Two modifiers reduce the level of expression of Corngrass1 (Cg1)
--Bruce G. Abedon and W.F. Tracy

Corngrass1 (Cg1) has a grasslike appearance when highly expressed due to retention of juvenile traits in distal vegetative organs. Variation in Cg1 expression has been attributed to modifying loci as well as instability at the Cg1 locus. Analysis of segregation ratios for high and low expression Cg1 segregants in 11 families and three genetic backgrounds indicates the presence of two modifiers that reduce the level of Cg1 expression.

Eleven families segregating for Cg1 were created in two inbred (De811 and Mo17) and one hybrid (B73xMo17) backgrounds. Two, four, and five families were evaluated for De811, Mo17, and B73 x Mo17, respectively. The number of backcross generations was three for De811 and four for Mo17 backgrounds. The hybrid was made by crossing B73 to Cg1 plants that were the result of three generations of backcrossing to Mo17. The source of Cg1 was the Maize Genetics Cooperation stock 310D. Selfed Cg1 plants were used to initiate the backcross program in De811 and Mo17. The final number of plants per family varied from 29 to 178.

After anthesis, the number of Cg1 and normal segregants was determined for each Cg1 family. Cg1 plants were divided into two phenotypic classes (high and low expression) for each background. For De811 and B73 x Mo17 backgrounds, Cg1 plants were classified as highly expressed if they had narrow leaves, multiple tillers (>1), and inflorescences that were more than 50% vegetative. In the Mo17 background, Cg1 expression is normally more reduced than in other backgrounds. Consequently, in this background, highly expressed Cg1 plants had narrow to intermediate width leaves, few tillers (<1), and inflorescences that ranged from 50% vegetative to fully reproductive but with only one central spike. In all backgrounds, plants with low expression had slightly reduced to normal width leaves, few tillers (<1), slightly reduced to normal inflorescences, and juvenile epicuticular wax sectors occurring on a greater number of transition leaves than inbred or hybrid recurrent parent controls. This last criterion reduced the chance of near-normal Cg1 plants being misclassified as normal.

All data were analyzed by chi-square analysis. In order to determine if segregation was occurring normally in each Cg1 family, we tested a 1:1 segregation ratio for Cg1/+ and +/+ progeny. In order to test and distinguish between one and two gene models for modifying loci, we tested 1:1, 3:1, and 1:3 segregation ratios for Cg1 high and low expression classes. Contingency chi-square tests were also performed for the combined data from all Cg1 families.

Segregation of Cg1/+ and +/+ progeny fit a 1:1 ratio for each Cg1 family analyzed individually as well as for the combined data among families (Table 1). In addition, the contingency chi-square for this segregation was not significant (Table 1), indicating homogeneity among families. These results indicate that segregation occurred normally in the Cg1 families used in this study.

Table 1. Segregation of Cg1/+ and +/+ progeny in 11 families and 3 genetic backgrounds.
 
# of plants
Genetic background Family Cg1/+ +/+ 1:1 segregation - Estimateda X2
B73 x Mo17 A 19 13 1.13
B 32 35 0.13
C 12 17 0.86
D 22 13 2.31
De811 E 85 93 0.36
F 55 55 0.00
Mo17 G 26 33 0.83
H 28 19 1.72
I 24 31 0.89
J 32 24 1.14
K 25 30 0.45
Total 360 363 0.01
9.83b
a X2 at P 0.05, 1df= 3.84; P 0.01= 6.64
b Contingency X2 (at P 0.05, 10df= 18.31; P 0.01= 23.21 )

The contingency chi-square test for segregation of high and low Cg1 classes was significant (p<0.01), indicating non-homogeneity among families (Table 2). Consequently, segregation ratios for high and low Cg1 expression classes were tested on an individual family basis.

Table 2. Segregation of high and low Cg1 expression in 11 families and 3 genetic backgrounds.
 
# of plants
Genetic background Family Low expression High expression
Segregation - Estimateda X2
B73 x Mo17 A 10 9 0.05 7.74 5.07
B 9 23 6.13 0.17 37.50
C 2 10 5.33 0.44 21.78
D 6 16 4.54 0.06 26.73
De811 E 40 45 0.29 22.06 35.3 9
F 13 42 15.29 0.05 77.39
Mo17 G 14 12 0.15 11.54 6.21
H 16 12 0.57 15.43 4.76
I 5 19 8.17 0.22 37.56
J 10 22 4.50 0.67 32.67
K 7 18 4.84 0.12 29.45
Group 1c 80 78 0.03 55.37 50.03
Group 2d 52 150 47.54 0.06 261.39
26.13b
a X2 at P 0.05, 1 df = 3.84; P 0.01 = 6.64
b Contingency X2 (at P 0.05, 10 df = 18.31; P 0.01 = 23.21 )
c Pooled data from families A, E, G, and H
d Pooled data from families B, C, D, F, I, J, and K

All families fit hypothesized segregation ratios (Table 2). Families A, E, G, and H fit a 1:1 segregation ratio while families B, C, D, F, I, J, and K fit a 1:3 segregation ratio. In order to determine if the non-significant deviation from hypothesized ratios was due to sampling error resulting from small sample size in some families, data from families fitting a 1:1 segregation ratio (families A, E, G, and H) were pooled to form Group 1 and data from families fitting a 1:3 segregation ratio (families B, C, D, F, I, J, and K) were pooled to form Group 2. Group 1 and Group 2 fit a 1:1 and 1:3 segregation ratio, respectively. Since all families were derived from the same Cg1 source, these results indicate that all families segregated in hypothesized ratios.

The results from families B, C, D, F, I, J, and K indicate the presence of two unlinked, dominant, modifying genes that together reduce Cg1 expression. Data from families A, E, G, and H indicate the presence of one dominant modifying locus. Since all families were derived from the same source of Cg1, one might conclude that the 1:1 segregation ratio for families A, E, G, and H, resulted because one of the modifiers in families B, C, D, F, I, J, and K was fixed while the other was segregating. Due to the genetic structure of these families, we feel that self progenies are required to confirm the results of this experiment. These progenies are being generated in the greenhouse. 


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