Neurospora Genetic Backgrounds Differ in Meiotic Silencing by Unpaired DNA (MSUD) Strength: Implications for Dp-Mediated Suppression of Repeat-Induced Point Mutation (RIP)
Neurospora strains bearing chromosome segment duplications (Dp strains) can be generated in the laboratory and are also recoverable from natural populations. Two “genome defense” processes, RIP (repeat-induced point mutation) and MSUD (meiotic silencing by unpaired DNA), impart some unusual phenotypes to Dp-heterozygous crosses. MSUD is an RNAi-based gene silencing of any gene not properly paired with a homologous sequence during meiosis. Efficient MSUD, as in the highly inbred N. crassa Oak Ridge (OR) background used for most genetic studies, can make Dp-heterozygous crosses barren and very non-productive. Our recent studies showed, however, that strains with inefficient MSUD are more widespread in nature than OR-type strains, and in them, Dp-heterozygous crosses can be non-barren. RIP occurs in the haploid nuclei of the pre-meiotic dikaryon and alters, via multiple G:C to A:T transition mutations, duplicated DNA sequences present in an otherwise haploid genome, and thus destroys duplicated genes. RIP-induced mutants can be recovered among the progeny of Dp-heterozygous crosses. Additionally, the presence of large Dps (> 250 kb) suppresses RIP in smaller gene-sized duplications (< 5 kb) in the same cross, presumably by titration of the RIP machinery. Thus, in an inefficient MSUD background a large Dp would enable a small duplication to escape RIP, whereas efficient MSUD induces barrenness and thereby suppresses the production of escapees. Our findings enable us to explain why only one Neurospora strain was found to contain an active transposable element, whereas all the other strains examined contained only its RIP-inactivated copies.
KeywordsNeurospora Neurospora crassa Repeat-induced point mutation (RIP) Meiotic silencing Meiotic silencing by unpaired DNA (MSUD) Transposable element
I thank Dev Ashish Giri for preparing Fig. 8.1. I receive support as an Indian National Science Academy (INSA) Senior Scientist, in the Centre for DNA Fingerprinting and Diagnostics (CDFD), and I am an Honorary Visiting Scientist at the Centre for Cellular and Molecular Biology (CCMB). Neurospora strains were obtained from the Fungal Genetics Stock Center, Manhattan, Kansas, USA (McCluskey et al. 2010).
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