[Mode of serotype inheritance in exoconjugant progeny of the ciliate Dileptus anser]

Two clones of Dileptus anser, originally isolated from natural reservoirs and referred to below as B and D clones, were found to display different serotypes, when cultured under identical laboratory conditions. On being tested with two different polyclonal rabbit immune sera against each particular clone (the classic immobilization test) these clones showed no cross-reaction. At a standard dilution (1:50) and at a standard exposure time (4 h), either of the two immune sera immobilized 100% or commonly 0% of homologous and heterologous clone cells, respectively. In addition, the difference in serotypes was confirmed by the immunofluorescence analysis. By crossing (conjugation) between B (mating type I) and D (mating type III) cells, exconjugant F1 clones were obtained. Their serotypes were then tested (the same immobilization test) with antisera against both the "parental" clones: some clones were tested before their sexual maturation in ca. one month after conjugation, while others were examined in approximately 4 months after conjugation, i.e. after reaching maturity. Each of the F1 clones could react with both immune sera, which means that they possessed the intermediate, "hybrid" phenotype. Five different F1 clones were selected, and each of them was back-crossed to both "parental" clones, B and D. We succeeded in raising 25 exconjugant F2 (B1, to be more exact) clones from F1 x B crosses and 26 clones from F1 x D crosses. The conventional testing of these clones in 5-10 weeks after conjugation provided quite unexpected results, since among them no segregation for "parental" serotypes was observed. Each of the 51 tested clones demonstrated the "hybrid" serotype--seemingly the same as that of F1 clones. Such a non-Mendelian inheritance of the character is hardly to explain from the standard, canonical assumptions on the genetic control of serotype difference between original "parental" clones (different alleles in one locus? different loci?). Also it does not seem likely that the absence of segregation could result from differential survival of various phenotypes in F2 (although the total viability of exconjugant clones appeared rather low). The above data obviously need further confirmations and experimental analyses. We attempt to discuss the obtained results in terms of the epigene hypothesis (Tchuraev, 1975) and in relation to the epigenetic control of serotype expression in species of the Paramecium aurelia complex and in Tetrahymena thermophila, which are "the chosen few" subjects in ciliate genetics.