Sexual and Somatic Recombination in Common-ab Heterokaryons of Schizophyllum Commune.

NIEP (1918) and BENSAUDE (1918) discovered sexual incompatibility in the higher Basidiomycetes. KNIEP (1920) established that strains must differ at two unlinked incompatibility factors, A and B, to permit formation of a fully compatible heterokaryon, the dikaryon. Since the fruiting of a dikaryon produces progeny of four mating types, this pattern of sexuality is called tetrapolar incompatibility (KNIEP 1922). The common-B heterokaryon, formed by two strains with the same B factor but different A factors, was recognized morphologically by BRUNSWIK (1924), although such heterokaryons were for some time thought to be illegitimate dikaryons. The nuclear behavior of common-B heterokaryons has been subsequently examined (QUINTANILHA 1935; TAKEMARU 1959a,b; PARAG 1960; PARAG and RAPER 1960; SWIEZYNSKI and DAY 1960b). The distinctive morphology of common-A heterokaryons of Schizophyllum commune was described by PAPAZIAN (1950, 1951). Further investigations of these heterokaryons have been reported (RAPER and SAN ANTONIO 1954; PAPAZIAN 1956, 1958; RAPER, SAN ANTONIO and MILES 1958; SNIDER and RAPER 1958, 1964, DICK 1960; DICK and RAPER 1961), as have studies of the heterokaryons of another tetrapolar Hymenomycete, Coprinus lagopus ( SWIEZYNSKI and DAY 1960a, b) . The existence of common-AB heterokaryons, in which both mating type factors are identical in the parental homokaryons, had been suspected in S. commune on the basis of preliminary experimental results (RAPER and SAN ANTONIO 1954). Since the evidence for these heterokaryons was inconclusive, experiments to test for and to characterize these heterokaryons were performed. The presence of the two parental nuclear types in the same cytoplasmic system might be demonstrated in three ways by using strains which carry nonallelic nutritional deficiencies: (1) the isolation of hyphal tips which produce mycelia on minimal medium; (2) the recovery from a single fruit body of segregants carrying genetic markers from both parents; and ( 3 ) the discovery of vegetatively recombinant nuclei carrying genetic markers from both parents. Successful demonstration of the first experimental objective would indicate that a single hyphal system contained both auxotrophic nuclei, while demonstra-