EPIXENOSOMES, PECULIAR EPIBIONTS OF THE CILIATED PROTOZOAN EUPLOTIDIUM-ITOI - WHAT KIND OF ORGANISMS ARE THEY

Epixenosomes live on the dorsal surface of their ciliate host, Euploti dium itoi. They lack a nuclear envelope and divide like prokaryotes. O n the other hand they have a morphological and functional cell compart mentalization and possess tubules that are sensible to tubulin inhibit ors 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 lev el. Different prokaryotic and eukaryotic probes suitable for detecting rRNA genes were used. An additional test was performed with the gene encoding for beta tubulin in the ciliate Euplotes crassus. Positive re sults, evidenced by a precise localization of gold particles, were obt ained with all the eukaryotic probes used. These probes were obtained from organisms belonging to three different kingdoms (Protista, Animal ia, Plantae). On the contrary, no hybridization was obtained with prok aryotic probes, not even when the probe used was an oligonucleotide co mplementary to all bacterial 16S rRNA so far sequenced. On the basis o f these results and of the other observations so far accumulated, the possible eukaryotic nature of epixenosomes is discussed.