Duffy Siobain, Turner Paul E, Burch Christina L
Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut 06520-8106, USA.
Genetics. 2006 Feb;172(2):751-7. doi: 10.1534/genetics.105.051136. Epub 2005 Nov 19.
Natural and experimental systems have failed to universally demonstrate a trade-off between generalism and specialism. When a trade-off does occur it is difficult to attribute its cause to antagonistic pleiotropy without dissecting the genetic basis of adaptation, and few previous experiments provide these genetic data. Here we investigate the evolution of expanded host range (generalism) in the RNA virus phi6, an experimental model system allowing adaptive mutations to be readily identified. We isolated 10 spontaneous host range mutants on each of three novel Pseudomonas hosts and determined whether these mutations imposed fitness costs on the standard laboratory host. Sequencing revealed that each mutant had one of nine nonsynonymous mutations in the phi6 gene P3, important in host attachment. Seven of these nine mutations were costly on the original host, confirming the existence of antagonistic pleiotropy. In addition to this genetically imposed cost, we identified an epigenetic cost of generalism that occurs when phage transition between host types. Our results confirm the existence in phi6 of two costs of generalism, genetic and environmental, but they also indicate that the cost is not always large. The possibility for cost-free niche expansion implies that varied ecological conditions may favor host shifts in RNA viruses.
自然系统和实验系统均未能普遍证明泛化与特化之间存在权衡。当权衡确实发生时，如果不剖析适应的遗传基础，就很难将其原因归结为拮抗性多效性，而且之前很少有实验提供这些遗传数据。在此，我们研究了RNA病毒phi6中扩大宿主范围（泛化）的进化，phi6是一个实验模型系统，能够轻松识别适应性突变。我们在三种新型假单胞菌宿主上分别分离出10个自发的宿主范围突变体，并确定这些突变是否会对标准实验室宿主造成适应性代价。测序结果显示，每个突变体在phi6基因P3中都有九个非同义突变之一，该基因在宿主附着过程中起重要作用。这九个突变中的七个在原始宿主上具有代价，证实了拮抗性多效性的存在。除了这种遗传造成的代价外，我们还发现了泛化的表观遗传代价，这种代价在噬菌体在宿主类型之间转换时出现。我们的结果证实了phi6中存在泛化的两种代价，即遗传代价和环境代价，但它们也表明这种代价并不总是很大。无代价生态位扩张的可能性意味着不同的生态条件可能有利于RNA病毒的宿主转移。
