Determination and inheritance of mating type in Paramecium caudatum.

M A T I N G type determination and inheritance in Paramecium caudatum has been reported only for syngen 12 (HIWATASHI 1958). Inheritance was usually synclonal, i.e. the two clones derived from the two members of a conjugating pair are identical in mating type expression, indicating direct genic control of mating type. However the breeding analysis was incomplete and some of the observations, as will be pointed out below, were difficult to reconcile with simple direct genic control. In the hope that another syngen of this species might be more amenable to analysis and might help understand the difficulties in syngen 12, attention was directed to syngen 3 with the results here to be set forth. As will appear, the needed clarification has been obtained and the main features of the genetics of mating types in both syngens can now be confidently formulated. Special genetic interest attaches to the analysis of mating type genetics in Paramecium and certain other Ciliates because of certain remarkably regular and puzzling deviations from a one to one correspondence between phenotype and genotype (SONNEBORN 1937, 1947; BUTZEL 1955; TAUB 1963; BLEYMAN 1967; NANNEY and CAUGHEY 1953; NANNEY, CAUGHEY and TEFANKJIAN 1955; BARNETT 1966). While one gene may limit expression to one mating type, an allele may permit expression of this and the complementary type in species that have only two mating types per syngen. In species with multiple mating types per syngen, one gene may permit expression of any one of a certain array of mating types while an allele permits expression of a different array of types, with much overlap between the two arrays. In both kinds of species, determination of which mating type will be expressed by cell lineages of identical genotype may occur soon after fertilization by some ununderstood events which “differentiate” the developing macronucleus. Yet in other Ciliates differences of mating type appear to be strictly correlated with differences of genotype, e.g. in Euplotes ( KIMBALL 1942; POWERS 1943; SONNEBORN 1947; HECKMANN 1961 , 1964) , in P . bursaria ( SIEGEL and LARISON 1960), in syngen 13 of P. aurelia ( SONNEBORN 1966), and in syngen 8 of Tetrahymena pyriformis (ORIAS 1963). Both of the latter two species include other syngens that show the peculiarities mentioned above. Some of these peculiarities are found in P. caudatum. This encourages the hope that light may be thrown on them by studies of the system of mating type