Pseudo-Wild Types in Neurospora Crassa

Abstract

Three pyrimidine mutants, designated pyr-3a, pyr-3b (a temp. mutant) and pyr-3d, were used in expts. to obtain pseudo-wilds (strains which are wild phenotypically, but give mutant progeny). Isolates of pyr-3d with and without a translocation were used, as well as 2 other mutants, co, giving a colonial growth pattern, and arg, an arginine-requiring mutant. Progenies of crosses involving these mutants were analyzed by plating spores on minimal medium or minimal + cytidine or arginine. Arg and pyr mutants could be distinguished by limited hyphal growth on minimal and co could be classified on supplemented minimal when combined with arg. or pyr. Crosses involving the translocation gave many asci with aborted spores. Pseudo-wilds have not been obtained from crosses between pyr-3a and -3b. 40 phenotypically wild spores were isolated from 2 crosses involving pyr-3a and pyr-3d T(translocation). Tests on minimal and minimal + cytidine showed that the isolated strains closely resembled wild types isolated at random, but when 12 of them were grown on Westergaard''s basic medium supplemented with an autolysate of wild-type medium, they were stimulated by cytidine, indicating the presence of a pyrdependent component. When the 40 strains were crossed with both mating types of standard wild, 15 crosses were successful, giving an avg. of 44% mutant spores which behaved like pyr mutants in tests on minimal or minimal + cytidine. When pseudo-wild strains from pyr.-3b x pyr-3dT and from co x pyr-3dT were crossed with wild type, one or other, but not both, of the parent mutants were recovered. In crosses not involving the translocation both parental types were recovered. Phenotypically wild strains from pyr-3a x co pyr-3d segregated pyr mutants when crossed with wild, indicating that the pseudo-wild condition was not dependent on the translocation. The pseudo-wild strains showed the co character in a diluted and varying expression, and crosses of the pseudo-wilds with co indicated that they had 2 types of nuclei, co pyr and + pyr., one of which functioned in each perithecium. Crosses between co pyr-3d and arg gave a small frequency of pseudo-wilds, suggesting that arg was located between co and pyr. Pseudo-wilds were also obtained from co x pyr-3dT, along with the type of sterility associated with the translocation. One pseudo-wild strain later showed the co character. Of 13 asci which had 2 normal spore pairs in one half and 2 defective pairs in the other, 4 contained pseudo-wilds. Of 8 asci having a normal and an aborted spore pair in each half, none had pseudo-wilds. Observations made on asci segregating defective spores from crosses not involving a translocation suggest that spores giving rise to pseudo-wilds occur in asci which also contain defective spores. Attempts are made to explain the pseudo-wild condition on the following bases suppression of the mutant character by chance combinations of genes, back-mutation at the pyr locus giving a pyr-independent heterokaryon in which pyr nuclei predominate, formation of n + 1 gametes which give pry-independence but are not usually transmitted in crosses, and establishment of a heterokaryon, capable of wild type growth, by the inclusion of genetically different nuclei in an ascospore.