New results on the genetics of mating types inParamecium bursaria

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Abstract
The four mating types comprising syngen 1 of the ciliated protozoan P. bursaria are determined by specific combinations of pairs of alleles at two independently assorting loci. This hypothesis is verified by extending it to new clones isolated from nature and to clones synthesized in crosses. The results of certain crosses which seemed at first to be entirely exceptional are adequately explained on the basis of an unstable gene at one of the two loci which appears to "mutate" to its recessive allele. In sum, it was shown that mating type I is determined by the genotypes AABB, AABb, AaBB and AaBb; the genotypes controlling type II are aaBB and aaBb; type HI is determined by the double recessive aabb; genotypes AAbb and Aabb bring about type IV. The cytogenetic events of conjugation and the nuclear reorganization of exconjugants are verified using genetic markers at these two loci. No evidence for uniparental nuclear reorganization, autogamy, was found.