An efficient technique for the enrichment of mutants of Mucor racemosus by differential freeze-killing is described. Ungerminated spores were resistant to freeze-killing. During germination susceptibility increased up to a maximum coinciding with the appearance of germ tubes. Under optimum conditions a thousandfold preferential survival of growth-limited spores over actively germinating spores was obtained. Using this differential killing, cultures treated with mutagen were enriched with respect to auxotrophic and temperature-sensitive mutants, with the desired mutants constituting a large fraction of the survivors of the freeze-thaw treatments. This technique may be applicable to mutant enrichment in other filamentous fungi.
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