The EA is a very efficiently engineered structure. In its resting position, its organisation is compact, fitting well into the intact epixenosome. During the ejection, it unrolls starting from inside, whereby the layers slip relative to each other. Thus, a tube is formed which fits exactly through the hole of the DZ. The tube penetrates the hole, and reaches the opening of the cell membrane formed as the first step of the ejecting process. As a result, the apical portion of the epixenosome cytoplasm containing the DZ and the IB is translocated away from the ciliate host. At the end of the process, the tube is 40 |m long, and 150 nm in diameter with a spear-shaped head of 2 |im length (Fig. 4C). Since the tube consists of overlapping layers, it remains continuous. The head is devoid of a membrane and encloses the terminal open portion of the tube. The internal fibrils represent the very tip of the whole extruded structure. Some BT are often found associated with the ejected tube (Rosati et al. 1993a). To date, no information is available concerning the chemical nature of the tube wall. However, the tubular shape, together with the ratio between length and diameter, render the tube much more resistant than a completely unwound flat ribbon, independent of its chemical nature. The ejectisomes of the chloromonadine Pyramimonas are the most similar stuctures as they also "form a tube by a spiral rolling of the ribbon" (Kugrens et al. 1994).
Ejectisomes routinely discharge when the water is disturbed in the vicinity of the cells. It has been experimentally demonstrated (Rosati et al. 1997) that the ejecting process in epixenosomes is activated by the detection of external signals through membrane receptors and involve the activation of the adenylate cyclase-cyclic AMP system as a transducing mechanism. The membrane receptors are localised at the top of the epixenosomes, just where the membrane interruption appears as a first step of the whole extrusion process. Membrane receptors have been detected based on their affinity for soybean agglutinin, a lectin that inhibits the ejection in live epixenosomes. On the contrary, ejection was stimulated (and the binding of soybean agglutinin prevented) in the presence of adrenalin. The latter is known to bind to the receptors with a high affinity for the lectin. The presence of receptors for mammalian hormones has already been reported in prokaryotes (Lenard
1992). In addition, the ejection of the epixenosomes is also inhibited by treatments with antitubulin drugs like nocodazole. This has led to the hypothesis that BT are involved in the ejection process (Rosati et al. 1993b).
Which stimuli trigger the ejection in the natural environment? What is the significance of a process in which the genetic material of bacterial epibionts is flung out of the remnant of their cell body and away from the host surface? As a massive ejection was always observed when the ciliate host is dying, it may be hypothesised that under these conditions Euplotidium produces the molecule(s) which stimulate the activation of symbiont ejection. In this case, the ejection would have a dispersive function similar to that of the polar filament in microsporidia spores. However, the reinfection of the epixenosome-free cultures of Euplotidium was unsuccessful. Under natural conditions, ejection may also serve the purpose of regulating the number of epixenosomes during division of the ciliate host. In this case, the ejection would be activated by substances produced by the host during a specific phase of its cell cycle. On the other hand, the apical position of the specific receptors, as well as the observation that the extrusive process can be stimulated or inhibited by the addition of different compounds to the Euplotidium culture medium, may indicate that stimuli are detected from the environment. A third hypothesis, still largely unexplored, is that the synthesis of the extrusive apparatus, like that of some R-bodies, could be determined by viral elements. In this case, the ejection would function as a dispersive tool for the latter.
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