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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.

Summary In the present study the variations of the epixenosomal band and the behavior of epixenosomes during the divisional morphogenesis of Euplotodium itoi have been analyzed. From the results obtained it arises that the multiplication, the precise redistribution between the two daughter E. itoi cells, and the transformation of epixenosomes from form I to form II are all processes well coordinated with the reproductive cycle of the ciliate. Euplotidium can live and divide even without epixenosomes. It appears, however, that not only E. itoi has a strong influence on epixenosomes but that the epixenosomes themselves also influence the ciliate host in some way.