Unpredicted growth responses of single-ascospore colonies of Neurospora crassa

Abstract

Inheritance of growth characteristics, in two-marker crosses, was studied by comparing samples of ordered spore octets cultured on plates of differently supplemented medium. Two mutants which differ in the timing of their response to thiamine were among those giving unexpected results. Growth of thi-1 from ascospores duplicates that of wild type when thiamine is supplied, but thi-5 is entirely unresponsive during the first 20 h. On yeast extract medium, thi-5 mimics wild type, and conidial transfers from cultures thus established respond to thiamine. Unlike the single mutants, which grow slowly on minimal medium, the thi-1 thi-5 double mutant has an absolute growth requirement. It responds slowly to thiamine plus leucine. Individual wild-type recombinant colonies from thi-1 × thi-5 appear ditype in their response to tryptophan, as if some hyphae were completely inhibited and others unaffected. This condition, and the phenotypic shift characterizing thi-5, suggest unpredicted genetic heterogeneity in mycelia from single ascospores.