DISSERTATION RESEARCH: Characteristics of viral host niche evolution

虽然生物体有时可以扩大它们生活的环境范围，但促进扩张的遗传和生态条件尚不清楚。这里提出了两个实验，将使用病毒及其细菌宿主紫丁香假单胞菌来研究这些条件。这两个实验都将使病毒暴露于新的宿主，并确定病毒种群进化以使用新宿主的可能性的两个关键决定因素：1)使病毒适应这些宿主的突变数量，以及2)这些突变对病毒正常宿主的成本。在第一个实验中，将在一些新宿主上分离宿主范围突变体，以确定新宿主与正常宿主之间的相似性是否会影响宿主范围扩大的可能性。在第二个实验中，允许在新宿主上生长的突变将与使病毒进一步适应新宿主的突变进行比较。这种比较将使调查人员能够确定这两种生态情景在环境范围扩大的可能性上是否不同。了解促进有机体环境范围扩大的条件长期以来一直是生态学和进化生物学中的一个基本问题，对于解决诸如新宿主群体中出现疾病(如SARS何时在人类群体中出现)等应用问题也具有重要意义。虽然理论研究为解决这一问题提供了一个直观的框架，但调查影响范围扩大可能性的因素的实证工作相对较少。这项研究试图检查一些生态和遗传因素，这些因素被预测为影响环境范围扩大的可能性。

While organisms can sometimes expand the range of environments they live in, the genetic and ecological conditions which facilitate expansion are unclear. Proposed here are two experiments which will investigate these conditions using a virus and its bacterial host, Pseudomonas syringae. Both experiments will expose the virus to new hosts, and determine two critical determinants of the likelihood that viral populations will evolve to use the new host: 1) the number of mutations that adapt the virus to these hosts, and 2) the costs these mutations have on the virus' normal host. In the first experiment, host range mutants will be isolated on a number of new hosts to determine if the similarity between the new hosts and the normal host influences the likelihood of host range expansion. In the second experiment, mutations that allow growth on a new host will be compared to mutations that further adapt the virus to this new host. This comparison will enable the investigators to determine if these two ecological scenarios differ in their likelihood of environmental range expansion. Understanding the conditions that facilitate the expansion of an organism's environmental range has long been a fundamental question in ecology and evolutionary biology, and also has importance for addressing applied questions such as the emergence of disease in new host populations (as when SARS emerged in human populations). While theoretical studies have provided an intuitive framework to address this issue, there has been relatively little empirical work investigating factors that influence the likelihood of range expansion. This study seeks to examine some of the ecological and genetic factors that are predicted to influence the likelihood of environmental range expansion.