Meiotic Silencing by Unpaired Dna Msud

In N. crassa, mutation in the gene Asm-1 (ascus maturation) affects protoperithecial formation. When a deletion mutant, designated Asm-1, was crossed to wild asm-1+, the ascospores formed were all unmelanized and inviable (Aramayo and Metzenberg, 1996; Shiu et al., 2001). The defect was corrected by transformation of Asm-1 by a cloned Asm-1+ gene. Interestingly, the cross of two deletion mutants yielded black ascospores. Analysis of several crosses (Figure 9.6) involving the deleted asm-1+ gene and copies of normal genes in normal or ectopic locations led to the conclusion that the presence of an unpaired copy of asm-1 silenced the expression of all copies of Asm-1, whether paired or unpaired. This gene silencing phenomenon was called meiotic silencing of unpaired DNA (MSUD). Thus, whereas RIP scans the genome for duplicated sequences before karyogamy, MSUD operates after karyogamy, recognizing any unpaired DNA sequences and silencing them. This gene silencing mechanism might be important in holding down the genetic load due to transposable elements that move during meiosis.

Figure 9.5 A diagram showing timing and consequence of RIP. Based on Selker et al. (1987).
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