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The isolation and characterization of recessive meiotic mutants in Neurospora crassa DeLange, Aloysius


The study of the genetic control of meiosis has been initiated in Neurospora crassa by the isolation of recessive meiotic mutations. These mutations were detected by their reduced fertility or by the abortion of ascospores. To allow their expression, recessive meiotic mutations were made homozygous by selecting (n + 1) disomic ascospores. Cultures produced by each of these ascospores contain two types of nuclei with identical genes (including mutations) on all chromosomes except linkage group (LG) I, which contains the mating type locus. The simultaneous presence of these two types of nuclei allows the initiation of the sexual cycle, and therefore the detection of recessive mutations affecting the sexual cycle, including meiosis, on any chromosome except LG I. Using this method, three major classes of mutants have been detected. First, eleven mutants affecting perithecial development were expressed only as the maternal parent. Second, thirteen mutants produced perithecia with few or no ascospores. The infertility in two of these mutants was definitely caused by recessive mutations (asc-2 and asc-4). Finally, the abortion of many ascospores was detected in thirteen mutant strains. Among these strains, six recessive mutations (asc-1, asc-3, asc-5, asc-6, asc-7, and asc-8) caused the abortion of many ascospores. The dominant mutation SK(ad-3A) was detected in this screen for recessive mutations, because it caused ascospore abortion when crossed with an ad-3A mutant but not with a wild type strain. This mutation, apparently allelic to ad-3A, caused the abortion of all ad-3A-containing ascospores. The three ascospore abortion-type mutants asc-1, asc-3, and asc-6 were analyzed in more detail using both cytological and genetic methods. Ascospore abortion in these mutants was caused by abnormal disjunction of meiotic chromosomes. In mutants asc-1 and asc-6, the primary defect in pairing of homologs during the first meiotic prophase was followed by the formation of univalents at metaphase I. Observations on these mutants and on the mei-1 mutant (previously isolated; see Smith, 1975) suggested equational centromere division of many univalents at anaphase I. Subsequent irregular and prolonged separation of chromosomes at the second meiotic division appeared to be a secondary effect of the abnormal first division. The asc-3 mutant had a defect in ascus formation, and later in disjunction during the second meiotic and post-meiotic divisions. The first-acting defect before or during karyogamy resulted in the abortion of most cells. Some cells managed to proceed past this block. During the second meiotic division most chromosomes of the few resulting asci were attached to only one of the two spindle-pole bodies. Disjunction at the post-meiotic division was also highly irregular. This mutant appeared to be defective in the attachment of one spindle-pole body to a set of centromeres. The defect may involve either a centromere-associated product or a spindle-pole body.

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