UBC Theses and Dissertations
Studies of bisexual (A+ a) strains in Neurospora crassa DeLange, Aloysius
In the first part of this thesis, a system was constructed, which is selective for recessive meiotic mutants in Neurospora crassa. This system employs bisexual (A + a) Pseudo-Wild Type cultures (produced from disomic ascospores), which were selected by means of several closely linked auxotrophic markers on L.G.I (the mating type locus is also located on this linkage group). These Pseudo-Wild Type cultures consist of at least 2 nuclear components of opposite mating type (A and a). Consequently, they are self-fertile. Moreover, since the cultures are homozygous for genes on other linkage groups, recessive mutants affecting meiosis may be detected. The second part of this thesis deals with escape from self-incompatible bisexual heterokaryons. In Neurospora crassa, strains of opposite mating type generally do not form stable heterokaryons because the mating type locus acts as a hetero-karyon incompatibility locus. However, when one A and one a strain, having complementing auxotrophic mutants, are placed together on minimal medium, some slow growth may result. Escape from slow growth to that at a wild type or near-wild type rate was observed. Most escape products are stable heterokaryons, which have lost one or the other of the mating type alleles from one of the component nuclei. These nuclei have therefore become heterokaryon compatible. Alternatively, when one component strain is tol and the other tol⁺ (tol being a recessive mutant suppressing the heterokaryon incompatibility associated with mating type), escape may occur upon the deletion or mutation of tol⁺, similarly causing heterokaryon compatibility. Deletion of more than one locus, including the mating type locus, was demonstrated in 3 cases. One such deletion covered most of the left arm of LG I. An induction-type mechanism of escape is speculated upon.