Data from: Host coevolution alters the adaptive landscape of a virus

The origin of new and complex structures and functions is fundamental for shaping the diversity of life. Such key innovations are rare because they require multiple interacting changes. We sought to understand how the adaptive landscape led to an innovation whereby bacteriophage λ evolved the new ability to exploit a receptor, OmpF, on Escherichia coli cells. Previous work showed that this ability evolved repeatedly, despite requiring four mutations in one virus gene. Here, we examine how this innovation evolved by studying six intermediate genotypes of λ isolated during independent transitions to exploit OmpF and comparing them to their ancestor. All six intermediates showed large increases in their adsorption rates on the ancestral host. Improvements in adsorption were offset, in large part, by the evolution of host resistance, which occurred by reduced expression of LamB, the usual receptor for λ. As a consequence of host coevolution, the adaptive landscape of the virus changed such that selection favouring four of the six virus intermediates became stronger after the host evolved resistance, thereby accelerating virus populations along the path to using the new OmpF receptor. This dependency of viral fitness on host genotype thus shows an important role for coevolution in the origin of the new viral function.

全新且复杂的结构与功能的起源，是塑造生命多样性的核心基础。这类关键创新事件极为罕见，因其需要多组相互协同的突变改变。我们旨在解析适应性景观（adaptive landscape）如何推动一项创新的发生：即λ噬菌体（bacteriophage λ）获得了利用大肠杆菌（Escherichia coli）细胞表面受体OmpF的全新能力。此前的研究表明，尽管该功能的演化需要在某一病毒基因中积累4处突变，但其仍可多次独立发生。本研究通过分析6株在独立完成向利用OmpF受体转变的演化过程中分离得到的λ噬菌体中间基因型，并将其与原始祖先株进行对比，探究了该项创新的演化路径。所有6株中间基因型在原始宿主上的吸附率均实现了大幅提升。然而，吸附能力的提升在很大程度上被宿主的抗性演化所抵消——宿主通过降低λ噬菌体常规受体LamB的表达量获得了抗性。由于宿主的协同演化（coevolution），病毒的适应性景观发生了改变：当宿主演化出抗性后，对6株病毒中间基因型中4株的正向选择压力显著增强，进而加速了病毒种群沿新路径演化以利用OmpF受体的进程。由此可见，病毒适合度（fitness）对宿主基因型的依赖关系，证明了协同演化在新型病毒功能起源过程中发挥着至关重要的作用。