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 ofthi-1from ascospores duplicates that of wild type when thiamine is supplied, butthi-5is entirely unresponsive during the first 20ensp;h. On yeast extract medium,thi-5mimics wild type, and conidial transfers from cultures thus established respond to thiamine. Unlike the single mutants, which grow slowly on minimal medium, thethi-1 thi-5double mutant has an absolute growth requirement. It responds slowly to thiamine plus leucine. Individual wild-type recombinant colonies fromthi-1times;thi-5appear ditype in their response to tryptophan, as if some hyphae were completely inhibited and others unaffected. This condition, and the phenotypic shift characterizingthi-5, suggest unpredicted genetic heterogeneity in mycelia from single ascospores.
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