ABERRANT RECOMBINATION OF PYRIDOXINE MUTANTS OF Neurospora

Abstract

Two pyridoxine mutants, pdxp (pH-sensitive) and pdx (not pH-sensitive), were used in crosses with the markers co (colonial) and pyr1 (pyrimidine). Co is located 10-13 units from the centromere in linkage group D, pyrl is 4-5 units nearer the centromere, and pdx and pdxp at approximately the same position between the two markers (.5-1 unit from pyrl). The double mutants pyr1 pdx and pdxp co were obtained from appropriate crosses. Germinated spores from the crosses pdx X pdxp, pdx X pdx, and pdxp X pdxp, also using the markers, were counted on minimal medium + pyrimidine. Of the not pdx or pdxp progeny counted, there were 5 ++co, 7 pyr+co, 7 +++ and 13 pyr++, and these were obtained only from the pdx X pdxp crosses. When these types were crossed with wild the random spores were of the types expected assuming that the recombinants had genotypes corresponding to their phenotypes. The distance between pyr and co in the heterozygous crosses was approximately 4-5 units. Pseudo-wild types were also obtained from the pdx X pdxp crosses. Out of 988 asci dissected from the heterozygous crosses (+pdxp co X pyr pdx +) in 585 asci at least one member of each spore pair germinated, and among these 4 asci were recombinants. Among the segregants the double mutant pdx pdxp did not appear as would be expected if pdx and pdxp are independent loci. All segregants from 3 of the recombinant asci and 9 recombinants from random spores were grown on minimal substrate at pH 5.1 with and without pyridoxine and on minimal at pH 7.1, in all cases with pyrimidine. The pdx and pdxp isolates showed no significant differences from the parent strains in dry weight of mycelium, and the not pdx or pdxp recombinants gave no response to pyridoxine. Co was located 11 units from the centromere, based on 584 dissected asci. The hypothesis of gene conversion is discussed in relation to the absence of the double mutants pdx pdxp and the high frequency of recombinants in the region pyr to co.