Ecological drivers of evolutionary transitions in mutualistic symbioses

在自然环境中，不同生物体之间的密切和长期联系-共生-是广泛和重要的。共生的一种主要形式是光合生物的联合，它们从阳光中为宿主提供能量：所谓的光合共生。光共生的例子包括地衣，其中真菌宿主是绿色真菌，珊瑚，其中刺胞动物宿主是虫黄藻。通过光共生，成对的生物体可以在单独的环境中生存，因此，光共生增加了生物多样性，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益处取决于环境条件：例如，在光线充足的栖息地，共生体对宿主非常有益，而在黑暗的环境中，共生体对宿主的维持可能是昂贵的。在这里，我们想了解光强度的环境变化如何塑造光合共生的长期进化。尽管它们具有广泛的重要性，但对光合共生的进化起源知之甚少。可能的原因是地衣和珊瑚是古老的协会，生长非常缓慢，很难在实验室中培养。我们的方法是观察我们在实验室中创建的单细胞真核生物宿主（草履虫）和光合蓝藻共生体（集胞藻）之间的光共生的实时进化。尽管自然界中存在许多草履虫与藻类的共生体，但通过使用“合成”共生体，我们将捕捉共生体从诞生那一刻起的整个进化历史。我们将利用草履虫的短世代时间和大种群规模来观察100代的真实的进化。我们将发现和对比宿主和共生体的适应性，因为它们共同进化，从近暗到明亮的光强度梯度。为了充分了解适应的生理，生化和遗传基础，我们将采用尖端的细胞成像，质谱和基因组测序技术。- 我们的研究是新颖的，因为我们将第一次研究光共生从一开始就在真实的时间里进化了100代-我们的研究与自然环境有关，因为我们测试了影响光共生的一个关键生态变量的环境变化的影响：光强度。此外，我们的研究结果将有助于预测自然光共生对变化环境的反应-我们的研究是强大的，因为我们将使用实验方法来研究真实的时间进化。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来直接观察遗传，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.