THE AC2-BZ2M MUTABLE SYSTEM OF MAIZE

The Activator-2 controlling element in the Ac2-bz2m mutable system of maize differs from the Ac of McClintock in several ways: 1) one dose o f a single copy Ac2 has no effect in endosperms homozygous for the bz2 m responding allele, while higher doses produce exponential increases in frequency of excision of Ds from the Bz2 locus, 2) the timing of ex cision is late regardless of Ac2 dosage and the negative dosage effect of Ac is not observed, 3) a single copy of Ac2 has no activity in the sporophyte and 4) alleles which interact with Ac (wx-m1, c-m1, wx-m9) do not respond to a single dose of Ac2, and no breakage occurs at Ds in the standard location. In plants with the more potent Ac2Ac2 allele , a low level of activity is found in the anther wall of the sporophyt e and alleles of the Ac-Ds system show some response. The late timing of mutations and the absence of a negative dosage effect distinguish A c2 from Ac. Germinal mutations of bz2m to Bz2 occur almost exclusively in the staminate inflorescence and within a narrow time span, after m eiosis and before the second microspore division. Transpositions of Ac 2 have been studied in single copy Ac2 plants, where they are detected in kernels with higher dot number. Transpositions also occur in plant s with the Ac2Ac2 allele; they give rise to single copy Ac2 elements, either a remnant Ac2 at the original site on chromosome 8, or a tr-Ac2 on a chromosome other than chromosome 8 or on the chromosome 8 homolo g. The ratio of remnant Ac2 to tr-Ac2 cases favors the hypothesis that transposed elements are inserted at unreplicated recipient sites. Int ercrosses of 10 newly arisen tr-Ac2 alleles revealed three. and possib ly four, were inserted in chromosome 1. Two show linkage with Bz2 and one, originally believed to be close to Bz2, was later found to be lin ked to c2 on chromosome 4: the latter tr-Ac2 element may represent a s econdary transposition.