Epixenosomes, peculiar epibionts of the ciliated protozoon Euplotidium itoi: what kind of organisms are they?

Epixenosomes live on the dorsal surface of their ciliate host, Euplotidium itoi. They lack a nuclear envelope and divide like prokaryotes. On the other hand they have a morphological and functional cell compartmentalization and possess tubules that are sensible to tubulin inhibitors and positively react with different antitubulin antibodies. In the present paper, as a first step to investigate their real nature, the in situ hybridization technique was applied at the ultrastructural level. Different prokaryotic and eukaryotic probes suitable for detecting rRNA genes were used. An additional test was performed with the gene encoding for {3 tubulin in the ciliate Euplotes crassus. Positive results, evidenced by a precise localization of gold particles, were obtained with all the eukaryotic probes used. These probes were obtained from organisms belonging to three different kingdoms (Protista, Animalia, Plantae). On the contrary, no hybridization was obtained with prokaryotic probes, not even when the probe used was an oligonucleotide complementary to all bacterial 16S rRNA so far sequenced. On the basis of these results and of the other observations so far accumulated, the possible eukaryotic nature of epixenosomes is discussed.