Two new B-A translocations involving the long arm of chromosome 4

--J. B. Beckett

Several years ago, an effort was made to generate new B-A translocations on the short arm of chromosome 6. In order to use Bor-Yaw Lin, B-Y's procedure (MNL 46:193), a recessive endosperm factor was required on the short arm of chromosome 6. Because such factors were not available at that time, the reciprocal translocation T4-6(003-16) was employed to add a portion of the long arm of chromosome 4 onto 6S so that c2 could be used as the endosperm factor.

Immature tassels of a stock carrying T4-6(003-16), R-scm and several B chromosomes were X-rayed when regions with microspores at or just beyond the first division could be located. About a week later, when pollen from the appropriate regions began to shed, it was applied to silks of a c2 stock. The occasional colorless kernels, often with colorless scutella, were planted out and 87 individuals were testcrossed onto ears of a c2 stock. Two likely and four possible B-A translocations were identified. The two likely translocations were confirmed this summer.

One stock appears to carry a simple B-A translocation, hereby designated TB-4Lh. The only individual checked for pollen fertility had 5% aborted pollen. Because only colorless kernels with colored scutella had been planted, all plants should have been hyperploid heterozygotes, which often have little pollen abortion. Several testcrosses onto a hybrid tester stock carrying c2, dp1 and R-scm yielded ears segregating nearly normal-sized colorless kernels with colored scutella. In the sandbench, the colored kernels yielded progenies that segregated distal pale seedlings. Thus, the presence of a B-A translocation appears to be virtually confirmed, as the male parents did not carry dp1.

The second translocation, here temporarily designated TB-4L13474, appears to be compound rather than simple. Colorless kernels with colored scutella were planted. The only plant examined had approximately 60% aborted pollen, which is my usual estimate of pollen abortion in hyperploid compound translocation heterozygotes. Several plants were testcrossed onto the same hybrid tester stock mentioned in the preceding paragraph. The resulting ears segregated large and small kernels. Large colorless kernels had colorless scutella; small colorless kernels had colored scutella. In the sandbench, kernels from testcrosses of four plants were tested. The large colored kernels gave only normal seedlings at first, but after another day tiny narrow-leaved seedlings emerged, most of which displayed the distal pale phenotype. As with TB-4Lh above, the appearance of distal pale seedlings in testcross progenies virtually confirms the presence of a B-A translocation.

Although X-rays generate a variety of chromosomal aberrations, it is likely that TB-4L13474 is a compound involving the short arm of chromosome 6 and the the long arm of chromosome 4. If so, plants grown from the tiny colorless kernels with colored scutella should be hyperploid heterozygotes of the following constitution: 4, 6, 6-B, B-6S-4L, B-6S-4L. The appropriate designation for the translocation would then be TB-6Se-4L003-16. If the translocated segment of 4L can be removed by crossing over, the resulting simple B-A translocation should be designated TB-6Se. Testcrosses involving rgd and other likely 6S genes will be made in the greenhouse this winter in order to determine whether the translocation does indeed involve 6S.

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