New results on the genetics of mating types inParamecium bursaria

Abstract

1. The four mating-types I, II, III, IV known forP. bursaria, syngen 1 are determined by specific combinations of dominant and recessive alleles at two unlinked loci. Mating-type I is formed by cells with the genotypesAABB, AABb, AaBBandAaBb; type II is controlled by the genotypesaaBBandaaBb; the double recessive,aabbis type III, genotypesAAbbandAabbbring about type IV.2. Clones which are exceptions to these rules for mating-type determination occur in low frequency (2–3%) in one line of descent. All can be explained by assuming either a mutation of dominant geneBto its recessive allelebor to a loss of theBlocus. The data suggest that in four clones, themacronuclear but not themicronuclear genotype is affected; in two clones both nuclei are aberrant.3. The cytogenetic events of conjugation were verified by the use of the new genetic markers. No evidence for uniparental nuclear reorganization, autogamy, was found.