Maize hypoploids are derived from non-disjunction of the B-A chromosome from a B-A translocation stock. In a genetic sense, a hypoploid has part of one chromosome arm missing, and is hemizygous for that particular arm segment. Preliminary study showed that a B-A translocation per se will not have any significant effect on kernel weight, plant height, and leaf size. This result suggests that the B chromosome and the break point of the translocation will not have a critical deleterious effect on proper genetic function of the genome. But in hypoploids, when one chromosome arm is missing, different chromosome arms show different effects on kernel weight, plant height, and leaf size. Our studies show that the degree of effects is not highly correlated with the length of chromosome arm fragment, but is highly dependent on the missing arm. In other words, certain chromosome arms are more critical than others for the expression of those quantitative traits. This implies a non-random distribution of genes that are important for the expression of a quantitative trait. It also implies clearly that genetic dosage effects can be immediately observed, if gene doses are altered.
Materials used for this study came from progenies in an ongoing project for converging TB's to inbred lines. TB-carrying pollen parents at the BC3 level with A619 were crossed onto genetic testers for identification of hypoploids. Samples of 100 kernels were weighed and the percent reduction for hypoploid versus sib endosperms was calculated. Hypoploid plants were identified in the field and plant height and leaf width were measured. The physical arm length missing (Table 1, A) was calculated from data on breakpoints (Kindiger et al., MNL 60:50) and arm lengths (Maize for Biological Research, p. 52).
Results showed that the hypoploids of TB-1Sb, TB-1La, TB-5Sc, TB-7Lb, TB-9Lc and TB-10L19 have substantial effects on kernel weight (Table 1, B). A 0.48 correlation coefficient between physical chromosome arm length and kernel weight was obtained, which implies that kernel weight reduction was not directly proportional to physical arm loss. The plant height of hypoploids is consistently smaller than that of their normal sibs, but TB-1Sb, TB-1La and TB-9Lc have greater effects than others on plant height (Table 1, C) and on leaf width (Table 1, D).
Plant and leaf color of the hypoploids is also variable. Hypoploids for TB-7La and TB-8Lc are dark green, TB7Sc and TB-9Lc are yellow green, and those for TB-1Sb, TB-3Sb, and TB-5La are pale green. The leaf size and leaf shape of the hypoploids are modified also. TB-1Sb, TB-5Sc, TB-5La, and TB-6Lc have long, narrow leaves, TB-1La has short, narrow leaves, TB-4Sa and TB-8Lc have short, broad leaves, TB-3La and TB-7La have short, erect leaves.
Based on these preliminary results, it is evident that certain chromosome arms show a positive correlation of their genetic effects on kernel weight, kernel size, leaf width, and plant height. The long and short arms of chromosome one are very critical for normal kernel, normal leaf, and normal plant development. Since these preliminary results were based on F1 materials from crosses on different testers, the background and environment may have great effect on the phenotypic expressions. Also, the materials that were screened came only from one inbred line, and the hypoploid phenotypes cannot be used for a generalized description. In any event these results show that certain important quantitative traits are influenced by dosage of certain chromosome arm regions, and these regions are important for crop characters and their improvement.
Ming-Tang Chang, E.H. Coe and J.B. Beckett
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