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The genetic nature of the incompatibility factors in the higher fungi has been the subject of considerable interest for many years. Multiple-allelic incompatibility systems of two types, bifactorial 1, 2 and unifactorial,3 were described in 1920-1924 and have since been found to be common in all groups of the Basidiomycetes except the rusts and possibly the smuts. The number of alternate factors at the incompatibility loci varies from ten or so in the Gasteromycetes 4, to a hundred or more in the Hymenomycetes. 6, 7 The occurrence among the progeny of a single fruit of occasional non-parental factors, interfertile with both parental factors, was originally interpreted as mutation at the incompatibility loci.8Schizophyllum commune typifies the bifactorial incompatibility system. Alternate incompatibility factors of two series, A and B, assort independently at meiosis to yield progeny of four distinct, self-sterile, mating types.A'A2BB2 -> A1B1, A1B2, A2B', A2B2.Sexual interaction between mycelia belonging to different mating types to yield dikaryotic mycelia is restricted to those conbinations that are heterozygous for both A and B factors. This pattern of interaction is termed tetrapolar sexuality or tetrapolarity. The single-locus nature of the incompatibility factors and the mutative origin of non-parental factors were generally accepted until 1950, when Papazian 9, 10 found evidence, via tetrad analysis, that the A factor of S. commune comprised two or more loci, between which crossing over produced non-parental A factors. The six presumed recombinant classes reported by Papazian9 were interpreted by Raper 11 as reflecting an A factor of at least four loci. VOL. 44, 19D58 889

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