Spontaneous Duplications and Transpositions of a Large Chromosome Segment in Aspergillus nidulans

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Abstract
Spontaneous revertants of the leaky adE20 mutant of A. nidulans were obtained as vigorous sectors emerging from stunted colonies on adenine-free medium. Among the genetically heterogeneous sectors up to .apprx. 20% were recognized unequivocally as having an additional chromosome segment bearing adE20; 2 doses of this leaky allele permitted growth without added adenine. Eleven spontaneous duplication strains of independent origin were analyzed genetically. Eight carried the duplicate segment on chromosome IIR; 3 of these, phenotypically similar to all 8, were analyzed in detail and were shown, within the limits of such genetic analysis, to have a large terminal segment of chromosome IR duplicated and attached terminally and uninverted to IIR. One strain had a duplication, possibly tandem, on IR and 2 had duplications attached elsewhere in the genome. The results suggested a preferential site for the initiation of duplicate segments in this system and a preferential site for their attachment. Agents known to modify instability of a previously studied Dp(IR .fwdarw. 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 .fwdarw. IIR) instability in a specific way, and Co2+, which stabilizes Dp(IR .fwdarw. IIR), gave 14, 50 and 62% duplication sectors, respectively, among revertants. Duplications selected in the presence of Co2+ had mainly IIR attachments; of those from trypan blue and coumarin, .apprx. 1/4 were attached to IIL and none to IIR.

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