Rapid evolution of tolerance to pesticides under warming in an aquatic invertebrate and its gut microbiome

Do the gut microbiota contribute to rapid evolution in tolerance to stressors in its host? Organisms are increasingly exposed to multiple anthropogenic stressors. Pollutants and warming are among the two most important anthropogenic stressors, that moreover magnify each other’s effects: many pollutants are more toxic at higher temperatures. This is of huge concern both for current risk assessment of pollutants in warmer regions and for future risk assessment under global warming. Two interrelated research topics to improve risk assessment, and more general advance the emerging field of multistressor ecology remain unstudied. First, while organisms may rapidly evolve tolerance to a single stressor, it is largely unknown if organisms are also able to rapidly evolve tolerance against a combination of stressors. Using powerful experimental evolution trials we will select for increased tolerance to a pollutant, to warming, and to their combination. We will thereby test how increased tolerance to one or both stressors shapes how organisms cope with the increased toxicity to the pollutant under warming. To obtain mechanistic insights this will be combined with targeted analyses of the expression levels of genes known to be important in tolerance against each stressor separately. We will also test for consistency in the independent evolution of gene expression patterns in response to the stressors across the replicated evolution trials. Second, recent studies suggest that the gut microbiome may play an important role in generating tolerance against a stressor in host organisms. We will bring this to the next level and test whether the rapid evolution of host tolerance against a pollutant, warming and also their combination is associated with shifts in the gut microbiome communities. Therefore, we will determine the shifts in the composition of the gut bacterial communities and test whether these can be explained by shifts in the presence of tolerant bacteria and associated expression levels of genes involved in tolerance against these stressors. In addition, the identified differentially expressed genes will be compared between the gut microbiota and the host organism. In follow-up tests, we will demonstrate causal links between shifts in gut microbiome community structure and changes in the tolerance of both the gut microbiome and its host. In a first approach, we will rear gut microbial communities of sensitive hosts outside the host under different combinations of exposure to a pollutant and/or to warming, test whether these bacteria evolve tolerance, and when transplanted to sensitive hosts increase tolerance against these stressors in the hosts. In a second approach, we will run reciprocal transplant experiments of gut microbiota and test whether transplanting the gut microbiome of hosts that evolved tolerance to a given stressor into sensitive hosts increases tolerance to that stressor in the sensitive hosts (and vice versa). We will address these innovative research topics at the interface of ecotoxicology, global change biology and microbiology using the water flea Daphnia magna, a keystone species in aquatic food webs and a model organism in ecotoxicology, evolutionary biology and ecological genomics. Several key aspects make it an ideal study system to for the first time address both integrated research topics. This model species has recently been shown to evolve rapidly when exposed to warming and to pollutants separately. Its gut microbiome is well-studied and shown to contribute to the build-up of resistance against xenobiotics. Moreover, Daphnia can produce clonally, making it possible to contrast specific host genotypes and their associated gut microbiome.