The filamentous fungus Aspergillus nidulans has two genes encoding a-tubulin, tubA and tubB. Mutational analysis of tubA has demonstrated that the tubA gene is essential for mitosis and nuclear migration. In this study we have deleted the tubB gene by replacing it with a selectable marker and have named this new allele tubBA. The results demonstrate that the tubB gene is not required for vegetative growth or asexual reproduction, nor is it required for the initiation or early stages of sexual

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... fferentiation. Deletion of tubB, however, completely prevents ascosporogenesis, because tubBA strains produce no sexual spores when self-crossed. These strains produce viable ascospores when outcrossed to tubB + strains, indicating that the tubBA mutation is recessive. We have studied the cytology of sexual development in wild-type strains and in the tubB mutant and have observed that tubBA, strains develop normally to the stage of ascus formation. However, only a single nuclear mass is observed in the tubBA ascus, indicating that either the two zygotic haploid nuclei are blocked in karyogamy or that karyogamy occurs but the resulting diploid nucleus is subsequently blocked in meiosis I.