Anaerobic Ciliates and Their Methanogenic Endosymbionts

Many anaerobic ciliates possess hydrogenosomes, and consequently, they have the potential to host endosymbiotic methanogens. The endosymbiotic methanogens are vertically transmitted and even the cyst stages carry methanogens. Accordingly, the analysis of the SSU rRNA genes of ciliates and their methanogenic endosymbionts revealed that the endosymbionts are specific for their hosts and not identical with free-living methanogens. Notably, the endosymbionts of a monophyletic group of ciliates that thrive in either freshwater environments or intestinal tracts are substantially different. Ciliates from freshwater sediments host methanogens belonging to the Methanomicrobiales, while ciliates thriving in the intestinal tracts of cockroaches, millipedes and frogs host methanogens that belong to the Methanobacteriales. Comparative analysis of free-living and gut-dwelling ciliates and their corresponding endosymbionts reveals only a limited co-evolution suggesting infrequent endosymbiont replacements. Such an endosymbiont replacement is clearly the reason for the very distant endosymbionts of free-living and gutdwelling ciliates: the endosymbionts are related to the methanogens in the particular environments, in which the hosts live. J.H.P. Hackstein IWWR, Faculty of Science, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, Netherlands e-mail: [email protected] J.H.P. Hackstein (ed.), (Endo)symbiotic Methanogenic Archaea, Microbiology Monographs 19, DOI 10.1007/978-3-642-13615-3_2, # Springer-Verlag Berlin Heidelberg 2010 13