Host-parasite interactions in hybridizing Daphnia, from correlations to experiments

富营养化是全球变暖加速的世界性环境问题，影响水生生态系统的稳定，并具有长期后果。我们建议调查富营养化是否以及如何影响两个相互作用的进化过程：疾病传播和种间杂交。我们将研究富营养化对杂交系统中宿主-寄生虫相互作用的影响以及对杂交成功本身的影响。我们将使用水蚤(Daphnia)作为杂交宿主，以鱼孢子虫Caullerya作为病原体。该牵头机构项目结合了对两个湖泊(Müggelsee，DE和Greifensee，CH)的实地调查，大规模池塘实验(CH)，应激暴露的水蚤(CH)的转录图谱，以及分子标记(DE)的开发和应用，这将使我们能够研究杂交水蚤的寄生虫进化。18个实验池塘将接种100个水蚤克隆，然后它们将经历高、中、低营养条件和有无寄生虫。这些克隆是从Greifensee的水柱中分离出来的，或者是从湖泊沉积物中保存的滞育卵孵化出来的，代表了不同的起源年龄、不同的物种和杂种。我们将测试曾经生活在格雷芬塞高度富营养化条件下(20世纪60年代至70年代)的水蚤是否比后来经历更清洁条件的克隆人更好地适应高营养负荷。然后，我们预计富营养化池塘的流行病将会更严重，因为蓝藻的存在似乎促进了Caullerya的感染。我们的目标是为Caullerya寄生虫开发一套多态的分子标记，以便能够测试这种寄生虫是否以菌株特有的方式感染宿主基因型，以及这些关联是否进一步受到营养条件的影响。这些标记将在池塘和实地研究中使用。为了从机制上了解以前观察到的寄生虫流行与蓝藻(富营养化的副产品)水华之间的关系模式，将在有或没有蓝藻的情况下，在寄生虫和无寄生虫的条件下转录表达谱。最后，使用长期现场采集的水蚤，我们将验证病原体的传播是否在蓝藻存在的情况下加剧。这项拟议的研究是瑞士和德国科学团队密切合作的结果，将增进我们对推动自然系统进化过程的环境力量的理解。总体而言，通过专业知识和方法的独特结合，该项目将提高我们对不同营养条件下疾病传播和生物多样性丧失的了解，从而可能为水生生态系统的管理提供工具。

Eutrophication is a worldwide environmental problem accelerated by global warming, affecting the stability of aquatic ecosystems and having long-lasting consequences. We propose to investigate if and how eutrophication affects two interacting evolutionary processes: disease spread and interspecific hybridization. We will study the influence of eutrophication on host-parasite interactions in a hybridizing system and on hybridization success per se. We will use waterfleas (Daphnia) as a hybridizing host and the ichthyosporean Caullerya as a pathogen. This Lead Agency Project is a combination of field surveys of two lakes (Müggelsee, DE and Greifensee, CH), a large scale pond experiment (CH), the transcriptional profiling of stress-exposed Daphnia (CH), and the development and application of molecular markers (DE) that will allow us to study parasite evolution in hybridizing Daphnia. Eighteen experimental ponds will be inoculated with 100 Daphnia clones, which will then experience high, moderate or low trophic conditions and presence or absence of parasites. These clones were isolated from the water column of Greifensee or hatched from diapausing eggs preserved in the lake sediment, representing different ages of origin, and different species and hybrids. We will test whether Daphnia that used to live in Greifensee under highly eutrophic conditions (1960s-70s) are better adapted to high nutrient load than clones that experienced cleaner conditions later on. Then, we expect that epidemics will be greater in eutrophic ponds, because of the presence of cyanobacteria which seem to facilitate Caullerya infection. We aim to develop a set of polymorphic molecular markers for the parasite Caullerya, to be able to test if the parasite infects host genotypes in a strain-specific manner and if these associations are further shaped by trophic conditions. These markers will be used in the pond and in the field studies. To gain a mechanistic understanding of the previously observed correlational pattern between parasite epidemics and blooms of cyanobacteria (a by-product of eutrophication), Daphnia will be transcriptionally profiled under parasite and parasite-free conditions while in the presence or absence of cyanobacteria. Finally, using long-term field collections of Daphnia, we will verify if pathogen spread is intensified under the presence of cyanobacteria. The proposed research, a result of close collaboration between Swiss and German scientific teams, will advance our understanding of the environmental forces driving evolutionary processes in natural systems. Overall, through the unique combination of expertise and methods, this project will improve our understanding of disease spread and biodiversity loss under different trophic conditions and, consequently, might provide tools for management of aquatic ecosystems.