Endosymbiotic interactions in anaerobic protozoa

Several aspects of the endosymbiosis of methanogenic archaea with anaerobic protozoa are reviewed. Special attention is payed to the role of hydrogenosomes and plastid-like organelles that seem to provide the substrates for the methanogenic endosymbionts. Evidence is presented that hydrogenosomes evolved several times in the various protoctistan taxa. Hydrogenosomes are seemingly different, and their common denominator is the production of hydrogen. The absence of nucleic acids and a protein-synthesizing machineny hampers the analysis of their divergent evolutionary history, and molecular genetic data argue not only for different but even a chimeric origin of the hydrogenosomes.