Dominant spore color mutants of Aspergillus nidulans defective in germination and sexual development

Abstract

The ascomycete A. nidulans produces green conidia (asexual spores). Recessive mutants which produce yellow conidia have been previously isolated from haploid strains and have been shown to be deficient in laccase (diphenol oxidase), an enzyme that requires Cu for activity. Using a diploid parent strain, dominant yellow conidial mutants were isolated which, in the haploid state, produced even less laccase activity than a recessive mutant. Three isolates of such mutants behaved similarly and define a single complementation group (yB) on chromosome VIII distinct from the yA locus on chromosome I defined by recessive mutants. Unlike yA mutants, whose only discernable phenotype is their conidial color, yB mutants are pleiotropic: conidial germination was delayed relative to the wild type, and sexual development was blocked at an early stage. The 3 phenotypes of yB mutants were expressed on yeast extract-glucose medium containing 1.6 .mu.M of added Cu. When Cu was added to above 5 .mu.M, all 3 phenotypes were remediated, and near wild-type levels of laccase were produced. yB mutants have a reduced availability of Cu. The dominance of yB mutants could result, for example, from an alteration in transport or storage of Cu. Using an immunological assay, no laccase antigenic cross-reacting material was detected in yB mutants grown on medium of low Cu content. Either the synthesis or the stability of laccase is Cu dependent.