Interspecific Pair Formation Induced by Natural Mating Reaction in Paramecium

The species of Paramecium are sexually isolated primarily by the specificity of mating reaction. By this reason, it has been thought that interspecific pair formation does not occur when cells of different species are mixed. However we discovered that interspecific pair formation can be induced simply by mixing the mating reactive cells of two species. Among four species of Paramecium, P. tetraurelia, P. multimicronucleatum, P. caudatum, and P. bursaria, the interspecific pairs were observed in the former three species which belong to the “aurelia” group, but those three species did not mate with P. bursaria which belongs to the “bursaria” group. The percentage of interspecific pair formation was less than 10% in all positive cases. Macronuclear fragmentation, one of the remarkable nuclear changes in normal conjugation, was also observed in the interspecific pairs. The time course of the pair formation and macronuclear changes were similar to those of intraspecific conjugation.