rec 1+, rec 2+ and rec 3+ are dominant trans-acting genes that suppress meiotic recombination in specific regions of the Neurospora crassa genome. As an example, up to 1% of progeny from a rec 2 by rec 2 cross experience recombination at his 3 but this falls to about 0.005% when one or both parents carries rec 2+. Our attempt to clone rec 2+ has encountered substantial difficulty and the main reason for this has recently become obvious – rec 2+ does not exist!

Approximately 10kb of the rec 2+ chromosome is replaced with a unique 3kb stretch in rec 2 strains. However, rec-2+ DNA seems to lack function since putting rec 2+ sequence into rec 2 strains fails to yield a rec 2+ phenotype while deletion of rec-2+ sequence does not give a rec-2 phenotype. With the discovery of meiotic silencing by unpaired DNA (MSUD), we wondered whether rec 2+ might simply appear dominant in rec 2+/rec 2 heterozygotes because it deprives rec 2 of an opportunity to pair during meiosis. This appears to be the case.

Disabling MSUD in rec 2+/rec 2 heterozygotes substantially increases recombination at his 3. Similarly, in the absence of MSUD, the amount of recombination at his 1 in rec 1+/rec 1 heterozygotes and at am in rec-3+/rec-3 heterozygotes is indistinguishable from that in rec 1 and rec-3 homozygotes respectively. Thus, rather than rec+ genes producing suppressors of recombination it now appears likely that the products of rec 1, rec 2 and possibly rec 3 act to promote recombination.