Most of the genetic, cellular, and biochemical diversity of life rests within single-celled organisms - the prokaryotes (bacteria and archaea) and microbial eukaryotes (protists). Very close interactions, or symbioses, between protists and prokaryotes are ubiquitous, ecologically significant, and date back at least two billion years ago to the origin of mitochondria. However, most of our knowledge about the evolution and functions of eukaryotic symbioses comes from the study of animal hosts, which represent only a small subset of eukaryotic diversity. Here, we take a broad view of bacterial and archaeal symbioses with protist hosts, focusing on their evolution, ecology, and cell biology, and also explore what functions (if any) the symbionts provide to their hosts. With the immense diversity of protist symbioses starting to come into focus, we can now begin to see how these systems will impact symbiosis theory more broadly.

Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada

Institute of Parasitology Biology Centre Czech Academy of Sciences 370 05 České Budějovice Czech Republic

Institute of Parasitology Biology Centre Czech Academy of Sciences 370 05 České Budějovice Czech Republic; Faculty of Science University of South Bohemia 370 05 České Budějovice Czech Republic

Okinawa Institute of Science and Technology Okinawa 904 0495 Japan; Department of Botany University of British Columbia Vancouver BC V6T 1Z4 Canada

References provided by Crossref.org

A unique symbiosome in an anaerobic single-celled eukaryote

Genome reduction and horizontal gene transfer in the evolution of Endomicrobia-rise and fall of an intracellular symbiosis with termite gut flagellates

Multiple and frequent trypanosomatid co-infections of insects: the Cuban case study

Methanogenic symbionts of anaerobic ciliates are host and habitat specific

Divergent marine anaerobic ciliates harbor closely related Methanocorpusculum endosymbionts

Gene Transfer Agents in Bacterial Endosymbionts of Microbial Eukaryotes

MIxS-SA: a MIxS extension defining the minimum information standard for sequence data from symbiont-associated micro-organisms

Organellar Evolution: A Path from Benefit to Dependence

Editorial: Symbioses Between Protists and Bacteria/Archaea