A second R1 allele-specific aleurone color inhibitor, Inr2, is located on 9L --Stinard, P As reported in last year's MNL (MNL 74:70-71), two R1 allele-specific aleurone color inhibitors were isolated from the novelty maize variety "John Deere." One inhibitor, Inr1, had been previously mapped to the long arm of chromosome 10 (MNL 73:89-90) and an allele of Inr1 is also found in the Maize Genetic Stock Center's da1 stock. Here we report the mapping of the second inhibitor, Inr2, to the long arm of chromosome 9.

Because Inr2 exhibits dominant inhibition of aleurone color, easily scored on the ear in the presence of appropriate R1 alleles (herein referred to as R1-S, or susceptible alleles), we chose to map inr2 using a set of wx1 marked A-A translocations. Plants homozygous for Inr2 and R1-S were crossed to a series of wx1-marked translocations in a colorless aleurone (r1) background. F1 plants were backcrossed by a homozygous inr2 wx1 R1-S line, and the resulting ears were scored for colorless (Inr) vs. colored (inr) and waxy (wx) vs. starchy (Wx) kernels (Table 1). In crosses involving wx1 y1 T6-9e, some of the backcrosses were made by plants homozygous for inr2, wx1, y1, and R1-S, so three-point linkage data for wx1, y1, and inr2 were obtained (Table 2). All crosses demonstrated linkage of inr2 with wx1, indicating that inr2 is located on chromosome 9. The distance between wx1 and inr2 showed variability (from 7.6 centimorgans to 30.3 centimorgans) depending upon which A-A translocation was used in the linkage cross. This kind of linkage variability is not unusual in crosses involving translocation stocks (E. B. Patterson, 1952. Ph.D. Thesis, California Institute of Technology). Furthermore, the production of viable duplicate-deficient eggs by adjacent disjunction when plants heterozygous for certain translocations are used as females in linkage crosses can also distort linkage data somewhat.

The linkage data obtained from the crosses with T6-9e (breakpoints 6L.18 9L.24) provide sufficient information to fix the location of inr2 on chromosome 9 with respect to wx1. Because y1 is located on the 9-6 chromosome very close to the breakpoint in T6-9e, and the gene order determined from the linkage data is clearly wx1 y1 inr2, inr2 has to be located on 9L distal to the T6-9e 9L breakpoint (9L.24). If we take the T6-9e linkage data for the wx1--inr2 distance (25.2 centimorgans) as a minimum value for the distance between wx1 and inr2, inr2 is located near or distal to bk2 on 9L. Additional tests using B-A translocations and 9L linkage markers will be conducted to confirm and refine the location of inr2 on 9L.

Table 1. Two point linkage data for wx1 inr2 in crosses involving various A-A translocations.
Testcross: [wx1 T inr2 r1 / Wx1 N Inr2 R1-S] X wx1 N inr2 R1-S.
 
 
Region 0
Region 1
 
Translocation Wx Inr wx inr wx Inr Wx inr % recombination wx1--inr2
T1-9(5622) 246 182 60 82 24.9 +/- 1.8
T1-9(8389) 292 289 47 49 14.1 +/- 1.3
T2-9c 215 385 120 69 24.0 +/- 1.5
T2-9b 177 131 48 86 30.3 +/- 2.2
T2-9d 205 200 34 83 22.4 +/- 1.8
T3-9(8447) 204 245 13 24 7.6 +/- 1.2
T3-9(8562) 75 74 16 8 13.9 +/- 2.6
T4-9(5657) 296 212 63 48 17.9 +/- 1.5
T5-9(022-11) 254 250 22 46 11.9 +/- 1.4
T5-9a 214 184 59 47 21.0 +/- 1.8
T7-9a 250 248 41 78 19.3 +/- 1.6
T8-9d 213 194 47 66 21.7 +/- 1.8

Table 2. Three point linkage data for wx1 y1 inr2 in crosses involving T6-9e.
Testcross: (wx1 y1 T6-9e inr2 r1 / Wx1 Y1 N Inr2 R1-S) X wx1 y1 N inr2 R1-S.
 
Region Phenotype No. Totals
0 wx y inr 459  
  Wx Y Inr 497 956
       
1 wx y Inr 125  
  Wx Y inr 99 224
       
2 wx Y Inr 29  
  Wx y inr 24 53
       
1 + 2 wx Y inr 9  
  Wx y Inr 10 19
% recombination wx1--y1 = 5.8 +/- 0.7
% recombination y1--inr2 = 19.4 +/- 1.1
% recombination wx1--inr2 = 25.2 +/- 1.2
 
 


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