C population is contingent. We have determined the entire genomic consensus sequence of MARM-C and have identified only six mutations. Clearly, some or all of these mutations allowed the MARM-C quasispecies population to compete equally with wt in a defined constant host environment, but the period of neutrality was shortened when the environment was perturbed during competitions. Interestingly, when four passages of each population were carried out independently in the presence of 5-FU (but in the absence of competition), no significant differences were detected in the fitness changes of wt and MARM-C, nor was there a difference in their subsequent abilities to compete with each other in a standard fitness assay. We propose a model for this contingent neutrality. The conditions employed to generate the MARM-C quasispecies population selected a small number of mutations in the consensus sequence. It appears that the MARM-C quasispecies population has moved into a segment of sequence space in which the average fitness value is neutral but, under environmental stress, beneficial mutations cannot be generated rapidly enough to compete with those being generated concurrently by competing wt virus quasispecies populations.
在BHK - 21细胞的竞争试验中，检测了基因标记的（MARM - C）水泡性口炎病毒群体的复制适应性。在涉及混合群体多达八轮竞争传代的标准适应性试验中，MARM - C与野生型（wt）表现出同等竞争力，但非常长时间的竞争总是导致wt以非常缓慢的非线性方式在MARM - C之上占据优势（J. Quer等人，《分子生物学杂志》264:465 - 471，1996）。在本研究中，我们表明一些完全不相关的环境干扰，在竞争期间降低了病毒复制，都导致wt在MARM - C之上的优势加速显现。这些干扰因素包括：（i）添加的（或内源性产生的）缺陷干扰颗粒的存在；（ii）化学诱变剂5 - 氟尿嘧啶（5 - FU）的存在；或（iii）温度升高至40.5摄氏度。因此，MARM - C群体的“中性适应性”是有条件的。我们已经确定了MARM - C的整个基因组共有序列，并且仅鉴定出六个突变。显然，这些突变中的一些或全部使得MARM - C准种群体在特定的恒定宿主环境中能够与wt同等竞争，但当竞争期间环境受到干扰时，中性期会缩短。有趣的是，当每个群体在5 - FU存在的情况下（但不存在竞争）独立进行四轮传代时，wt和MARM - C的适应性变化未检测到显著差异，它们随后在标准适应性试验中相互竞争的能力也没有差异。我们提出了一个关于这种有条件中性的模型。用于产生MARM - C准种群体的条件在共有序列中选择了少量突变。似乎MARM - C准种群体已经进入了一段序列空间，其中平均适应性值处于中性，但在环境压力下，有益突变无法足够快速地产生以与竞争的wt病毒准种群体同时产生的突变竞争。
