THE GENIC CONTROL OF MATING TYPES IN PARAMECIUM BURSARIA

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Abstract
Multiple mating types have long been known in Paramecium bursaria. It has now been established that a system of complementary genes at 2 unlinked loci controls each of the mating types in variety 1. Thus specific genotypes can be assigned to most, but not all, clones. The exceptions do not necessarily invalidate the two-locus hypothesis for they may represent instances of altered genic expression or dominance. If so, the mechanism of mating type determination in P. bursaria requires the operation of both "genetic" and "epigenetic" control systems. An evolutionary sequence originating in the genetic control of mating type, as in Euplotes patella, through the condition exemplified by P. bursaria and Tetrahymena pyriformis (variety 8), to epigenetic control, as in P. aurelia, P. multimicronucleatum, and T. pyriformis (variety 1), may be suggested.