--Philip S. Stinard
The duplicate factor pair lw3 and lw4 conditions lemon (pale yellow) kernels that give rise to white seedlings. The two factors are located on 5L, and on chromosome 4 near the centromere, respectively. Since a lw3 lw4 stock was available in our cold storage facilities, but not at the Stock Center, we undertook to characterize our stock for the purpose of donating it to the Stock Center. We thought it most appropriate to isolate lines homozygous for each factor individually. As this had not been done before, we developed the theoretical approach for using B-A translocations to locate duplicate factors outlined in an accompanying article, and applied it to the lw3 lw4 system.
A stock segregating 3:1 for yellow:lemon-white kernels was obtained by self-pollination of plants grown from yellow kernels of a selfed ear segregating 15:1. The 3:1 stock was maintained for several generations by self-pollination of plants grown from yellow kernels of selfed 3:1 ears. As it was not known which duplicate factor was homozygous in the 3:1 stock, plants producing 3:1 ears were assigned the genotype symbol lwx lwx Lwy lwy, where x=3 and y=4, or vice versa. A homozygous lwx lwx Lwy Lwy stock was in turn derived by self-pollination of plants grown from yellow kernels of 3:1 ears, and selection of ears producing only yellow kernels. The lwx lwx Lwy Lwy stock was crossed onto the B-A translocation stocks TB-4Lf and TB-5La, and the F1's crossed onto independently derived 3:1 lw3 lw4 stocks as well as the appropriate TB testers (c2 and pr gl8, respectively).
Of four crosses involving TB-5La, two segregated for lemon kernels, suggesting that the duplicate factor uncovered in the cross is on 5L (and is therefore lw3). Yellow kernels from the two segregating ears were sown in the sandbench, and gave rise to seedlings, some of which were white. If lw3 is indeed uncovered by TB-5La, then the lemon kernels with putative hypoploid mutant endosperms should have hyperploid germs that carry TB-5La as well as lw3. To test this, three lemon kernels from each ear were sown in the field, and the resulting plants selfed and outcrossed to the 5L tester pr gl8. The selfed ears segregated for a low frequency of lemon-white kernels (expected of the selfs of segmental trisomics involving duplicate factors; see Neuffer and Beckett, MNL 61:49-50). The outcrosses to pr gl8 all segregated for pr kernels giving rise to wildtype seedlings, and Pr kernels giving rise to seedlings segregating for glossy, thus confirming the hyperploid status of the male parent and the location of lw3 to 5L.
Since the factor uncovered by TB-5La was lw3, the stock crossed onto TB-5La must have been homozygous for lw4. Thus, in the stock lwx lwx Lwy lwy, x=4 and y=3.
Linkage studies were conducted on the homozygous lw4 stock in order to confirm the location of lw4 on chromosome 4. The lw4 lw4 Lw3 Lw3 stock was crossed onto a homozygous su1 gl4 tester. The F1 was pollinated by lw4 lw4 Lw3 lw3 plants, and the progeny grown in our selfing block and self-pollinated. From the manner in which the cross was set up, only half of the selfed ears should carry lw3 and thus segregate for lemon kernels. It is only such ears that provide meaningful linkage data. Ears segregating for lemon kernels were scored for su1 and for the ratio of yellow:lemon kernels. Kernel subsamples from these same ears were planted in the sandbench, and the resulting seedlings scored for gl4. The data from this linkage test, summarized in Table 1, yield the following linkage relationship: su1-10.5-lw4-10.5-gl4. Thus, lw4 is approximately equidistant between su1 and gl4 on chromosome 4. These data are in good agreement with the F2 data of Tulpule (Am. J. Bot. 41:294-301) that place lw4 13cM distal to su1. These data would also increase the distance from su1 to gl4 from 15cm (current linkage map distance) to 21cM. Since the number of individuals in our linkage study is rather small (n=86), this experiment will be repeated in order to better place lw4 on the chromosome 4 linkage map. For now the suggested map revision is:
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