Genetic control of chromosome instability in Aspergillus nidulans as a mean for gene amplification in eukaryotic microorganisms.

A haploid strain of Aspergillus nidulans carrying I-II duplication homozygous for the leaky mutation adE20 shows impreved growth on minimal medium. The duplication, though more stable than disomics, still shows instability. Several methods were used for detecting genetic control of improved stability. (a) visual selection, using a duplicated strain which is very unstable due to UV sensitivity, (adE20, biAl/dp yA2; uvsB). One stable strain showed a deletion (or a lethal mutation?) DISTAL TO BIA on the segment at the original position (on chromosome I). This deletion reduces crossing over frequency between the two homologous segments. As the deletion of the non-translocated segment (yelow sectors) must be preceded by crossing over, the above reduces the frequency of yellow sectors. A deletion of the translocated segment (green sectors) results in non-viability due to the deletion, and such sectors do not appear. The net result is a stable duplication involving only 12 C.O. units carrying the gene in concern. (b) Suppressors of UV sensitivity (su-uvsB) were attempted using the above uvs duplicated strain. Phenotypic revertants were easily obtained, but all were back mutations at the uvsB locus. (c) Mutations for UV resistance higher than that of the wild type were not obtained, in spite of the strong selective pressure inserted. (d) Recombination deficient mutations (rec), six altogether, all uvs+, did not have any effect on stability.