Use of two B-A translocations associated with the same chromosome arm but with different break positions, provides a method for isolation of molecular markers in a specific A-chromosome region. A B-A translocation, when crossed as staminate plant with a pistillate plant carrying the normal chromosome complement, produces a hypoploid whose A-B chromosome is deficient for the portion of the A chromosome distal to the breakpoint. The extent of the deficiency associated with an A-B chromosome is dependent on the breakpoint of a translocation. The A-B chromosome from a proximal translocation has a long deficiency, and that from a distal translocation has a short one. The two deficiencies cover a region in common--the region distal to the breakpoint of the distal translocation. They are different in the region between the breakpoints of the two translocations; the region is absent in the former, but it is present in the latter. Therefore, a molecular marker whose paternal signal appears on the first hypoploid but not on the second, is located in the region delimited by the two breakpoints. TB-10L19 and TB-10L32 were used to define the region on the long arm of chromosome 10 (10L) for isolation of AFLPs. Hypoploids from the two translocations were produced by crossing the two translocation-carrying inbred W22s as male onto inbred B73. TB-10L19 breaks very proximally to the 10th centromere (Lin, MNL 48:182-184), and its 10-B chromosome is deficient for almost the entire 10L. TB-10L32 has a breakpoint between g1 and r on 10L (Lin, MNL 48:182-184), and its 10-B chromosome is deficient for about the proximal half of 10L. Thus, the two 10-B chromosomes are different in the deletion region: the proximal half of 10L is deleted from the first 10-B chromosome, but it is not deleted from the second. Accordingly, an AFLP signal appearing on the hypoploid of TB-10L32, but not on the hypoploid of TB-10L19, is located on the proximal half of 10L. 47 AFLPs of this nature were identified in this study. These AFLPs are located on 10L but not on the B chromosome for the following reasons: TB-10L19 breaks in the proximal one fifth of the distal heterochromatic region on the B chromosome (Lin, Genetics 92:931-945), and TB-10L32 breaks in the proximal one fourth of the same region (Lin, unpublished); therefore, the 10-B(19) carries more distal B heterochromatic region than the 10-B(32) does. If an AFLP were located on the B chromosome, it would be present on the hypoploid of TB-10L19, but not on that of TB-10L32. None of the 47 AFLPs behave in this pattern; thus, these 47 AFLPs must be 10L-specific.

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