Directed replacement of mt A by mt a-1 effects a mating type switch in Neurospora crassa.
To test the functions of a mating type genes, we developed an efficient strategy to select transformants of Neurospora crassa in which resident A mating type DNA was replaced by cloned DNA from the mt a idiomorph. Cloned a idiomorphic DNA could specify all functions, including fertility, of a mating type, but only when it replaced A DNA at the mating type locus. Only the mt a-1 region of the a idiomorph was necessary in order to specify a mating type. Gene replacement events involved the homologous sequences flanking the unique mating type idiomorphic DNA, resulting in apparently isogenic a and A strains. These isogenic strains were fertile when crossed with one another, indicating that no determinants outside the transforming DNA are necessary for fertility as a and that no host sequences of A strains interfere with fertility as a. One a replacement strain bore a duplication of the transforming mt a-1 and hph DNA. The duplication strain had unexpected properties. Although mating type segregated 1:1 in crosses of this strain to A, the duplicated regions were efficiently altered during the sexual process to generate a single copy in the progeny. No progeny were recovered that had undergone RIP (repeat induced point mutation) sufficient to inactivate the mt a-1 gene. We infer that the mt a-1 gene is necessary and sufficient to specify a mating type identity in all vegetative and sexual activities. Mt a-1 may also play an essential role in ascosporogenesis after fertilization.
Chang
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T. H. Morgan School of Biological Sciences, University of Kentucky, Lexington 40506-0225.
Staben
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D003001:Cloning, Molecular; D004271:DNA, Fungal / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D005298:Fertility / Q000235:genetics; D005656:Fungal Proteins / Q000235:genetics / Q000502:physiology; D005800:Genes, Fungal; D049770:Genes, Mating Type, Fungal; D008540:Meiosis / Q000235:genetics; D005810:Multigene Family; D009492:Neurospora crassa / Q000235:genetics* / Q000254:growth & development / Q000502:physiology; D010641:Phenotype; D015183:Restriction Mapping; D014170:Transformation, Genetic
D004271:DNA, Fungal; D004268:DNA-Binding Proteins; D005656:Fungal Proteins