Evolution of phenotypic plasticity in an emerging pathogen

人们越来越担心，由于我们过度使用抗生素，毒性更强、耐药性更强的病原体的进化将很快导致一场全球危机。进化医学的进步主张，开发有效的抗生素替代品的关键在于更好地了解病原体的行为和进化方式，特别是对宿主防御的反应。例如，我们对细菌实时调整其行为（复制率）以应对不同程度宿主耐药性的能力一无所知，也不知道这对疾病传播的影响。这种能力将从根本上改变我们对宿主-病原体相互作用的理解。特别是，因为病原体复制的速度会影响它们的毒力（即它们对宿主造成的伤害程度），一种预测是，这种兼性行为实际上会降低毒力进化的速度，这对人类、牲畜和野生动物的抗生素项目有明显的影响。在这里，我们提供了病原体兼性复制率的原因和后果的第一次调查，通过使用一个特别充分记录的，新出现的鸡败支原体（Mg）的传染性爆发，它最近从家禽跳到一种野生北美鸣禽，家雀。这次暴发特别严重，导致数亿雀类死亡，尽管宿主耐药性在短短10年内变得普遍。细菌在新雀寄主定植后所经历的环境变化，以及随后在抗性传播期间所经历的环境变化，代表了理论上应该引起行为灵活性的典型成分，称为可塑性。我们利用野生家雀的新型感染实验，结合尖端的分子技术，来测试Mg在雀类流行过程中可塑性调节复制率的能力是如何进化的，并确定这种可塑性的环境线索和遗传基础。这个系统允许对自然种群的可塑性进化进行罕见的调查，原因有二。首先，我们可以获得在动物流行病暴发和随后宿主耐药性传播期间取样的Mg菌株的全面收集。因此，有可能使用这些不同的Mg菌株进行实验感染，以测量菌株之间的可塑性差异，并测试可塑性在野外如何演变。其次，我们可以使用抗生素、疫苗和免疫抑制剂来实验性地操纵野生捕获的雀类的抗性水平，从而重现Mg在动物流行过程中所经历的环境条件。具体来说，我们将回答以下四个问题。(1)病原体对宿主抗性反应的可塑性是否对病原体有利，因为它允许病原体在被免疫系统清除之前感染更多的次级宿主？(2)随着新宿主的定殖以及随后对宿主抗性扩散的反应，可塑性是如何进化的？这个问题将使我们能够测试环境中的突然变化（即新宿主的定植）和/或逐渐的环境变化（即宿主抗性的传播）是否驱动了病原体可塑性的进化。(3)细菌诱发表型可塑性的环境线索是什么？众所周知，细菌能感知环境中其他细菌分泌的分子。然而，它们是否使用细菌密度或细菌压力的信号来评估其环境质量尚不清楚。(4)可塑性的遗传基础是什么？这个问题将通过最新的基因测序技术来确定不同Mg菌株之间可塑性差异的基因和过程。

There is growing concern that the evolution of more virulent and more resistant pathogens in response to our overuse of antibiotics will soon lead to a global crisis. Advances in evolutionary medicine have advocated that the key to developing effective alternatives to antibiotics relies on an improved understanding of how pathogens behave and evolve, in particular in response to host defences. For example, nothing is known about the ability of bacteria to facultatively adjust their behaviour (replication rates) in real time in response to different levels of host resistance, or the consequences of this for disease transmission. Such ability would radically change our understanding of host-pathogen interactions. In particular, because the rate at which pathogens replicate has repercussions for their virulence (i.e., how much damage they do to a host), one prediction is that this facultative behaviour will actually reduce the speed at which virulence evolves, with obvious implications for antibiotics programs in humans, livestock and wildlife. Here we provide the first investigation of the causes and consequences of facultative replication rates in pathogens by using a particularly well-documented, emerging infectious outbreak of the bacterium Mycoplasma gallisepticum (Mg), which recently jumped from poultry into a wild North American songbird, the House finch. This outbreak was particularly severe, leading to the death of hundreds of millions of finches, although host resistance became widespread within just 10 years. The environmental changes experienced by the bacteria upon colonisation of the novel finch host, and subsequently during the spread of resistance, represent the typical ingredients that should theoretically give rise to behavioural flexibility, termed plasticity. We use novel infection experiments of wild-caught house finches, combined with cutting-edge molecular techniques, to test how the ability to plastically adjust replication rates evolved in Mg over the course of the finch epizootic and to identify the environmental cue and genetic basis of this plasticity. This system allows a rare investigation of the evolution of plasticity in natural populations for two reasons. First, we have access to a comprehensive collection of Mg strains sampled at epizootic outbreak and subsequently during the spread of host resistance. It is therefore possible to conduct experimental infections using these different strains of Mg to measure differences in plasticity among strains and to test how plasticity evolves in the wild. Second, we can use antibiotics, vaccines and immune-suppressants to experimentally manipulate the level of resistance of wild-caught finches and thereby recreate the environmental conditions experienced by Mg over the course of the epizootic. Specifically, we will answer the following four questions. (1) Is pathogen plasticity in response to host resistance beneficial for the pathogen in that it allows the pathogen to infect more secondary hosts before it is cleared by the immune system? (2) How does plasticity evolve following colonisation of a novel host and, subsequently, in response to the spread of host resistance? This question will allow us to test whether an abrupt change in the environment (i.e., colonisation of a new host) and/or whether gradual environmental changes (i.e., spread of host resistance) drive the evolution of pathogen plasticity. (3) What is the environmental cue used by bacteria to elicit phenotypic plasticity? Bacteria are known to sense molecules secreted by other bacteria in the environment. Whether they use signals of bacterial density or of bacterial stress to assess the quality of their environment, however, is unknown. (4) What is the genetic basis of plasticity? This question will be determined using the very latest genetic sequencing technology by identifying genes and processes underlying difference in plasticity between different strains of Mg.