Crossing the divide: population genomics of adaptation to salinity in a model protist.

单细胞生物（微生物）对全球生态系统的健康和功能至关重要。特别是，海洋微生物保证了约50%的行星氧气的生产，因此与世界热带雨林一样重要，并为海洋食物网提供燃料，从而维持对人类至关重要的渔业。了解海洋微生物种群的动态显然很重要，例如，了解微生物种群的大小和边界，决定这些种群的因素，以及哪些环境特征影响区域之间的扩散。然而，由于其体积小，几乎不可能直接跟踪自然环境中的海洋微生物并研究这些参数，其生态学和进化的这一方面仍然研究不足，而是因为自由生活的海洋微生物被认为分布广泛，种群规模大，在大多数海洋中缺乏明显的扩散障碍，人们普遍假设，海洋微生物种群分布广泛，而且在很远的距离上具有遗传多样性和同质性。然而，这种种群结构模型过于简单：最近的研究发现，海洋微生物具有复杂的种群结构，例如，由洋流决定。在许多海洋物种中，特别是在鱼类等经过充分研究的群体中，有证据表明，环境（如盐度）对构成独特种群的因素有很大影响，特别是通过选择特定数量和类型的遗传多样性。这种对环境的适应可能会阻止个人成功地迁移到其他地方（即迁移到他们不适合的环境），反过来也会阻止来自环境不同的其他地区的个人移民。因此，了解种群对环境的适应对于预测种群对干扰的响应和未来环境变化至关重要。事实上，“环境在选择某些类型的遗传多样性（即驱动种群适应）方面的效率如何？我们建议在北海-波罗的海过渡区收集自由生活的海洋微生物Oxyrrhis marina的样本，那里的盐度沿着500公里的横断面从盐水（~32 psu）条件变化到微咸水（<10 psu）。我们选择这个区域作为环境梯度是研究适应性分化的有效手段，也因为盐度是海洋进化最重要的非生物驱动因素之一：许多研究揭示了北海和波罗的海海洋鱼类种群之间的强烈遗传差异，以及波罗的海种群遗传多样性的典型减少。通过对我们的样本进行大量基因（350个）的测序，我们可以量化遗传多样性的数量和类型如何沿着沿着这个环境梯度变化。这些数据不仅使我们能够确定海洋微生物具有特定种群边界的可能性，而且它们表明海洋微生物种群，甚至DNA的特定区域如何直接受到环境条件的影响。这将使人们更好地了解海洋微生物种群的动态和进化机制及其承受环境变化的能力。

Single celled organisms (microbes) are extremely important to the health and function of global ecosystems. In particular, marine microbes assure the production of about 50% of the planetary oxygen, thus are as important as the world's rainforests, and fuel the marine food web, thereby maintaining the fisheries that are essential to mankind. It is clearly important to understand the dynamics of marine microbial populations-for example, to know the sizes and boundaries of microbial populations, the factors that determine these populations, and what environmental features affect dispersal between areas. However, becase of their small size it is almost impossible to directly track marine microbes in the natural environment and study these parameters and this aspect of their ecology and evolution remains understudied.Rather, because free-living marine microbes are assumed to have broad distributions and large population sizes, and there are a lack of obvious barriers to dispersal in most seas, there is the common assumption that marine microbial populations are widespread and also genetically diverse and homogeneous over large distances. However, this model of population structure is too simplistic: recent studies have discovered that marine microbes have complex population structures, for example, determined by the ocean currents. In many marine species, especially well-studied groups such as fish, there is evidence that the environment (e.g. salinity) has a strong effect upon what constitutes a distinct population, particularly by selecting for a specific amount and type of genetic diversity present. Such adaptation to the environment can prevent individuals from successfully moving elsewhere (i.e. to an environment that they are not suited to) and conversely prevent immigration by individuals from other areas that have different environments. Hence, understanding population adaptation to the enviroment is crucial to predict response to disturbance and future environment change. Indeed, the question of "how efficient is the environment in selecting for certain types of genetic diversity (i.e. driving population adaptation)?" has not been studied for any species of marine microbe: this represents a fundamental gap in our knowledge.We propose to collect samples of the free-living, marine microbe, Oxyrrhis marina, across the North Sea-Baltic Sea transition zone, where salinity changes along a 500 km transect from saline (~32 psu) conditions to brackish water (<10 psu). We selected this region as environmental gradients are an efficient means of studying adaptative divergence and also because salinity is one of the most important abiotic drivers for evolution in the oceans: many studies have revealed strong genetic differences between marine fish populations in the North Sea and the Baltic Sea, as well as a typical reduction in genetic diversity in Baltic Sea populations. By sequencing our samples at a large number of genes (350), we can quantify how the amount and type of genetic diversity changes along this environmental gradient. Not only do these data allow us to determine the potential for marine microbes have specific population boundaries, but they indicate how marine microbial populations, and even how specific regions of DNA, can be directly affected by the environment conditions. This will provide a better understanding of the dynamics and evolutionary mechanisms of marine microbial populations, and their capacity to withstand environment change.