Lethal ovule lo2 is distal to TB-9Sb

When plants carrying c lo2 and TB-9Sb were pollinated by a normal stock, most of the offspring carried, as expected, the B-A translocation, since lo2 ovules abort. A confirmed tertiary trisomic plant (9 9 B-9Sb) derived from this material shed normal pollen but gave less than 50% seed set when selfpollinated. Except for occasional recombinants, all kernels were colored.

Since TB-9Sb and lo2 are 0.7 and 6.1 map units, respectively, distal to wx (D. S. Robertson, MNL 52:134, 1978; 0. Nelson, MNL 46:203-205, 1972), the breakpoint of TB-9Sb must be close to lo2 and likely between wx and lo2. Since colored kernels and normal ovules were transmitted together to the progeny of the self-pollinated tertiary trisomic plant, and c is already known to be beyond the breakpoint, both C and Lo2 must be on the B-9S chromosome. The trisomic plant was therefore of the constitution (c lo2)/(c lo2)/B-9S(C Lo2). Although the B-9Sb chromosome is transmitted through the pollen 7.8% of the time (D. S. Robertson, Egypt. Genet. Cytol. 4:189-200, 1975), the fact that the tertiary trisomic plant gave less than 50% seed set may indicate that the B-9Sb element is lost during megasporogenesis more than 7.8% of the time.

The tertiary trisomic stock can be maintained by self-pollination. By selecting semisterile plants that continue to segregate occasional colorless kernels, it should seldom be necessary to confirm the trisomic condition by root tip chromosome counts. Pollen from this stock should give progenies consisting almost entirely of +/lo2 plants. because crossing over between the breakpoint and lo2 should be rare. However, some of the plants will also carry the B-9Sb chromosome, which sometimes may be advantageous.

J. B. Beckett and Bryan Kindiger

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

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