sugary-3 (su3), a new endosperm mutant with sugary/shrunken phenotype

--Philip S. Stinard

A new recessive mutant conditioning sugary/shrunken endosperm was found segregating on the selfed ear of a plant (84-5081-3) grown from the outcross to standard of a putative Mutator-induced wx mutant, wx-Mus3. Mutant kernels have the glassy appearance of su1, but are smoother than su1 kernels. Mutant kernels are also shrunken in a manner reminiscent of sh1. Mutant kernels give rise to normal, healthy green plants. Analysis of the starch of mutant endosperms reveals that mutant endosperms have a higher amylose content (around 40%) compared to wildtype (typically 25%; Jay-Lin Jane, J-L, Iowa State University, unpublished data). This mutant, originally designated su-sh*-5081, was allele tested with all mutants with similar phenotype, including sh1, su1, su2, ae, du, sh2, bt1, and bt2. All allele tests proved negative. Since the endosperm phenotype is most similar to su1 and su2, su-sh*-5081 was given the designation sugary-3 (su3). Although the endosperms of su3 kernels have elevated amylose content, su3 was not designated as ae2 because su3 lacks the unique phenotypic interaction that ae has with wx. Kernels homozygous for both ae and wx in the genetic backgrounds we work with are small, severely wrinkled, and translucent. Kernels homozygous for both su3 and wx are shrunken like sh1, but have the opaque appearance of wx.

We attempted to locate su3 to chromosome arm using a series of B-A translocations covering 19 of the 20 maize chromosome arms. We obtained multiple negative results for all chromosome arms tested. We next crossed su3 to a series of wx marked translocations with breakpoints near the centromeres of the non-9 chromosomes. The F1's were selfed and were outcrossed to either standard wx testers or su3 testers. Only selfed ears of the F1 between su3 and wx T4-9g (4S.27, 9L.27) showed evidence of linkage (repulsion) between su3 and wx. The outcrosses of su3 / wx T4-9g to wx and su3 were grown in our 1991 nursery, the resulting plants self-pollinated, and the selfed ears scored for su3 and wx. The data from both types of outcrosses were combined in order to calculate linkage values. The results, reported in Table 1, indicate tight linkage between su3 and wx T4-9g (p = 6.5 ± 2.5cM). It should be noted that the parental and crossover classes are unbalanced, with a shortage of kernels in the wx classes. This is most likely due to poor seed set on semisterile ears carrying the translocation, leading to the rejection of any ear that had too few kernels to be properly classified.

Table 1. Two-point linkage data for su3-wx T4-9g (classification for su3 and wx only). Testcross: Su3 Su3 wx wx n n X (su3 Wx n / Su3 wx T4-9g) or su3 su3 Wx Wx n n X (su3 Wx n / Su3 wx T4-9g).
 
Reg. Phenotype No. Totals
0 su3 + 59  
  + wx 28 87
1 su3 wx 1  
  + + 5 6
% recombination su3--wx=6.5±2.5

Because of the evidence of linkage between su3 and wx T4-9g, we conducted a three-point linkage test involving su3 and the chromosome 4 linkage markers su1 and gl4. The test was set up as a modified backcross as indicated in Table 2. Kernels from the backcross ears were planted in the field, the resulting plants self-pollinated, and the selfed ears scored for su1 and su3. Kernel samples from each ear were planted in the sandbench, and the resulting seedlings were scored for gl4. The results (Table 2) indicate independent segregation between su3 and su1, and between su3 and gl4. The linkage observed between su1 and gl4 (p = 18.6cM) is close to that reported on the 1991 maize linkage map (15cM). Since su3 is not uncovered by either TB-4Sa or TB-4Lf, we conclude that su3 is probably not located on chromosome 4.

Table 2. Three-point linkage data for su3, su1, and gl4. Testcross: Su3 Su3 Su1 Su1 Gl4 Gl4 X (su3 Su1 Gl4 / Su3 su1 gl4)
 
Reg. Phenotype No. Totals
0 su3 + + 38  
  + su1 gl 31 69
1 su3 su1 gl 37  
  + + + 25 62
2 su3 + gl 11  
  + su1 + 7 18
1+2 su3 su1 + 7  
  + + gl 5 12
% recombination su3--su1=50.3±3.9
% recombination su1--gl4=18.6±3.1
% recombination su3--gl4=49.7±3.9

Further corroboration that su3 is not located on chromosome 4 was obtained by conducting testcrosses of hypoploid plants grown from crosses of su3 by TB-4Sa and TB-4Lf. Crosses of hypoploid plants by su3 produced ears segregating approximately 1:1 for mutant and wildtype kernels, confirming that su3 is not uncovered by TB-4Sa or TB-4Lf. Since su3 is linked to wx T4-9g, but is not located on chromosome 4, the following possibilities remain: (1) su3 is located on chromosome 9, or (2) our wx T4-9g stock (obtained from the Maize Stock Center) is misidentified and instead is a translocation stock involving chromosome 9 and a chromosome other than chromosome 4. Both of these possibilities are currently being investigated.


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