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Meiotic silencing by unpaired DNA (MSUD) in Neurospora crassa: a new approach to studying recombination-independent homologous pairing
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International audience. Homologous chromosome pairing represents a critical aspect of meiosis in nearly all sexually reproducing species. While meiotic pairing relies on the formation of double-strand DNA breaks in some organisms, in many others it can proceed in the apparent absence of DNA breakage and recombination. The mechanistic nature of such recombination-independent pairing represents a fundamental question in molecular biology. Using “meiotic silencing by unpaired DNA” (MSUD) in the fungus Neurospora crassa as a model system, we demonstrate the existence of a principally new solution to the problem of inter-chromosomal homology recognition during meiosis. Here we take advantage of the unique ability of MSUD to efficiently detect and silence (by RNA interference) any relatively short DNA fragment lacking a homologous allelic partner. We show that MSUD does not require the function of eukaryotic RecA proteins and the type II topoisomerase-like protein Spo11. We further show that MSUD recognizes weak interspersed homology in which units of sequence identity as short as 3 base-pairs (bp) are spaced apart with a periodicity of 11 bp, approximating double-helical DNA pitch and corresponding to an overall sequence identity of only 27%. Taken together, these results reveal the role of a recombination-independent homology-directed process in guiding the expression of small interfering RNAs and suggest that meiotic chromosomes can be evaluated for sequence homology at base-pair resolution by a mechanism that operates on intact DNA molecules.
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