Molecular basis of mutations at the waxy locus of maize: correlation with the fine structure genetic map

More than 40 mutant alleles of the waxy (Wx) locus of maize are available for molecular analysis. Previous studies have examined the nature of phenotypically unstable Wx mutant alleles caused by insertion of the maize transposable activator (Ac) and dissociation (Ds) elements. In this study we have used Southern blot analysis to characterize the locus in 22 strains harboring wx alleles with stable mutant phenotypes. Of these mutations, 17 are of spontaneous origin, 4 were induced by gamma rays, and 1 was induced by ethyl methanesulfonate. Of these 22 alleles, we find that 13 have either insertions or deletions within the Wx transcription unit. The insertions range in size from 150 base pairs to 6.1 kilobases. For 4 of the 6 deletions identified, the two breakpoints are within the Wx gene. For 9 other alleles we can detect no obvious lesions within or around the transcription unit. Evidence is presented that the insertions and deletions result in the mutant phenotype and are not polymorphisms. This conclusion is based on two findings: (i) a survey of inbred lines revealed only a single instance of polymorphism within the transcription unit, whereas all of the lesions described alter the transcription unit; and (ii) there is an excellent correlation between the position of these lesions on the physical map and their relative position on a fine structure genetic map of the locus.