The Neurospora crassa standard Oak Ridge background exhibits an atypically efficient meiotic silencing by unpaired DNA

Abstract Meiotic silencing by unpaired DNA (MSUD) was discovered in crosses made in the standard Oak Ridge (OR) genetic background of Neurospora crassa . However, MSUD often was decidedly less efficient when the OR-derived MSUD tester strains were crossed with wild-isolated strains (W), which suggested either that sequence heterozygosity in tester x W crosses suppresses MSUD, or that OR represents the MSUD-conducive extreme in the range of genetic variation in MSUD efficiency. Our results support the latter model. MSUD was much less efficient in near-isogenic crosses made in a novel N. crassa B/S1 and the N. tetrasperma 85 genetic backgrounds. Possibly, regulatory cues that in other genetic backgrounds calibrate the MSUD response are missing from OR. The OR versus B/S1 difference appears to be determined by loci on chromosomes 1, 2, and 5. OR crosses heterozygous for a duplicated chromosome segment ( Dp ) have for long been known to exhibit an MSUD-dependent barren phenotype. However, inefficient MSUD in N. tetrasperma 85 made Dp -heterozygous crosses non-barren. This is germane to our earlier demonstration that Dp s can act as dominant suppressors of repeat-induced point mutation (RIP). Occasionally, during ascospore partitioning rare asci contained >8 nuclei, and round ascospores dispersed less efficiently than spindle-shaped ones. General abstract In crosses made in the standard OR genetic background of Neurospora crassa , an RNAi-mediated process called MSUD efficiently silences any gene not properly paired with its homologue during meiosis. We found that MSUD was not as efficient in comparable crosses made in the N. crassa B/S1 and N. tetrasperma 85 backgrounds, suggesting that efficient MSUD is not necessarily the norm in Neurospora. Indeed, using OR strains for genetic studies probably fortuitously facilitated the discovery of MSUD and its suppressors. As few as three unlinked loci appear to underlie the OR versus B/S1 difference in MSUD.