Variable penetrance and expressivity of grassy tillers, gt

A line homozygous for grassy tillers in a B37 background, supplied by Dr. W. C. Galinat, was used as a pollen parent on seven inbreds: 0h43, 0h51A, R53, W64A, W117, W153R and W182BN. The penetrance and expressivity of the grassy tiller phenotype was variable both among and within the seven F2 populations.

When 0h43 was the maternal parent, gt behaved as a single recessive gene. A pooled ratio of 126+/-:46 gt/gt for nine F2 lines was observed. This fit the expected 3:1 ratio, X2 = 0.28 (Table 1). The F2 generations of other crosses did not segregate for grassy tillers in a 3:1 ratio. An excess of grassy tiller plants was observed in the cross with W182BN. The ratio of 38 normal to 54 grassy tiller plants was a poor fit to a 3:1 ratio (X2 = 55.71***). A deficiency of grassy tiller types as well as within-cross heterogeneity was observed in the F2 of the cross between gt/gt and W64a. The pooled ratio for 9 ear to row families was 187 normal:14 grassy tiller plants, a poor fit for a 3:1 ratio (X2 = 34.867**). While six of the 9 F2 families contained 0 grassy tiller plants out of 20 plants per family, in one F2 family 10 of 20 plants had grassy tillers. A significant heterogeneity X2 was found among the 9 segregating families (heterogeneity X2 = 23.591**). Of the seven crosses observed, six deviated significantly from a 3:1 ratio either in total X2 or pooled X2 or both (Table 1). Three of the seven were also found to have significant heterogeneity X2's.

The differences in penetrance were related to differences in the expression of the grassy tiller trait among the 7 crosses. In the F2 generations in which there was abundance of grassy tiller plants, such as the W182BN crosses, the grassy tiller phenotype was extreme. There were many tillers per plant and they were quite robust, at times reaching 75% of the height of the main culm. In those plants which were apparently gt/gt, from the crosses that had too few grassy tiller plants, the grassy tiller trait was expressed weakly. Only a few tillers were present in such plants and the tillers were usually a foot or less in height.

The variability in the penetrance and expressivity of the grassy tiller types did not seem to be due to any obvious environmental effects. It appears more likely that other genes may be suppressing or enhancing the expression of the grassy tiller allele. Considering the amount of variability in the expression of gt/gt observed in just seven crosses, selection for desirable phenotypes of gt/gt, especially for silage purposes, should be possible.

Table 1.

W. F. Tracy and H. L. Everett


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