Spontaneous revertants of the leaky mutant of were obtained as vigorous sectors emerging from stunted colonies on adenine-free medium. Among the genetically heterogeneous sectors up to about 20% were recognized unequivocally as having an additional chromosome segment bearing ; two doses of this leaky allele permitted growth without added adenine. Eleven spontaneous duplication strains of independent origin were analysed genetically. Eight carried the duplicate segment on chromosome IIR; three of these, phenotypically similar to all eight, were analysed in detail and were shown - within the limits of such genetic analysis - to have a large, terminal segment of IR duplicated and attached terminally and uninverted to IIR. One strain had a duplication, possibly tandem, on IR and two had duplications attached elsewhere in the genome. The results suggested a preferential site for the initiation of duplicate segments in this system, as well as a preferential site for their attachment. Agents known to modify instability of a previously studied Dp(IR → IIR) strain affected the frequency of duplications among selected revertant sectors and/or the genomic locations of duplicated segments. Trypan blue and coumarin, which enhance Dp(IR → IIR) instability in a specific way, and Co, which stabilizes Dp(IR → IIR), gave 14, 50 and 62% duplication sectors respectively, among revertants. Duplications selected in the presence of Co had mainly IIR attachments; of those from trypan blue and coumarin, about one-quarter were attached to IIL and none to IIR.


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