Host-symbiont coevolution: Exploring the parasitism-mutualism continuum

在自然环境中，不同有机体之间密切而持久的联系--共生--是广泛而重要的。共生关联可以是寄生的，其中一个有机体以牺牲另一个的利益为代价，也可以是互惠互利的，两个有机体都从这种关联中受益。这种共生关系支撑了生态系统的功能：互惠共生可以让成对的有机体在其他情况下都不能生存，而寄生可以限制宿主种群的增长。这些共生体在微生物群落中特别重要，但人们对这些共生体如何影响微生物共生体的进化和多样性知之甚少。在这里，我们想要了解在从寄生到互惠的条件下，共生生物的进化是如何不同的。我们的方法是研究细菌-质粒共生的进化，其中关联的性质根据环境从寄生到互惠：在汞存在的情况下，这种关联是互惠的，因为质粒具有抗汞的基因；而在没有汞的情况下，这种关联是寄生的，因为携带质粒会减缓细菌的生长。在自然界中，携带该质粒的细菌可以在其他有毒的汞污染土壤中生存。我们将利用细菌较短的世代时间和较大的种群规模来观察实际的进化。我们将发现并对比细菌和质粒基因组中发生的遗传变化，因为它们在一系列环境中共同进化，将共生从寄生改变为互惠共生。-我们的研究是新颖的，因为我们将首次使用单一的共生联系来研究寄生-互惠共生连续体的进化。-我们的研究与自然环境有关，因为这种细菌-质粒联系在微生物群落中广泛存在，但人们对此知之甚少，并可能在维持生物多样性和生态系统功能方面发挥重要作用，特别是在应对土壤重金属污染方面。-我们的研究是强大的，因为我们将使用实验方法来研究行动中的进化。-我们的研究是及时的，因为我们将利用DNA测序的最新技术来直接观察细菌和质粒基因组中遗传序列的进化。

Intimate and prolonged associations between different organisms - symbioses - are widespread and important in the natural environment. Symbiotic associations can range from being parasitic, where one organism benefits at the expense of the other, to being mutualistic, where both organisms benefit from the association. Such symbiotic associations underpin the functioning of ecosystems: mutualism can allow pairs of organisms to survive where otherwise neither would and parasitism can limit the growth of host populations. These symbioses are particularly important in microbial communities but little is known about how these associations shape the evolution and diversity of microbial symbionts. Here we want to understand how the evolution of symbiotic organisms is different under conditions that range from parasitic to mutualistic. Our approach is to study the evolution of a bacterial-plasmid symbiosis where the nature of the association ranges from parasitic to mutualistic depending on the environment: in the presence of mercury the association is mutualistic because the plasmid has a gene for mercury resistance; and in the absence of mercury the association is parasitic because carrying the plasmid slows the growth of the bacteria. In nature carrying the plasmid allows bacteria to survive in otherwise toxic mercury contaminated soils. We will exploit the short generation times, and large population sizes of bacteria to observe evolution in action. We will discover and contrast the genetic changes that occur in the genomes of both the bacteria and the plasmid as they co-evolve on a continuum of environments that change the symbiosis from parasitic to mutualistic. - Our study is novel because we will, for the first time, study evolution across a parasitic-mutualistic symbiosis continuum using a single symbiotic association. - Our study is relevant to the natural environment because such bacterial-plasmid associations are widespread, but poorly understood, in microbial communities and are likely to play an important role in maintaining biodiversity and ecosystem function, particularly in response to heavy metal contamination of soils. - Our study is powerful because we will use an experimental approach to study evolution in action. - Our study is timely because we will exploit the latest technologies in DNA sequencing to directly observe evolution of genetic sequences in bacterial and plasmid genomes.