The allele C-m925408U is characterized by the coordinate loss of Yg2, C, Sh, Bz and Wx genes (sectored phenotype) present on the short arm of chromosome 9. This coordinate loss is ascribed to chromosome breakage. This breakage is initiated late in kernel development and is evidenced by small colorless or bronze (bz) sectors (depending on the genotype). Some of the sectors are so small that the bronze sectors (resulting from the loss of the Bz gene) are not readily revealed because of the diffusion of the Bz product from the surrounding Bz tissue into the bz sectors. Large sectors illustrative of early breakage are observed occasionally.
As expected, the sectored phenotype is more obvious when the mutant is crossed as male than when crossed as female. Kernels which are colored (with no discernible bronze sectors) when stained with I/KI solution revealed the loss of the Wx gene. We presume that the lack of bronze sectors might be due to diffusion of Bz product rather than due to the insertion of breaker or related elements into the Wx gene. However, the number of waxy sectors is less than observed when the mutant is crossed as male. Thus the dosage of breaker (which is thought to be autonomous) does not seem to influence the frequency of breakage. We recently obtained kernels with the C-I allele linked to the breaker. The loss of C-I in these kernels can be identified easily and we hope the pattern of breakage in these kernels can be used to identify more clearly the male-female differences observed in this mutant.
In plants grown from the Yg2 C Sh Wx bk/ yg2 c sh wx kernels, the loss of Yg2 is revealed when the plants are as young as 7-8 day old seedlings (two leaf stage). Among twenty plants screened, one plant showed 3 to 4 very early occurring yg2 sectors. In the remaining plants the yg2 sectors range from very late occurring loss to early occurring loss. The number of yg2 sectors ranges from 1-4. The colored round waxy kernels segregating on the same ear as the above kernels are used as control. Three out of sixteen control plants show 1 to 3 late occurring yg2 sectors. A much more thorough quantitation of the yg2 loss will be undertaken using one month old seedlings.
The loss of C in kernels is always correlated with the loss of Wx, indicating that the chromosome breaking structure is located proximal to the Wx locus. The limited mapping data obtained indicate that the breaker is located about 3 to 5 map units proximal to the Wx locus. A few exceptional kernels were obtained in which the breakage is initiated between the Bz and Wx genes. This phenotype indicates that the breaker carrying unit is transposable. The genetic ratios of the sectored kernels suggest that the breakage is caused by an autonomous transposable element.
This breakage mutant was screened for the presence of Ac, Uq, En and Cy transposable elements. System tests revealed that C-m925408U contains only En and Cy elements. Thus Ac, which is known to cause chromosome breakage, is not involved in causing the breakage observed in this mutant. Our initial attempts to correlate the breakage either with En or Cy were not successful because of high copy number of both En and Cy. Further tests will be undertaken to establish the system relationship.
In maize most of the transposable element mediated chromosome breakage alleles are caused by the Ac-Ds system (McClintock, 1946, 1947, 1948, 1949; Dooner and Belachew, 1991; Weil and Wessler, 1993). Apart from Ac only En/Spm is known to cause chromosome breakage (Cormack and Peterson, in press). Since the Mu element is shown not to cause chromosome breakage (Rowland et al., 1989) it will be interesting to see whether the chromosome breakage in C-m925408U is caused by Cy.
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