The inheritance of penicillin titre in crosses between lines of Aspergillus nidulans selected for increased productivity.

Selection for increased penicillin production among the progeny of pairwise crosses between wild-type isolates of Aspergillus nidulans resulted in the production of high-titre strains. Three crosses were made between strains with increased titre derived from independent selection lines. Significant genetic variation was found among the progeny of each cross, indicating that different genes for increased titre had been selected in each line. Gene action was additive in each cross. Renewed selection from the progeny of one 'between-line' cross resulted in further increases in titre, over three successive generations.