GENETICS OF THE ST SEROTYPE SYSTEM IN TETRAHYMENA PYRIFORMIS, SYNGEN 1

Genetic analyses using lines of Tetrahymena pyroformis manifesting different serotypes indicate that the St serotypes are governed by alleles at a single genetic locus. These alleles are termed St(A) and St(C). The St locus is not closely linked to any of the other well-studied loci examined. Differentiation in St(A)/St(C) heterozygotes follows a pattern very similar to that observed with lines heterozygous at the other loci. Initially both alleles are expressed, but as the synclone divides, lines develop that manifest one allele or the other but not both. The time of differentiation is very early in the clonal life cycle, and the output ratio is eccentric. The pattern of development of the St locus places it in a category with the mating type and H serotype loci.