The Genic Control of Mating Types in Paramecium Bursaria.

Jennings"l discovery of a system of multiple mating types in Paramecium bursaria in 1938 followed closely Sonneborn's2 initial description of mating types in P. aurelia. The detailed and brilliant experimental analyses carried out by Jennings3 and directed toward an understanding of mating type inheritance and determination in this organism did not result in a satisfactory explanation for the various phenomena. The new investigations to be reported here, however, permit a resolution of some of the major difficulties. In particular, they demonstrate that the four complementary mating types long recognized for variety 1 are determined by specific combinations of complementary genes at two unlinked loci. Cells whose macronuclei carry dominant genes at both loci are mating type A; mating type C is brought about by the combination of homozygous recessives at both loci. The two alternative combinations of homozygous recessive alleles at one locus and at least one dominant gene at the other determine in a specific way mating types B and D. Jennings considered a similar two-gene hypothesis untenable because it failed to account for certain of his observations; but since the new and more extensive results to be reported here support the hypothesis without ambiguity, an attempt will be made to reconcile, at least formally, the apparent exceptions originally discovered and reported by Jennings. Since in ciliates the microand macronuclei of a clone are formed from mitotic products of a single initial nucleus, it can be inferred that they are isogenic. It has also been established that the micronuclei alone furnish the nuclear apparatus for new clones and that the mating type expressed by a cell is ultimately governed by its macronucleus. Within this framework, two systems of mating type control have been recognized. In Euplotes patella4 and Tetrahymena pyriformis (variety 8),6 (and perhaps Paramecium caudatum),6 conventional genetic analyses reveal that specific mating types are determined by the genes brought together at the origin of the clone; thus, cells with a particular genotype express a predictable phenotype. On the other hand, in P. aurelia,2 P. multimicronucleatum,7 and T. pyriformis (variety 1)8 macronuclei with presumably identical genotypes are characteristically found to govern the expression of different mating types; a single clone commonly contains cells expressing complementary types. Here a given mating type is not determined by a unique genotype per se, but instead the genetically pluripotent macronucleus is induced to express only one of the two (or more) possible types. The mechanism of nuclear induction or "mutation" and final mating type expression have been studied by Sonneborn9 and Nanney.8 Establishment of the two-gene hypothesis for mating type determination in P. bursaria aligns this organism with Euplotes and T. pyriformis (variety 8); as a rule each mating type reflects a specific genie combination. However, in certain exceptional clones, Jennings reported mating type instability ("self-differentiation"),