Analysis of natural diversity of symbiotic relationships in the Paramecium bursaria--Holospora curviuscula system

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.