Pseudo-Wild Types in Neurospora crassa

Abstract

In Neurospora the test for non-allelism which is usually considered conclusive consists of the recovery of the double mutant from a cross between the two mutants in question and the demonstration that the two parent mutants segregate in asci from a cross of the double mutant to wild type. [1] Carrying out this test is usually a simple matter if Lertile crosses can be obtained and if close linkage between the two mutants does not exist. Crosses between two pyrimidine mutants to be considered here, however, gave no complete asci and only a small percentage of phenotypically wild progeny. Since the mutants are not distinguishable by known physiological tests, the double mutants, if they occurred, could presumably be detected only by out-crossing mutant progeny at random and examining asci. This procedure seemed impracticable, and, instead, phenotypically wild progeny were studied with the possibility in mind that they might be found to differ from standard wilds in a way which would indicate that they did not arise from crossing-over between the mutant loci. Such a difference was readily demonstrated by crossing these strains to standard wild, since these crosses regularly gave about 45% pyrimidine mutant progeny. Strains which behaved in this fashion have been termed pseudo-wilds, since they appear to be wild phenotypically, but give mutant progeny. Pseudo-wilds from several other crosses have been isolated and studied in an attempt to explain the origin and behavior of these strains.