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Bacteria of the genus Holospora belong to obligatory endonucleobionts of ciliates of the genus Paramecium. The bacteria show specificity towards the particular host species and the types of nuclei they infect: macro- or micronuclei. During a long-term screening of P. bursaria clones, belonging to three different syngens, Holospora inhibited cells of two syngens only. Using the number of host clones and symbiont isolates, it was shown that H. curviuscula was unable to pass successfully through the syngen barrier even under experimental infection. Considering the species level of specificity in Holospora associations of P. caudatum we suggest the existence of a greater evolutionary divergence in P. bursaria syngens than in syngens of P. caudatum. We have revealed that in incompatible combinations "host clone--symbionts isolate" the complicated bacterial life cycle may be blocked at definite stages depending on genetic features of both partners. Thus, the recognition of the full block spectrum could break the continuous infection process down to independently controlled steps. The block spectrum revealed in the system of P. bursaria--H. curviuscula demonstrates its significant similarity to block spectra of other systems within the Holospora--Paramecium complex. A block of transverse binding formation has been first revealed in Holospora dividing in the nucleus.

The work is the first attempt to study nuclear dualism of ciliates with ultraviolet microbeam (UV-beam), which was not applied earlier for these purposes. The special advantage of the UV-beam is that it allow to inactivate selectively of the particular elements of nuclear apparatus of living ciliates is to observe consequences of operation on distant descendants of irradiated cell. On Paramecium caudatum the ways of UV-beam have been devised specifically. The original rotocompression chamber was employed for immobilization of cell in space of irradiation. The effective UV-doses were selected, the relative resistance of cytoplasm, macro-, micronucleus (Mi), and symbiotic bacteria of Mi -- omega particles -- was detected. The "pure" and infected cells were irradiated, in some different variants. The effect of UV-beam depends on dose and consists in a reduction the effectiveness of cell cloning, the loss of omega-particles from Mi, the appearance of the cells with 2--3 Mi, and, possibly, of amicronucleate cells. The injuries due to UV-beam are restored by photoreactivation of visible light.

Particles of a newly described endosymbiont of Paramecium caudatum, clone M-115, here referred to as iota--particles, are Gram--negative bacteria. The symbionts are only present within macronuclei and not in the cytoplasm or in the micronuclei. The cells of clone M-115 iota do not display any killer-effect when mixed with "clean" cultures of Paramecia. In life cycle of the symbiont, there is an alternation of two morphological forms: a spindle-shaped form 2.0--2.5 microns long and a rod-shaped form -- about 18 microns long. The data obtained revealed a high ability to infect cell of "clean" clones of P. caudatum with iota-particles. The antagonistic relationships between two different species of endonucleosymbionts: iota- and omega-particles, are detected. The revealed biological features of iota can be used for studying the mechanisms of nuclear differentiation in ciliates.