A genetic investigation of some adenine requiring mutants of Aspergillus nidulans

1. Tests for allelism between adenine-requiring mutants showed that they could be divided into five groups. Mutants of different groups were complementary to those of the same group were physiologically allelic. 2. The mutants of each group were found to be located at approximately the same position on the chromosome map. Different groups occupied different map positions, but two of the groups, each represented by a single mutant, were extremely closely linked. 3. Two mutants, both X - ray induced, were found to be associated with a rearrangement. 4. Four physiologically allelic mutants of one group were tested against each other and shown to occupy different butt extremely closely linked loci. Three others of the same group were shown to occupy loci different from two of the first four, but their location with respect to the other two and to each other has not yet been worked out. 5. Two different suppressors of one mutant were found, one is specific for this mutant; even mutants which are physiologically allelic with the suppressed mutant are not suppressed. 6. It was discovered that about 0.1 per cent of ascospores A. nidulans are diploid. In crosses involving strains carrying -65- rearrangements the frequency may be as high as 1.0 per cent of the viable ascospores. An attempt has been made to ascertain the origin of diploid ascospores. The results, although not conclusive, suggest that two mechanisms are largely responsible (a) absence of a meiotic division of the zygote nucleus and (b) non-disjunction at anaphase II of meiosis in the zygote. 7. A duplication of the terminal part of one chromosome, about 20 units in length and attached to a second chromosome, has been found. Grossing over occurs between the duplication and the homologous segment of the duplicated chromosome. The recombination fraction is greater than expected from the known distance between the loci which are duplicated. It therefore appears that the presence of the duplication results in an increase in the amount of crossing over in the duplicated region of the chromosome