Primary and secondary mutations at the incompatibility loci in Schizophyllum.

Abstract

Mating competence in a majority of the higher Basidiomycetes, typified by Schizophyllum commune, is determined by a bifactorial incompatibility system.1' 2 The determination of mating competence, however, is only one result of a more inclusive role played by the incompatibility factors in the determination of internuclear relationships in four distinctive types of heterokaryons that may be formed whenever two haploid, homokaryotic strains interact. Only one of the four heterokaryons, the dikaryon, normally leads to sexual fertility and the completion of the life cycle.' -6The genetic basis of incompatibility in these forms resides in two independently assorting and segregating factors, A and B, each of which is constituted of two distinct, linked loci having multiple alleles.7-9 In S. commune, 9 and 26 alleles have been identified at Aa and AA, respectively, and series of 9 A a and 50 A: alleles have been projected for the total world-wide population. Each unique combination of A a and AS alleles determines a specific A factor phenotype, which is reflected in its compatibility with all 350-450 other A factors.10 11 The B factor is similarly constituted of two (or possibly three") component loci, each having a less extensive series of multiple alleles to yield ca. 65 factor-phenotypes.11The heterokaryons formed in interactions between homokaryons, each carrying a specific A factor and a specific B factor, are related to the incompatibility system as follows 3-6 (1) AH B$, e.g., Al Bi X A2 B2, yields the dikaryon, a specialized heterokaryon having a nuclear ratio of 1:1 maintained by synchronous nuclear-division, and leads to the conmpletion of the sexual cycle.(2) Other combinations, A = B., e.g., Al BL X Al B2, A B=, e.g., Al Bi X

Isozyme Patterns and Sexual Morphogenesis in Schizophyllum

“…Similarly, when only the B factors are different, only the initial step of the process, nuclear migration, occurs; this is the main event of the B-sequence, and the result is the common-A heterokaryon. -Morphologically, the common factor heterokaryons are quite distinct from the homokaryon and the dikaryon, as well as from each other.3 It is a significanMit feature of this system that the A-and B-sequences can be "turned on" by meais other thaIs the interaction of pairs of different A and different B factors.4 5 A mutation in either of the factors has the same effect in the homokaryon as a pair of compatible factors in the heterokaryoil: a mutant-B strain is thus a close mimic of the common-A heterokaryon; a mutant-A 882 \OL. 66,1970 GENETICS: HAANG ANAD RAPER 883 strain.…”

Secondary mutations at the Bβ incompatibility locus of Schizophyllum

“…by G. Simchen (unpublished). The first secondary mutations (Raper, Boyd and Raper, 1965) were derived from the original Bfl mutation recovered by Parag, who in 1960 provided us with the mutant strain. Parag and Simchen kindly made the three additional mutations available.…”

“…A homokaryon having a mutant BJ3 allele is thus a phenotypic mimic of the A = B heterokaryon. The subsequent recovery of four secondary Bfl mutations that restored normal morphology and self-sterility in the homokaryon and were compatible with the progenitor Bfl allele suggested the origin of new functional alleles from existing alleles by successive mutations (Raper, Boyd and Raper, 1965). These mutations, however, differed from wild-type alleles in details of the mating reaction.…”

“…These considerations led to a search for new alleles as the result of mutations induced in known alleles. To date, more than 20 mutations have been found in two (Afi and Bfl) of the four basic incompatibility loci of S. commune and Coprinus lagopus (Day, 1963;Parag, 1962;Raper, Boyd and Raper, 1965;G. Simchen, unpublished).…”

Extrachromosomal factors affecting mating reactions in Schizophyllum commune

“…Changes in mating specificities followed by changes in mating reactions and in the types of the resulting heterokaryons have been found to occur as a result of mutations in the incompatibility factors (Parag,1 962a;Raper, Boyd and Raper, 1965;Raper, 1966) or by modifiers unlinked to the incompatibility factors (Raper and Raper, 1968). In two series of experiments, two apparent extrachromosomal factors have been found, which have 61 62 Y. PARAG a pronounced effect on mating specificity and on the heterokaryons formed following such matings.…”