As described above, the association between Euplotidium and epixenosomes appears to be obligatory in the natural environment. In a non-competitive environment, however, the ciliate survives well without its symbionts. Epixenosomes may therefore provide their host with an important ecological advantage, such as defence against predators. This hypothesis was experimentally verified by comparing the behaviour of Litonotus lamella when preying upon E. itoi with and without epixenosomes. L. lamella, a raptorial feeding ciliate that shares its habitat with E. itoi, was chosen because its feeding behaviour is well known (Ricci and Verni 1988; Ricci et al. 1996). It has been observed (Rosati et al. 1999) that, upon direct cell-to-cell contact, L. lamella discharges its toxicysts and thereby paralyses E. itoi both in the presence as well as the absence of epixenosomes. However, while L. lamella is able to ingest prey cells without epixenosomes, it never eats those with epibionts. It can be inferred that the presence of epixenosomes prevents the engulfment of euplotidia stricken by the predator, leading to a high probability of survival.
Indeed, about 60% of these prey cells are capable of resuming a normal behaviour.
How can epixenosomes mediate this defensive function? The possible role of the epixenosomal EA in defence was tested. For this purpose, the feeding behaviour of L. lamella against E. itoi with unaltered epixensomes and against E. itoi with epixenosomes whose ejecting capability was inhibited were compared (Rosati et al. 1997). Ejection was prevented by a treatment with the adenylate cyclase inhibitor alloxan. While, as expected, L. lamella did not ingest untreated euplotidia, it consumed some of those treated. Thus the ejection itself, rather than the mere presence of epixenosomes is critical for the defence. It can be hypothesised that the discharge of the predator toxicysts triggers the ejection and that this process interferes with the recognition of the prey by the predator, which is necessary for finding and engulfing the stricken prey (Rosati et al. 1999). Toxic substances are unlikely to be involved, however, since neither predators nor other ciliate species put in the same container with Euplotidium appear to be inhibited.
The never-ending predator-prey struggle has certainly provided a powerful stimulus for evolution also in the microbial world. Indeed several types of anti-predator defences have been reported for ciliates. It has been demonstrated that trichocyst discharge of Paramecium quickly dislocates the cell out of the range of the offensive organelles of an attacking predator (Miyake and Harumoto 1996). A similar function has been supposed for flagellate ejectisomes (Kugrens et al. 1994). The extrusive process of epixenosomes is involved in the host defence, although, epixenosome ejection does not dislocate Euplotidium.
As far as we know, the only case in ciliates in which protection against predation is provided by symbionts is that of Paramecium bursaria and its endosymbiotic zoochlorellae. The latter organisms in some way discourage predation by Didinium nasutum by releasing distasteful metabolites that repel the latter (Berger 1980). The defensive role of episymbiotic organisms described here is presently the only one reported for ciliates. The defensive function could account for the absence of E. itoi and E. arenarium cells devoid of epixenosomes in the tide pools from which all our experimental organisms have been collected, and very likely is an important factor for stabilising and maintaining such a specialised symbiotic relationship.
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