C-I-b836024: Ac induces chromosome breakage at the shrunken locus

--Etienne Kaszas and Peter A. Peterson

The chromosomal breaker linked to C-I and described as C-I-b836024 (M. Muszynski, M and P. A. Peterson, PA, MNL 64:9), has been further characterized. System tests show that the breaker is identified to the Ac/Ds family (observation of the BFB cycle is another confirmation). According to linkage analysis, the breaker is tightly linked to the shrunken-1 locus, or even inserted in the shrunken gene (original mutant occurred as a variegated and shrunken kernel; transposed breakers give in most cases, a low variegated round phenotype).

Segregation ratios indicate that it is an autonomous element (1 sect.:1 non-sect. from crosses to Line C or C bz). Therefore, the structure of the breaker could involve two tightly linked Acs, or an Ac-Ds complex, similar to the structure of the macrotransposon described by Dooner, HK and Belachew (1991). Crosses were made with recessive distal markers (yg2: yellow green and wd1: deletion giving rise to lethal seedlings), to detect breakage in leaf tissue. No yellow green or white sectors were observed, which is in contrast to the original C-I Ds in the presence of Ac (McClintock,, B 1951). This may be a case of tissue specific expression of Ac.

A number of derivatives have been recovered from the original breaker. These include:

1) transposed breakers showing a low pattern of mutability (smaller sectors: 1-3 a), but the BFB cycle still occurs; transposed breakers were mapped and found to be linked to the original site, proximal or distal to the shrunken locus; the segregation (1 sect.:1 non-sect.), characteristic of autonomous control, suggests that transposition could involve the complex structure, able to induce breakage.

2) changes of state (C-I-b912009W: small sectors and a corresponding low mutability of the wxm9::Ds allele); the shrunken phenotype, together with a failure to detect any cross-over products carrying the recessive shrunken allele without the breaker, indicates that this breaker derivative did not move from its original site. Further tests are underway to identify the composition of the breaker.

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