Recent studies show that soil eukaryotic diversity is immense and dominated by micro-organisms. However, it is unclear to what extent the processes that shape the distribution of diversity in plants and animals also apply to micro-organisms. Major diversification events in multicellular organisms have often been attributed to long-term climatic and geological processes, but the impact of such processes on protist diversity has received much less attention as their distribution has often been believed to be largely cosmopolitan. Here, we quantified phylogeographical patterns in Hyalosphenia papilio, a large testate amoeba restricted to Holarctic Sphagnum-dominated peatlands, to test if the current distribution of its genetic diversity can be explained by historical factors or by the current distribution of suitable habitats. Phylogenetic diversity was higher in Western North America, corresponding to the inferred geographical origin of the H. papilio complex, and was lower in Eurasia despite extensive suitable habitats. These results suggest that patterns of phylogenetic diversity and distribution can be explained by the history of Holarctic Sphagnum peatland range expansions and contractions in response to Quaternary glaciations that promoted cladogenetic range evolution, rather than the contemporary distribution of suitable habitats. Species distributions were positively correlated with climatic niche breadth, suggesting that climatic tolerance is key to dispersal ability in H. papilio. This implies that, at least for large and specialized terrestrial micro-organisms, propagule dispersal is slow enough that historical processes may contribute to their diversification and phylogeographical patterns and may partly explain their very high overall diversity.

Community and Quantitative Ecology Laboratory Department of Biology Concordia University Montreal QC Canada

Department of Ecology and Genetics Evolutionary Biology Centre Uppsala University Uppsala Sweden

Department of Taxonomy and Ecology Faculty of Biology and Geology Babeș Bolyai University Cluj Napoca Romania

Environment University of York York UK

Facultad de Ciencias Naturales y Oceanográficas Departamento de Zoología Universidad de Concepción Barrio Universitario de Concepción Chile

Institute of Ecology and Earth Sciences Natural History Museum University of Tartu Tartu Estonia

Institute of Parasitology Biology Center Czech Academy of Sciences České Budĕjovice Czech Republic

Laboratory of Biogeocenology Institute of Soil Science and Agrochemistry Siberian Branch of Russian Academy of Sciences Novosibirsk Russia

Laboratory of Soil Biodiversity Institute of Biology University of Neuchâtel Neuchâtel Switzerland

Laboratory of Soil Biodiversity Institute of Biology University of Neuchâtel Neuchâtel Switzerland Centro de Investigación en Recursos Naturales y Sustentabilidad Universidad Bernardo O'Higgins Santiago Chile

Laboratory of Soil Biodiversity Institute of Biology University of Neuchâtel Neuchâtel Switzerland Department of Zoology Institute of Biosciences University of São Paulo São Paulo Brazil

Laboratory of Soil Biodiversity Institute of Biology University of Neuchâtel Neuchâtel Switzerland Jardin Botanique de Neuchâtel Neuchâtel Switzerland

Laboratory of Soil Biodiversity Institute of Biology University of Neuchâtel Neuchâtel Switzerland Real Jardín Botánico CSIC Madrid Spain

Laboratory of Wetland Ecology and Monitoring Faculty of Geographical and Geological Sciences and Department of Biogeography and Paleoecology Adam Mickiewicz University Poznań Poland

School of Geography and Earth Sciences McMaster University Hamilton ON Canada

WSL Swiss Federal Institute for Forest Snow and Landscape Research Lausanne Switzerland Laboratory of Ecological Systems Lausanne Switzerland Department of Life Science and Biotechnologies University of Ferrara Ferrara Italy

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