Bull J J, Molineux Ian J
Department of Zoology, University of Texas, Austin, TX, 78712, USA.
Department of Microbiology, University of Texas, Austin, TX, 78712, USA.
Evolution. 1992 Aug;46(4):882-895. doi: 10.1111/j.1558-5646.1992.tb00606.x.
The evolution of cooperation was studied in an empirical system utilizing a parasitic bacteriophage (f1) and a bacterial host. Infected cells were propagated by serial passage so that a phage could increase its representation among infected hosts only by enhancing the rate of growth of its host. Loss of infectivity was therefore without selective penalty, and phage benevolence could potentially evolve through a variety of genetic changes. The infected hosts evolved to grow faster over the course of the study, but the genetic bases of this phenotypic change were more difficult to anticipate. Two fundamentally different types of genetic changes in the phage were revealed. One involved the loss of some phage genes, resulting in a noninfectious plasmid that continued to replicate via the parental phage replicon. The second change involved integration of the phage genome into host DNA by a process that, at low frequency, could be reversed to produce infectious phage particles. Integration is a previously unknown property of wild-type f1, and in the system studied, may have resulted from the use of a phage bearing an insert containing nonfunctional DNA. The evolution of this novel function apparently depended only on the presence of a small region in the phage genome that provided some homology to the host DNA, with the host providing all necessary functions. Although f1 is one of the simplest phages known, these observations suggest that host-parasite interactions of the filamentous phages are more complicated than previously thought. More generally, the f1 system offers a useful model for many problems concerning the genetic basis of adaptation.
利用寄生噬菌体（f1）和细菌宿主的实验系统研究了合作的进化。受感染的细胞通过连续传代进行繁殖，这样噬菌体只有通过提高其宿主的生长速率才能增加其在受感染宿主中的占比。因此，感染力的丧失没有选择压力，噬菌体的利他行为可能通过多种基因变化而进化。在研究过程中，受感染的宿主进化得生长更快，但这种表型变化的遗传基础更难预测。研究揭示了噬菌体中两种根本不同类型的基因变化。一种涉及一些噬菌体基因的丧失，产生一种非感染性质粒，该质粒继续通过亲本噬菌体复制子进行复制。第二种变化涉及噬菌体基因组通过一种过程整合到宿主DNA中，这种过程在低频情况下可以逆转以产生感染性噬菌体颗粒。整合是野生型f1以前未知的特性，在所研究的系统中，可能是由于使用了携带含有无功能DNA插入片段的噬菌体导致的。这种新功能的进化显然仅取决于噬菌体基因组中一个小区域的存在，该区域与宿主DNA具有一些同源性，而宿主提供所有必要功能。虽然f1是已知最简单的噬菌体之一，但这些观察结果表明丝状噬菌体的宿主 - 寄生虫相互作用比以前认为的更复杂。更普遍地说，f1系统为许多关于适应遗传基础的问题提供了一个有用的模型。
