We previously reported (MGNL 54:89-90) undertaking the production of a series of compound B-A translocations for the long arm of chromosome 5. At that time we had obtained presumptive compound translocation kernels for segments of 5L between breakpoints 0.1 (the breakpoint of TB-5La) and 0.21, 0.48, 0.57, 0.60, 0.61, 0.72 and 0.87. We have positively verified three of these (0.48, 0.57, 0.61) and feel confident about two more (0.21 and 0.60). All of the compound B-A translocation kernels producing plants exhibiting partial sterility were backcrossed to their respective testers used to originally identify them, and also to various inbred and hybrid lines. Compound translocations for segments of 5L between 0.1 and 0.48, 0.57 and 0.61 were backcrossed to an a1 Dt tester. When ears from such crosses segregated for the marker, kernels with colorless endosperm and pigmented embryos consistently yielded chromosome counts of 22, while kernels with colorless endosperm and non-pigmented embryos yielded chromosome counts of 20. The former are interpreted to be hyperploids containing two copies of the compound B-A translocated chromosome, while the latter are interpreted to be diploids. Pachytene analysis of microsporocytes of the original 5-3 translocations used to generate the compounds were performed to verify the breakpoints previously catalogued by A. E. Longley. T3-5(5874), 5L 0.21, 3L 0.16, and T3-5(8351), 5L 0.60, 3L 0.75, were both confirmed to their approximate positions. T3-5(8528), 5L 0.72, 3L 0.06, turned out to involve a diminutive chromosome 9 with the breakpoints at or near the centromeres. T5-9(8386), 5L 0.87, 9S 0.13, turned out to have the breakpoint in 5S at approximately 0.8-0.9. The findings on the latter two translocations would explain the aberrant results obtained from the crosses involving presumptive compounds generated using those translocations.
Presumptive B-A translocation kernels for breakpoints in 5L at 0.08 and 0.14 have been obtained from the previous summer. The former possesses a breakpoint proximal to that of the original TB-5La chromosome. The compound could be generated if recombination followed mispairing between the B5La chromosome and the 53 chromosome. Alternatively the actual breakpoint in 5L for T3-5(8104) may be distal to 0.08. No pachytene analysis has been done for these two compounds. The genetic and cytological confirmation should come from crosses made in our winter nursery.
This series of compound B-A translocations for 5L is currently being used to locate the position of newly induced mutants in 5L between the individual breakpoints. We are also using it to position a dosage dependent factor previously located to 5L which alters the levels of oleic and linoleic acids in the embryo. The tables below show the data obtained (% + standard error) when the dosage of 5L is altered between breakpoints 0.1 and 0.57 or 0.61. In both cases comparisons are made between sibling embryos either hyperploid or diploid for the designated segments.
S80 a1 Dt x 11 5-2 (B5L 0.1-0.57, 3L)
S80 a1 Dt x 11 6a-4 (B5L 0.1-0.61, 3L)
ns--Not significantly different from the diploid at p=0.05
Since no significant differences are found in these comparisons the dosage dependent factor must lie distal to the segments altered, beyond 5L 0.61.
J. D. Shadley and D. F. Weber
Return to the MNL 55 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page