The isolation and characterization of su3 and su4 duplicate loci --Philip Stinard In 1992 (Stinard, PS. 1992. MNL 66:4-5), we reported the isolation of a new sugary endosperm mutation, su3. At that time, we presented data indicating linkage of su3 with wx1 T4-9g (6.5 centimorgans between su3 and wx1), but not with other wx1-marked translocations. This seemed to indicate a location of su3 on chromosome 4. However, linkage tests of su3 with the chromosome 4 markers su1 and gl4 showed no linkage of su3 with these markers. Since su1 is tightly linked to wx1 in the translocation T4-9g (p ~ .04, Jackson, J and Stinard, P, MNL 72:79-81, 1998; p~.14, Jackson, JD et al., MNL 75:68-71, 2001), these results were inconsistent with our observations of linkage of su3 with wx1 T4-9g.

Other anomalies were also observed. F2�s of su3 with inbreds W64A and B73 consistently yielded ears segregating for a low frequency of su3 mutant kernels, approximating 15:1 ratios. At the time, we thought that this was due to poor penetrance since homozygous su3 lines often give a range of phenotypes from extreme to near normal. However, the 15:1 F2 ratios held up after several generations of backcrossing. Since the 15:1 ratios were suggestive of the involvement of duplicate loci, both of which need to be homozygous mutant in order to produce a mutant phenotype, we tested this hypothesis.

From the self-pollinated 15:1 segregating ear of a fifth generation outcross of su3 to W64A, nonmutant kernels were planted in our 1999 winter nursery, and the resulting plants self-pollinated. From the resulting ears, four ears with good 3:1 segregation of nonmutant:mutant kernels were selected and labeled A, B, C, and D. These ears would presumably be from plants homozygous mutant at one duplicate locus, and heterozygous mutant at the second duplicate locus. Nonmutant kernels from these ears were planted in our summer 2000 nursery, and the resulting plants were self-pollinated. Ears segregating 3:1 as well as ears with only nonmutant kernels were obtained. The latter ears would be expected to be homozygous mutant at one duplicate locus and homozygous nonmutant at the other duplicate locus; such ears will be referred to as single factor lines. Kernels from these four single factor lines (A, B, C, and D) were planted in our 2000 winter nursery, and intercrosses were made between these lines in all possible combinations.

During the summer of 2001, kernels from these intercrosses were planted, and the resulting plants were self-pollinated. The following results were obtained: F2�s of intercrosses A X C (8 ears) and B X D (8 ears) yielded nonmutant kernels only. F2�s of intercrosses A X B (8 ears), A X D (8 ears), B X C (9 ears), and C X D (8 ears) yielded 15:1 ratios of nonmutant: mutant kernels. We can conclude that the su3 mutant does indeed require two homozygous mutant loci for expression, and that lines A and C are homozygous mutant at one locus, and lines B and D are homozygous mutant at the other locus.

The existence of duplicate loci for su3 could explain the anomalous linkage results reported in 1992. The linkage experiment involving wx1 T4-9g might have been tracking one locus, and the linkage experiment involving su1 and gl4 might have been tracking the second locus. We have given the name su3 to the locus linked to wx1 T4-9g, and the name su4 to the unlinked locus. We are currently conducting a new round of linkage experiments using the separated factor lines in order to determine which are homozygous for su3 and which are homozygous for su4.
 
 

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