Interspecific mating reactions between Euplotes octocarinatus and Euplotes patella syngen 2.

Euplotes octocarinatus and E. patella syngen 2 were tested for their capacity to undergo interspecific mating reactions. Pheromones which the cells of both species secrete to induce cells of other mating types of the same species to prepare for conjugation by changing their cell surface properties were found to act only species-specific. Cells which have changed their surface properties under the influence of their species-specific pheromones, however, are able to form interspecific conjugant pairs. Our cytological investigations show that the micronuclear processes of about 75% of interspecific conjugant pairs become arrested in the course of the premeiotic division. In these cases the co-conjugants separate from each other and return to the vegetative cell cycle. About 25 % of the interspecific pairs enter meiosis, but the nuclear processes in the co-conjugants are not well synchronized and finally become blocked at metaphase of the first meiotic division. The fate of such co-conjugants is always cell death. The results show that sexual incompatibility between the two species is not only due to different pheromones; in addition, it appears to be necessary that the progress of the nuclear events in a cell is stimulated repeatedly by species-specific signals provided by the co-conjugant.