Modification of Chromosome Instability in Aspergillus nidulans

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Abstract
Strains of A. nidulans with a chromosome segment in duplicate show instability at mitosis; their colonies produce faster-growing sectors which arise from nuclei with spontaneous deletions in either duplicate segment. In an attempt to probe the depletion process, the effects of mutations causing sensitivity to UV treatment, and those of manganous ions, were studied in strains carrying either Dp(I,II) or Dp(III,VIII). For comparison, the effects of Mn2+ on balanced and unbalanced diploids were also examined. The uvsE allele, which decreases intragenic mitotic crossing over in diploids, increased deletion frequency in strains with either duplication. The uvsB allele, which increases intragenic mitotic crossing over in diploids, increased deletion frequency only in Dp(I,II) strains; in addition, by causing early mitotic crossing over between the homologous segments, it produced some novel deletion products. Mn2+ substantially decreased the deletion frequency in Dp(I,II) strains and decreased mitotic crossing over in diploids; it had no effect on Dp(III,VIII) strains. In haploid duplication strains there are apparently 2 classes of spontaneous DNA lesions, recombinogenic and non-recombinogenic, both of which, failing repair, lead to deletion.