The organization of the ribosomal DNA (rDNA) repeat unit in the standard wild-type strain of Neurospora crassa, 74-OR23-1A, and in 30 other wild-type strains and wild-collected strains of N. crassa, . tetrasperma, N. sitophila, N. intermedia, and N. discreta isolated from nature, was investigated by restriction enzyme digestion of genomic DNA, and probing of the Southern-blotted DNA fragments with specific cloned pieces of the rDNA unit from 74-OR23-1A. The size of the rDNA unit in 74-OR23-1A was shown to be 9.20 kilobase pairs (kb) from blotting data, and the average for all strains was 9.11 + 0.21 kb; standard error = 0.038; coefficient of variation (C.V.) = 2.34%. These data indicate that the rDNA repeat unit size has been highly conserved among the Neurospora strains investigated. However, while all strains have a conserved HindIII site near the 5' end of the 25 S rDNA coding sequence, a polymorphism in the number and/or position of HindIII sites in the nontranscribed spacer region was found between strains. The 74-OR23-1A strain has two HindIII sites in the spacer, while others have from 0 to at least 3. This restriction site polymorphism is strain-specific and not species-specific. It was confirmed for some strains by restriction analysis of clones containing most of the rDNA repeat unit. The current restriction map of the 74-OR23-1A rDNA repeat unit is presented.

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The genetic segregation of ribosomal DNA (rDNA) in Neurospora crassa was analyzed by exploiting restriction fragment length polymorphisms in the nontranscribed spacer (NTS) sequences of nine laboratory wild-type strains and wild-collected strains. In an analysis of random spore progeny from seven crosses, and of ordered tetrads from two of those crosses the rDNA was shown to be inherited in a simple, stable Mendelian fashion, exhibiting an approximately 1:1 ratio of the two parental rDNA types. No meiotic recombinants were detected among the progeny, indicating that non-sister-chromatid crossing over is highly suppressed in the rDNA region. The basis for this suppression of meiotic recombination is not known.