Ecological drivers of evolutionary transitions in mutualistic symbioses

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。共生的一种关键形式是涉及光合作用的有机体的联合，它们从阳光中向宿主提供能量：所谓的光共生。光共生的例子包括地衣和珊瑚，地衣中的真菌是绿藻的宿主，珊瑚中的线虫是虫黄藻的宿主。通过光共生，成对的有机体可以在两者都不能单独生存的环境中生存，因此光共生增加了生物多样性，并巩固了生态系统的功能。光共生的一个重要特征是，携带共生体对宿主的好处取决于环境条件：例如，在光照较好的生境中，共生体对宿主非常有利，而在黑暗环境中，共生体对宿主的维持成本可能很高。在这里，我们想要了解环境中光强度的变化如何塑造光共生生物的长期进化。尽管它们具有广泛的重要性，但人们对光共生生物的进化起源知之甚少。可能的原因是地衣和珊瑚是古老的群落，生长非常缓慢，很难在实验室培养。我们的方法是观察我们在实验室创造的单细胞真核生物宿主(草履虫)和光合作用蓝藻共生体(聚球藻)之间的光共生的实时进化。虽然自然界中存在许多草履虫-藻类共生体，但通过使用一种‘人工’共生体，我们将从共生开始的那一刻起捕捉到共生体的整个进化历史。我们将利用草履虫世代时间短、种群规模大的特点，实时观察百代的进化。我们将发现并对比宿主和共生体在从近暗到亮光的光强度梯度上共同进化时的适应情况。为了充分了解适应的生理、生化和遗传基础，我们将使用尖端的细胞成像、质谱学和基因组测序技术。-我们的研究是新颖的，因为我们将第一次实时研究光共生从开始到100s世代的进化-我们的研究与自然环境相关，因为我们测试了影响光共生的一个关键生态变量：光强度的环境变化的影响。此外，我们的发现将有助于预测自然光共生生物对不断变化的环境的反应--我们的研究很强大，因为我们将使用实验方法实时研究进化。-我们的研究是及时的，因为我们将利用DNA测序、生物化学和细胞成像的最新技术，直接观察遗传、生理和生化适应的进化。

Intimate and prolonged associations between different organisms - symbioses - are widespread and important in the natural environment. A key form of symbiosis are associations involving photosynthetic organisms which provide their hosts with energy from sunlight: so called photosymbioses. Examples of photosymbioses include lichens, where a fungus hosts an green alga, and corals, where a cnidaria hosts a zooxanthellae alga. Through photosymbiosis pairs of organisms can survive in environments where neither would alone, therefore photosymbioses increase biodiversity and underpin the functioning of ecosystems. An important feature of photosymbiosis is that the benefits to hosts of carrying symbionts depend upon the environmental conditions: for instance in well-lit habitats symbionts are highly beneficial to hosts whereas in dark environments symbionts may be costly for hosts to maintain. Here, we want to understand how environmental variation in light intensity shapes the long-term evolution of photosymbioses.Despite their widespread importance, little is known about the evolutionary origins of photosymbioses. Possible reasons for this are that lichens and corals are ancient associations and are very slow growing and hard to cultivate in the lab. Our approach is to observe the real-time evolution of a photosymbiosis created by us in the lab between a single-celled eukaryote host (Paramecium) and a photosynthetic cyanobacteria symbiont (Synechocystis). Although many Paramecium-alga symbioses exist in nature, by using a 'synthetic' symbiosis we will capture the entire evolutionary history of the symbiosis from the moment of its inception. We will exploit the short generation times, and large population sizes of Paramecium to observe evolution in real time for 100s of generations. We will discover and contrast the adaptations of both hosts and symbionts that occur as they co-evolve across a gradient of light intensity from near dark to bright light. To fully understand the physiological, biochemical and genetic bases of adaptations we will employ cutting edge cell-imaging, mass spectrometry and genome sequencing technologies. - Our study is novel because we will, for the first time, study the evolution of a photosymbiosis from inception for 100s of generations in real time- Our study is relevant to the natural environment because we test the effect of environmental variation of a crucial ecological variable affecting photosymbioses: light intensity. Moreover, our findings will help to predict responses of natural photosymbioses to changing environments - Our study is powerful because we will use an experimental approach to study evolution in real time.- Our study is timely because we will exploit the latest technologies in DNA sequencing, biochemistry and cell-imaging to directly observe evolution of genetic, physiological and biochemical adaptations.