University of Missouri and USDA/ARS
The 16 B-A translocations in A619, A632, B73 and Mo17 were planted in summer 1988. Reciprocal crosses of normal-A619 x TB-A632, normal-A632 x TB-A619, normal-B73 x TB-Mo17, and normal-Mo17 x TB-B73 were made. For each B-A translocation, 10 plants were used as male and crossed onto the normal elite line and also onto a TB tester. Results from the TB tester will separate these F1 hybrid ears into two classes: normal-elite x normal-elite and normal-elite x TB-elite.
The first quantitative genetic trait that we have studied is the kernel size/weight. Because of the non-disjunction of the B-A chromosomes, the endosperms of the F1 hybrid (normal-elite x TB-elite) will carry 2 doses, 3 doses, or 4 doses of the translocated chromosome arm. The change in genetic dose of a codominant gene specific for kernel size/weight will change the kernel size/weight distribution. The mean value should usually be greater in hyperploid endosperm than in hypoploid endosperm, if the gene action is greater with additive effect. The variance should always be greater than in normal F1 crosses. B. Y. Lin found that kernel size/weight reduction is caused by paternal imprinting, in which an endosperm factor (Ef) from the male is responsible for kernel size reduction. For this study the kernel size reduction of a specific translocation is determined by comparison of variance (F test) between F1-TB and F1-normal seed samples. Random 100-seed samples from each ear were used for this comparison. A significant F test implies that an endosperm-related factor is located on that chromosome arm segment. If the F test is insignificant, the chromosome arm segment is not carrying an endosperm-related factor.
Results from normal-ear vs. TB-ear kernel weight comparison can be categorized for the B-A translocation effect or gene action into four different types: 1) kernel weight distribution is skewed to lower values and the mean value is reduced as shown in Figure 1. An endosperm factor is located on the chromosome arm. 2) kernel weight distribution is skewed to higher values and the mean value is increased as shown in Figure 2. An endosperm factor is located on the chromosome arm. 3) kernel weight distribution is as shown in Figure 3. No endosperm factor on the chromosome arm. 4) kernel weight distribution is greater but mean value does not change as shown in Figure 4. An endosperm factor is located on the chromosome arm. A summary result of the endosperm factor study is listed in Table 1. Results showed that the distribution of endosperm factors in the maize genome is quite different between elite lines. For example, Mo17 carries more endosperm factors than A619 and A632, which may imply that Mo17 will have larger effect than A619 and A632 in determining kernel size/weight in hybrid production.
The other quantitative genetic traits that were studied were tassel length, tassel branches, leaf length, leaf width, plant height, ear height, ear length and row numbers. Fifty-seed samples from selected B-A translocation F1 ears of A619 x A632-TB, A632 x A619-TB, B73 x Mo17-TB and Mo17 x B73-TB were planted in summer 1989. The hypoploid plants from each of the B-A translocations were identified by their smaller size and 50% pollen sterility. A few hypoploids were confirmed by RFLP (restriction fragment length polymorphism) polymorphic clones on that chromosome arm. Because the hypoploid plants are missing one chromosome arm, all genes in that particular chromosome arm will be hemizygous, and the additive genetic effect will be reduced to half. It is possible to determine a quantitative genetic trait by comparing the genetic effect of normal plants vs. hypoploid plants. By selfing the hypoploid plants, genes on that chromosome arm will be fixed. From each of the B-A translocations, five normal plants and five hypoploid plants were used for quantitative genetic trait measurments. A significant size reduction of a particular quantitative trait implies a quantative gene is located on that chromosome arm, otherwise the size reduction should be insignificant.
Results showed that the distribution of quantitative genetic factors in the maize genome is quite different between elite lines. The analysis of difference between elite lines is not finished. Therefore, we will be using results from A632 x A619-TB crosses as a model to describe the genetic effects of the B-A translocations on those quantitative genetic traits. The average tassel length of the normal F1 hybrid is about 15 inches and the hypoploid is about 6 inches. Tassel length ratio of normal/hypoploid is about 2.5 and not all the B-A translocations have dramatic effect as shown in Figure 5. The tassel branches of TB-1Sb hypoploids are dramatically reduced (13.33 in normal vs. 1.67 in hypoploids). In addition, TB-3La and TB-4Lc hypoploids had significant effect in reduction of tassel branches.
6 shows the analysis of normal/hypoploid ratio of leaf length, leaf
width and plant height of A632 x A619-TB F1 hybrids. Results show that
chromosome arm 1L had a significant effect on leaf length reduction, chromosome
arm 1S, 5S and 5L had significant effect on leaf width reduction, and chromosome
1L had significant effect on plant height reduction. Figure
7 shows the analysis of normal/hypoploid ratio of ear height. The chromosome
arm of 3L had clear effect in lowering the ear height of A632 x A619-TB
F1 hybrids (36 inches in normal and 4.67 inches in hypoploids). Figure
8 shows the analysis of normal/hypoploid ratio of row numbers of A632
x A619-TB. Results show that row numbers were reduced from sixteen rows
(normal) to twelve-fourteen rows (hypoploids) but the difference is not
significant. The effect on ear length was the same as on row numbers. These
results are very encouraging in showing that B-A translocations can be
used as a tool to study quantitative genetic traits by dissecting each
chromosome arm. The power of success of this study is highly dependent
on stock purity and proper experimental design. Otherwise, results will
not be repeatable.
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