Multiple acquisition of methanogenic archaeal symbionts by anaerobic ciliates

Anaerobic heterotrichous ciliates (Armophoridae and Clevelandellidae) posse ss hydrogenosomes that generate molecular hydrogen and ATP. This intracellu lar source of hydrogen provides the basis for a stable endosymbiotic associ ation with methanogenic archaea. We analyzed the SSU rRNA genes of 18 heter otrichous anaerobic ciliates and their methanogenic endosymbionts in order to unravel the evolution of this mutualistic association. Here, we show tha t the anaerobic heterotrichous ciliates constitute at least three evolution ary lines. One group consists predominantly of gut-dwelling ciliates, and t wo to three, potentially four, additional clades comprise ciliates that thr ive in freshwater sediments. Their methanogenic endosymbionts belong to onl y two different taxa that are closely related to free-living methanogenic a rchaea from the particular ecological niches. The close phylogenetic relati onships between the endosymbionts and free-living methanogenic archaea argu e for multiple acquisitions from environmental sources, notwithstanding the strictly vertical transmission of the endosymbionts. Since phylogenetic an alysis of the small-subunit (SSU) rRNA genes of the hydrogenosomes of these ciliates indicates a descent from the mitochondria of aerobic ciliates, it is likely that anaerobic heterotrichous ciliates hosted endosymbiotic meth anogens prior to their radiation. Therefore, our data strongly suggest mult iple acquisitions and replacements of endosymbiotic methanogenic archaea du ring their host's adaptation to the various ecological niches.