Regulation of surface antigens expression in Paramecium primaurelia: genetic and physiological factors involved in allelic exclusion.

In paramecium aurelia, allelic exclusion can be considered as a basic feature of the surface antigens system in the same way as intergenic exclusion. Our studies on allelic exclusion in G156/G168 heterozygotes show that (1) allelic exclusion does not depend on discrete regulatory genes dispersed throughout the genome; (2) it does not seem to be influenced by cytoplasmic factors; (3) it occurs regardless of the surface antigen expressed by the parental strains at the time of the cross. These results are discussed in relation to both intergenic and interallelic exclusion for which a common basis is proposed.