Using compound B-A translocations in maize to segment chromosomes

The synthesis and use of compound B-A translocations has been described by Rahka and Robertson (Genetics 65:223-240, 1970). To date, they have been utilized almost exclusively to uncover regions in specific chromosome arms for which no simple B-A translocation was previously available. Both simple and compound B-A translocations have been extremely valuable in mapping genes to chromosome arms and for altering gene dosage in specific chromosomal segments.

The construction of a series of compound B-A translocations for a specific chromosome arm using a single precursor B-A translocation could provide a powerful tool for mapping genes within a specific chromosome arm and in testing for dosage effects for specific segments of that chromosome arm. Such a series can be generated by crossing plants containing a simple B-A translocation with plants containing reciprocal A-A translocations which have one of their breakpoints distal to the breakpoint in the A chromosome of the B-A translocation. For instance, one could cross a TB-5La plant (breakpoint in chromosome 5 is at 0.1 in the long arm) with T3-5(8528) which has its breakpoint in 5L at .72. When recombination occurs in 5L between the breakpoints, a chromosome is generated which contains a B centromere, proximal B chromatin, chromosome 5L between the two breakpoints in chromosome 5 (0.1L to 0.72L) and a segment of chromosome 3. When this plant is crossed as a male and the B centromere undergoes nondisjunction during the second microspore division, the number of copies of the segment of chromosome 5 between .1L and .72L is altered. When TB-5La chromosome undergoes nondisjunction, the entire segment distal to 0.1 is altered in gene dosage. The segment distal to 0.72 is unaltered in gene dosage in the former case while it is altered in the latter. By comparing these two, one can determine if a given gene is located proximal or distal to 0.72 in 5L. These can also be used for gene dosage comparisons.

Given a series of such translocations involving a common A chromosome, it would be possible to assign genes to specific segments of a given chromosome arm. This would be an extremely efficient method to place genes previously located to a specific chromosome arm with simple B-A translocations.

We have undertaken the production of a series of compound translocations for the long arm of chromosome 5. Presumptive compound translocation-containing kernels with segments of 5L between breakpoints 0.1 and 0.21, 0.48, 0.57, 0.60, 0.61, 0.72, and 0.87 have been recovered. We are in the process of further genetic and cytological verification of these as well as increasing them. We hope to illustrate the usefulness of this series of compound translocations in the manner discussed above.

We have determined that there is a dosage-dependent factor distal to the breakpoint of TB-5La which significantly alters the amounts of oleic and linoleic acid in maize embryos (Shadley and Weber, Can. J. Genet. Cytol. 22:11-19, 1980). The compound translocations we are generating are being utilized to further define the location of the factor(s) responsible for this alteration.

Several new mutations have been induced with ethyl methane sulfonate by Neuffer which are uncovered by TB-5La. We will be assigning these mutants to specific chromosomal segments utilizing this new series of compound translocations. If any additional unplaced mutants are available which are uncovered by TB-5La, we would appreciate it if they could be forwarded to us. Hopefully, the results will bear witness to the usefulness of such translocation stocks and lead to the generation of further series of compound translocations for other chromosome arms.

Jeff Shadley and David Weber


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