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Well-established mutualistic associations between ciliates and prokaryotes might be more widespread and diversified than so far supposed

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Four different associations between ciliates of the family Euplotidae and bacteria have been studied. The ectosymbiotic relationship between ciliates of the genus Euplotidium and epixenosomes, (bacteria related to Verrucomicrobia), offers cues for ecological and evolutionary studies. Epixenosomes, although not vital for their host, confer on them a selective advantage, i.e. defense against predation. Epixenosomes have a compartmentalized cell and possess microtubule-like structures. Interestingly, the presence of two tubulin-like genes has been reported in members of the free-living verrucomicrobial genus Prosthecobacter. We obtained evidence that well-known Polynucleobacter bacteria, upon which a group of fresh-water Euplotes species depends, interfere with the glycogen metabolism of their hosts. Euplotes magicirratus is no longer able to digest food-organisms and is destined to die, once deprived of its specific prokaryotic endosymbiont, likely a new species of the alpha-proteobacterial genus Devosia. A further intriguing finding is the regular presence of rickettsiae in the cytoplasm of a marine Diophrys species. This is the first report of these bacteria in protozoa. This short list suggests that specialized mutualism is more widespread in the phylum than so far supposed.

Document Type: Research Article


Affiliations: Dipartimento di Etologia, Ecologia, Evoluzione, Università di Pisa via A. Volta 4, 56126 Pisa, Italy;, Email:

Publication date: December 1, 2003


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