IMPROVED MEDIA FOR SELECTING NITRATE-NONUTILIZING MUTANTS IN ASPERGILLUS-FLAVUS

Selection and identification of nitrate-nonutilizing (nit) mutants of Aspergillus flavus was compared on six chlorate-containing agar media. Three isolates of A. flavus were used. Nit mutants isolated comprised three phenotypic classes, equivalent to niaD, nirA-, and cnx mutants of A. nidulans. There were significant differences among isolates and among media in number and types of nit mutants isolated. The most effective medium contained only nitrate as a nitrogen source, whereas most previously described media for isolation of nit mutants contain both nitrate and an amino acid. Morphology of nit mutants was more distinctive on this medium than on other media. The nitrate-only medium yielded mainly niaD mutants; other media yielded more nirA- mutants, which complement niaD mutants. Few cnx mutants were found. Use of a nitrate-only medium in conjunction with another medium should optimize isolation of nit mutants and complementary pairs of mutants.