Adaptation to environmental stress by evolution of non-genotypic heterogeneity within microbial populations

为了在自然环境中生存，微生物必须能够应对环境变化和扰动所产生的压力。环境压力的来源包括因气候波动和偶尔接触有害元素而产生的自然环境压力。此外，人类活动产生的污染释放出潜在的有毒污染物。人们普遍认为，帮助物种在这种环境压力下生存的一个关键因素是种群内存在的“遗传变异”，这是由同一物种的个体生物之间DNA序列的差异引起的。这种变异意味着个体会有一些略微不同的特征，使得一些种群可能会更好地适应特定的压力条件，从而使物种生存下来。虽然这种遗传变异显然非常重要，但近年来的科学进步表明，还有一个因素对环境压力的生存很重要。对单细胞微生物进行的实验表明，尽管具有相同的DNA组成（即基因一致），但种群中的单个细胞在环境胁迫下的生存能力明显不同。这一发现表明，这种被称为“非基因型异质性”的新的变异来源可能是微生物在自然界中生存和克服环境压力的一个主要因素。然而，迄今为止，几乎所有关于这一主题的研究都是在实验室生物的模型条件下进行的。我们最近由nerc资助的工作首次研究了生活在自然环境中的微生物的非基因型异质性的重要性，并取得了非常有希望的结果。迄今为止，我们的研究表明，在污染栖息地中，具有高水平非基因型异质性的野生酵母比异质性较低的酵母具有竞争优势。此外，当受到环境胁迫时，这些酵母随着时间的推移进化出增加的非基因型异质性。这些微生物生态学的重要突破支撑了我们的新提案。现在的目标是描述这些令人兴奋的发现的更广泛的重要性，并首次在环境胁迫下选择的野生细胞群中揭示非基因型异质性增加的机制。我们有以下三个具体目标。(1)确定我们迄今为止的发现在多大程度上适用于更广泛的领域。这将涉及在环境站点上测试非基因型异质性，这些站点受到相当多样化的选择压力，以及对包括细菌在内的广泛生物体的询问。(2)描述DNA序列的变化，使生物体能够通过进化增加非基因型异质性来适应环境压力。(3)阐明这些DNA序列变化是如何导致异质性增加的。我们将采用尖端的新技术来实现这些目标。这项工作也可能有重要的应用，例如通过开发异质性“标记”作为自然环境中污染物的新型生物报告。该项目的结果有望为非基因型异质性作为物种在环境变化中的生存策略提供重要的新见解。这将极大地帮助我们了解微生物如何在自然环境中持续存在，以及它们如何对人类通过污染造成的有害变化作出反应。

In order to survive in the natural environment, microorganisms must be able to cope with stresses arising from environmental change and perturbation. Sources of environmental stress include natural environmental pressures stemming from climate fluctuations and occasional exposure to harmful elements. In addition, pollution arising from human activities releases potentially-toxic contaminants. It is generally accepted that one key factor which helps species to survive such environmental stresses is the presence of 'genetic variation' within populations, arising from differences in DNA sequence among individual organisms of the same species. This variation means that individuals will have some slightly different characteristics, making it likely that some of the population will be better adapted to withstand particular stressful conditions and allow the species to survive. While such genetic variation is clearly very important, scientific advances over recent years have indicated that there is an additional factor important for the survival of environmental stresses. Experiments with single-cell microorganisms have shown that individual cells within a population have markedly different abilities to survive environmental stresses, despite having the same DNA composition (i.e. being genetically-uniform). This discovery has suggested that this new source of variation, known as 'non-genotypic heterogeneity', may be a major factor allowing microorganisms to survive and overcome environmental stress in nature. However, nearly all of the research into this topic has so far been performed only under model conditions with laboratory organisms. Our recent NERC-funded work has been the first to examine the importance of non-genotypic heterogeneity for microorganisms living in the natural environment, with extremely promising results. Our studies to date have indicated that wild yeasts with high levels of non-genotypic heterogeneity have a competitive advantage over yeasts with lower heterogeneity in polluted habitats. Furthermore, these yeasts evolve the property of increased non-genotypic heterogeneity over time when subject to environmental stress. These important breakthroughs for microbial ecology underpin our new proposal. The objective now is to describe the wider importance of these exciting findings and, for the first time in wild cell populations selected by environmental stress, to uncover the mechanisms that underlie increased non-genotypic heterogeneity. We have three particular aims, as follows. (1) To determine how applicable our findings-to-date are more broadly. This will involve testing non-genotypic heterogeneity at environmental sites subject to quite diverse selection pressures as well as interrogation of a wide range of organisms, including bacteria. (2) To characterise the changes in DNA sequence that allow organisms to adapt to environmental stress by evolving increased non-genotypic heterogeneity. (3) To elucidate how these DNA sequence changes cause increased heterogeneity. We will adopt cutting-edge new techniques to achieve these objectives. The work could also have important applications, for example through the development of heterogeneity "markers" as novel biological reporters of pollutants in the natural environment. The results of the proposed project are anticipated to provide major new insights into non-genotypic heterogeneity as a survival strategy for species during environmental change. This will help greatly in our understanding of how microorganisms persist in the natural environment, and how they may react to harmful changes caused by humans through pollution.