Recombination block in the Spore killer region of Neurospora

Spore killers Sk-2K and Sk-3K are chromosomal meiotic drive factors in Neurospora. In heterozygous crosses, ascospores not containing the Spore killer die. Sk-2K and Sk-3K, which differ in killing specificity, were found to be associated with suppression of recombination in a centromere-spanning region of linkage group III, and investigation of that recombination block is reported here. The block covers a region that is normally 30 to 40 map units long. A locus (r(Sk-2)) conferring resistance to Sk-2K maps to the left end of the recombination block. Recombination is normal in r(Sk-2) X Sk sensitive but blocked in Sk-2K X r(Sk-2); so the block does not depend upon killing. By selective plating, SkK stocks carrying genetic markers within the block were obtained at frequencies on the order of 10(-5) or 10(-6). Since this tight block is far beyond what has been observed for genetic reduction of recombination, a structural basis is assumed. No evidence of chromosome rearrangement was obtained. Crosses homozygous for Sk-2K show normal crossing-over and map order for the flanking markers cum and his-7 and three included markers (acr-7, acr-2, and leu-1). Results would be consistent with a divergence of sequence great enough to interfere with homologous pairing.