Evidence for frequent random disjunction of the A and AB elements of certain B-A translocations

It has been generally assumed that the A and AB chromosomes of a B-A translocation heterozygote usually pair at meiosis and go to opposite poles. Recent results indicate that disjunction sometimes may be more nearly at random. Known translocation heterozygotes of TB-9La, TB-9Lc, and TB-9Sd were crossed as female by a wx tester stock. Root tip counts were taken and the 21-chromosome plants were grown to maturity. Pollen samples of all plants were taken and each plant was crossed to an appropriate female tester. Each plant was crossed as female by a wx tester. This generated a set of TB's in which the wx allele is on the A chromosome, and Wx is on the BA of TB-9Sd and on the AB of TB-9La and TB-9Lc. A portion of the preliminary data is presented below:


Of the eleven plants listed above, nine uncovered c sh, Bf or gl15 Bf and thus must have been translocation heterozygotes.

The Wx:wx ratio of the TB-9 heterozygotes is significantly skewed in favor of the Wx class. The excess of Wx in the pollen of the TB-9Sd heterozygotes may be explained by the random movement of the BA to either pole during meiosis. However, the excess of Wx pollen in the TB-9La and TB-9Lc heterozygotes must have a different explanation. If the BA chromosome of translocation heterozygotes generally moves at random to the poles at meiosis 1, as studies have shown, then half of the pollen carrying the A chromosome should lack part of 9L and likely abort. Because these grains actually represent a portion of the Wx class, the excess of Wx pollen becomes even more striking.

Of the remaining two plants, 11026-2 appears to be a tertiary trisomic (AABA) and 11026-4 appears to be a new type (AAAB). Plant 11026-4 gave approximately 1Wx:1wx when crossed as female by a wx tester. However, when outcrossed onto a wx tester, it gave very few Wx kernels. It also failed to uncover gl15 and Bf, so it presumably did not possess an AB and a BA, both of which are usually required for nondisjunction to occur. If it was a tertiary trisomic (AABA), it should have been homozygous wx, so the few Wx kernels found on the testcross onto wx gl15 Bf ear would have been contaminants. However, if the plant was of the constitution AAAB, then the presence of Wx on the AB could explain the approximate 1Wx:1wx segregation observed in the pollen. Although confirmation is needed, it appears that the plant was an AAAB trisomic. The low percentage of Wx transmitted by the pollen is presumably a consequence of poor competitive ability of the aneuploid AAB pollen grains.

The generation of this AAAB plant from the TB-9Lc heterozygote may be due to the random disjunction of the AB to the poles at meiosis. This may also explain the excess of Wx pollen observed in the TB-9La and TB-9Lc heterozygotes. Root tip counts will be taken on the seedlings from the Wx kernels of plant 11026-4 to determine if they truly represent the AAAB class. Meiotic observations will also be undertaken.

B. Kindiger, J. B. Beckett and C. Curtis

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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