NEW DUPLICATION-GENERATING INVERSIONS INNEUROSPORA

Abstract

Inversion In(ILR)NM176 has one break point at the extreme right end of linkage group I and the other distal to mating type in the left arm. In crosses of Inversion × Normal the products of single crossing over within the inversion are complementary duplication-deficiency classes. One crossover product is viable, with a large segment of IL duplicated and the dispensable right tip presumably deficient. This class has low fertility and distinctive morphology. The complementary product has a large deficiency which results in a pair of white, inviable ascospores. Single exchanges within the heterozygous inversion thus produce asci with 6 Black: 2 White spores; four-strand double exchanges produce 4 B:4 W; and non-exchanges produce asci with 8 B:0 W. Approximate mapping of break points was accomplished by three-point crosses. Precise placement of the left break point between ser-3 and un(55701t), just left of mating type, is based on coverage of markers by the heterozygous duplication. No crossover has been obtained between mating type and the break point, despite extensive efforts. In(ILR)NM176 differs from the inversion In(ILR)H4250 described by Newmeyer and Taylor (1967) in one main respect: the mating type locus is included in the inverted segment of NM176. Consequently, when duplications are generated, the progeny are unisexual and do not have the unstable inhibited phenotype characteristic of H4250 duplication progeny, which are heterozygous for the mating type alleles A and a. Three other inversions which originated independently of In(ILR)NM176 resemble it closely and have similar or identical break points.