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Three chromosomal factors called Spore killer (Sk) have been found in wild populations of Neurospora sitophila and N . intermedia. Sk resembles other examples of meiotic drive such as Segregation Distorter in Drosophila, Pollen killer in wheat, and Gamete eliminator in tomato. In crosses heterozygous for Sk, each ascus contains four viable black ascospores and four inviable, undersize, clear ascospores, with second-division segregations infrequent. The survivors contain the killer allele Skk, while unlinked markers segregate normally. Reciprocal crosses are identical. When crosses are homozygous for an allele of Sk, all eight ascospores are viable and black in most asci. (Many homozygms crosses have a background level of randomly occurring inviable spores; however, the pattern of 4 viable : 4 small clear ascospores is not found in any of the asci of Sk-homozygous crosses.)—killer (Sk-1K) and sensitive (Sk-1S) alleles occur in about equal numbers among a worldwide sample of N. sitophila strains, following no geographic pattern. No killer allele has been found in N . crassa. Sk-2K and Sk-3k, found in N . intermedia, are rare. Most N. intermedia strains are Sk-2S and Sk-3S, but some are wholly o r partially resistant to one or both of the killer alleles, while not themselves acting as killers. Sk-2K and Sk-2R are both specific in conferring resistance to Sk-2K, but not to Sk-3K. Likewise Sk-3K and Sk-3R are resistant specifically to Sk-3K, but not to Sk-2K. Resistance segregates as an allele of SkK.——Sk-2 and Sk-3 have been mapped near the centromere of linkage group I11 after introgression into N . crassa, where crossing over is normally 11 % between the proximal I11 markers acr-2 and leu-1. But crossing over is absent in this region when either of the killer alleles is heterozygous (Sk-2K x Sk-2S, Sk-3K x Sk-3S and Sk-2K X Sk-2R have been examined).

SPORE KILLER, A CHROMOSOMAL FACTOR IN NEUROSPORA THAT KILLS MEIOTIC PRODUCTS NOT CONTAINING IT