Two-way selection of mutants and revertants to chloroneb resistance in Aspergillus nidulans

5 mutants of Aspergillus nidulans, selected for resistance to chloroneb, were also partially dependent on it. The resistance of these mutants to chloroneb was about 20-150 times higher than that of the original strain. The resistance marker was due to a mutation in a single gene, located in linkage group III, and behaved as a recessive character. This genetic marker was distal in relation to galAl with a recombination frequency of about 30-35%. The different levels of resistance were attributed to mutations at different sites in the same locus. Both stable and unstable sectors were obtained from resistant strains inoculated on chloroneb-free medium. The emergence of stable sectors was due to back mutation, suppressor mutation or another mutation, which allows growth to the full extent in the absence of the drug. The unstable sectors showed better growth when compared with the resistant strain, kept their resistance and produced both resistant and non-resistant secondary sectors. This procedure of 2-way selection of mutants and revertants to chloroneb resistance could be useful for studying forward and back mutation in A. nidulans.