--Detlef Piatkowski and Hans-Peter Döring

The sh-m6233-Dip allele arose in one of our maize cultures carrying Ac plus the sh-m6233 allele isolated by B. McClintock. Maize plants carrying the sh-m6233-Dip allele or the original sh-m6233 progenitor allele display Ac-dependent chromosome breakage. To analyse the chromosome breakage pattern, both strains which also carried Yg2 and C were crossed as male to a strain carrying yg2 c sh wx. Chromosome breakage was scored by the loss of C in the endosperm tissue and by the loss of Yg2 in the leaf tissue.

Chromosome breakage in the endosperm occurs very early during development, as the uncoloured sectors are very large and usually cover a larger area than the coloured tissue. In several cases there was no colour formation in the aleurone at all. This is indicative that chromosome breakage occurred during the last postmeiotic mitosis of the microspores or upon formation of the primary endosperm nucleus. Chromosome breakage in the leaves was displayed by the appearance of very many small yg2 streaks. This variegation pattern shows that the Ac acts with a high frequency but late during the development of the sporophyte. Only in three plants out of approx. 350 plants, we found larger yg2 sectors which comprised 1/6 of the leaf width and extended over the whole leaf length.

We examined the biochemical structure of the sh-m6233-Dip allele by Southern analysis. The authentic sh-m6233 allele carries a 4 kb double Ds structure in the first intron of the sucrose synthase gene (Weck et al., EMBO J. 3:1713-16, 1984). The sh-m6233-Dip allele had suffered a rearrangement at the double Ds structure, whereas sh sequences on either side of the double Ds structure remained unaltered with respect to their restriction map. The results of our Southern hybridizations indicate that the breakpoint of the rearrangement in the sh-m6233 allele is at the junction site between the double Ds structure and the 5' sh region or alternatively, at one of the junctions between the two Ds elements in the 4kb double Ds structure. For the sh-m6233-Dip allele it is very likely that at least a 3kb double Ds structure comprising one half and one complete Ds element is still linked to the 3' sequences of the sh gene. We have preliminary evidence that the missing other half Ds element is linked to the 5' sh region. To determine the nature of the rearrangement (inversion, insertion?) we are presently cloning the sh-m6233-Dip allele.

Upon analysis of approx. 20 progenitor cultures of the pedigree of the sh-m6233-Dip/sh-m6233 alleles, we found another two independent rearrangements at the sh-m6233 allele in the presence of Ac . Apparently, the sh-m6233 allele is highly unstable in the presence of Ac.

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