Spontaneous duplications and transpositions of a large chromosome segment in Aspergillus nidulans

Department of Genetics, The University, Sheffield S10 2TN, UK ABSTRACT Spontaneous revertants of the leaky adE20 mutant of Aspergillus nidulans 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 adE20 ; 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 adE20 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 2+ , which stabilizes Dp(IR IIR), gave 14, 50 and 62% duplication sectors respectively, among revertants. Duplications selected in the presence of Co 2+ had mainly IIR attachments; of those from trypan blue and coumarin, about one-quarter were attached to IIL and none to IIR.