Defensive extrusive ectosymbionts of Euplotidium (Ciliophora) that containmicrotubule-like structures are bacteria related to Verrucomicrobia

Epixenosomes, ectosymbionts on hypotrich ciliates (genus Euplotidium) defen d their host against the ciliate predator Litonotus lamella, Although here only Euplotidium itoi and Euplotidium arenarium from tide pools along a roc ky shore near Leghorn (Ligurian sea) were studied in detail, these epibiont s are certainly present on specimens of E, itoi and on other Euplotidium sp ecies in similar north coastal habitats. The complex life history of epixen osomes has two main stages. In stage I, cells with typical prokaryotic stru cture divide by binary fission. Stage II cells show complex organization wi th different cytoplasmic compartments where an extrusive apparatus within a proteinaceous matrix, although not membrane-bounded. differs from the rema ining cytoplasm. The ejection process is involved in defense; extrusive app aratus is surrounded by a basket consisting of bundles of tubules. These tu bules, 22 +/- 3 nm in diameter, delimited by a wall made up of globular str uctures, are sensitive to inhibitor of tubulin polymerization (nocodazole/4 degrees C temperature) and react positively with different antitubulin ant ibodies, two of which are monoclonal, The prokaryotic vs. eukaryotic nature of epixenosomes was resolved by comparative sequence analysis of amplified small subunit rRNA genes and in situ hybridization with fluorescently labe led rRNA-targeted polynucleotide probes. These unique ectosymbionts are phy logenetically related to Verrucomicrobia. Epixenosomes represent marine sym bionts in this recently discovered division of the Bacteria.