THE GENERAL INCIDENCE OF PSEUDO-WILD TYPES IN NEUROSPORA CRASSA

Abstract

N haploid organisms such as Neurospora wild-type progeny from crosses I involving linked mutants in repulsion are usually interpreted as resulting from recombination. However, in a study of certain pyrimidine-requiring mutants, it has recently been shown that phenotypically wild-type strains found in the progeny of mutant x mutant crosses were not the result of crossing over nor could they readily be explained as resulting from back mutation or suppression of the mutant character Crosses between these phenotypically wild-type strains and standard wild-type strains gave approximately 50% mutant progeny. Such strains have been called pseudo-wild types ( P W T ) . A large number of different PWT's recovered from the progeny of mutant x mutant crosses were wild types insofar as their ability to grow on minimal medium was concerned. All gave from 40 to 50% mutant progeny when outcrossed to standard wild-type strains, and both mutants of the parental strains were recovered in the outcross progeny providing a translocation involving the pyrimidine ( D ) linkage group was not present. However, if a translocation linked to the fiyr mutants was present in one of the strains involved in a cross from which PWT's were isolated, then either one or the other, but never both, of the parental mutants was recovered in the outcross progeny of any one P W T . P W T strains involving the pyr mutants were usually found by plating random spores from mutant x mutant crosses and selecting wild-type progeny. However, PWT's were also recovered from individual asci and such asci contained either four adjacent normal spores and four adjacent aborted spores or six normal and two aborted spores.