Many organisms have geographical distributions extending from the tropics to near polar regions or can experience up to 308C temperature variation within the lifespan of an individual. Two forms of evolutionary adaptation to such wide ranges in ambient temperatures are frequently discussed: local adaptation and phenotypic plasticity. The freshwater planktonic crustacean Daphnia magna, whose range extends from South Africa to near arctic sites, shows strong phenotypic and genotypic variation in response to temperature. In this study, we use D. magna clones from 22 populations (one clone per population) ranging from latitude 08 (Kenya) to 668 North (White Sea) to explore the contributions of phenotypic plasticity and local adaptation to high temperature tolerance. Temperature tolerance was studied as knockout time (time until immobilization, T imm ) at 378C in clones acclimatized to either 208C or 288C. Acclimatization to 288C strongly increased T imm , testifying to adaptive phenotypic plasticity. At the same time, T imm significantly correlated with average high temperature at the clones' sites of origin, suggesting local adaptation. As earlier studies have found that haemoglobin expression contributes to temperature tolerance, we also quantified haemoglobin concentration in experimental animals and found that both acclimatization temperature (AccT) and temperature at the site of origin are positively correlated with haemoglobin concentration. Furthermore, Daphnia from warmer climates upregulate haemoglobin much more strongly in response to AccT, suggesting local adaptation for plasticity in haemoglobin expression. Our results show that both local adaptation and phenotypic plasticity contribute to temperature tolerance, and elucidate a possible role of haemoglobin in mediating these effects that differs along a cold-warm gradient.

Significance Intracellular obligate parasitism results in extreme adaptations, whose evolutionary history is difficult to understand, because intermediate forms are hardly ever found. Microsporidia are highly derived intracellular parasites that are related to fungi. We describe the evolutionary history of a new microsporidian parasite found in the hindgut epithelium of the crustacean Daphnia and conclude that the new species has retained ancestral features that were lost in other microsporidia, whose hallmarks are the evolution of a unique infection apparatus, extreme genome reduction, and loss of mitochondrial respiration. The first evolutionary steps leading to the extreme metabolic and genomic simplification of microsporidia involved the adoption of a parasitic lifestyle, the development of a specialized infection apparatus, and the loss of diverse regulatory proteins.

Host-associated microbiota vary across host individuals and environmental conditions, but the relative importance of their genetic background versus their environment is difficult to disentangle. We sought to experimentally determine the factors shaping the microbiota of the planktonic Crustacean, Daphnia magna. We used clonal lines from a wide geographic distribution, which had been kept under standardized conditions for over 75 generations. Replicate populations were kept for three generations at 20 and 28 °C. The interaction of the host clonal line and environment (i.e., temperature) influenced microbiota community characteristics, including structure, the relative abundance of common microbial species, and the microbial richness and phylogenetic diversity. We did not find any correlation between host-associated microbiota and the geographic origin of the clones or their temperature tolerance. Our results highlight the prominent effects that host clonal lineage and its interaction with the environment has on host-associated microbiota composition.

Pathogen genomes provide insights into their evolution and epidemic spread. We sequenced 1,439 SARS-CoV-2 genomes from Switzerland, representing 3-7% of all confirmed cases per week. Using these data, we demonstrate that no one lineage became dominant, pointing against evolution towards general lower virulence. On an epidemiological level, we report no evidence of cryptic transmission before the first confirmed case. We find many early viral introductions from Germany, France, and Italy and many recent introductions from Germany and France. Over the summer, we quantify the number of non-traceable infections stemming from introductions, quantify the effective reproductive number, and estimate the degree of undersampling. Our framework can be applied to quantify evolution and epidemiology in other locations or for other pathogens based on genomic data.