SUMMARY: Strains of 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 deletion process, the effects of mutations causing sensitivity to UV treatment, and those of manganous ions, have been studied in strains carrying either Dp(I, II) or Dp(III, VIII). For comparison, the effects of Mn on balanced and unbalanced diploids have also been examined. The allele, which decreases intragenic mitotic crossing over in diploids, increased deletion frequency in strains with either duplication. The 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. Mn 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. The results suggest that in haploid duplication strains there are two classes of spontaneous DNA lesions, recombinogenic and non-recombinogenic, both of which, failing repair, lead to deletion.


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