Since about 10 years ago, Giemsa banding procedure has been developed and successfully employed in karyotype analysis of various organisms. The advantage of this technique lies in its characterization and localization of heterochromatins in the somatic chromosomes and the interphase nuclei through DNA denaturation and reannealing procedures. These heterochromatins are usually not visible through the standard aceto-carmine staining technique.
Diploid perennial teosinte (Zea diploperennis) has aroused a great deal of interest among plant scientists. However, its mitotic chromosomes have not yet been examined with the Giemsa procedure. This report deals with the results of our most recent experiments on the recognition of mitotic chromosomes of diploid perennial teosinte and its hybrids with maize with the above technique.
At the premetaphase stage of root-tip cells, distinct terminal bands, varying in size, appeared in eight of the 10 chromosome pairs. The short arms of chromosomes 1 and 2 had a small band, while the long arms of chromosomes 3, 4, 6, 7, 8 and 9 had a medium-sized band. Most of the bands were homozygous. However, chromosomes 5 and 10 were without any visible bands. The nucleolar organizer regions (NOR) of chromosome 6 were consistently differentiated and were densely stained. The satellites of these chromosomes were also darkly differentiated and frequently folded back on the NORs.
Mitotic chromosomes of the root-tip cells from F1 hybrids of the diploid perennial teosinte and an inbred maize, Zhi-35 from China, were stained using the same procedure as used for the teosinte parent. It was observed that recognizable terminal bands of the eight chromosomes of the teosinte parent reappeared. In addition, three intercalary bands on the long arms of chromosomes 4, 6 and 7 were identified. They came from the maize parental plant. The difference between the bands of the two parental species was very distinct.
The recognition of the parental chromosomes in the root-tip mitoses of the F1 hybrids between diploid perennial teosinte and maize was further investigated and confirmed by crossing the same teosinte with two other maize inbreds, Qi-330 and Huang-tzo-4, both from China. It was consistently found that the bands corresponded in shape, number and size with the knobs appearing on the pachytene chromosomes stained with conventional techniques. Furthermore, nucleolar organizer regions were persistently stained using the Giemsa banding procedure. This differs from the results reported by Ward on the Giemsa C-banding patterns of two varieties of maize (1980, Can. J. Genet. Cytol.). He could not stain the NORs of chromosome 6. This discrepancy could be accounted for by variations of technique and stage of mitosis.
In the past, knob counts have been used as reliable cytological markers to relate races of maize and teosinte. Because of the lack of a suitable technique to recognize knobs in the mitotic chromosomes and because of the small size of these chromosomes, the counts were mainly made on the basis of pachytene chromosomes. Nevertheless, due to the difficulty in distinguishing homozygous knobs from heterozygous ones, the results of the knob counts were often controversial and hard to confirm. From now on, it appears feasible that knob counts of maize and teosinte be done with Giemsa banding technique because the bands correspond exactly with the knobs. Thus it may better facilitate our understanding of the somatic chromosome characteristics of both maize and teosinte. It may also better reveal the relationships not only between these two species, but among the various races within each species as well. Therefore, it is conceivable that Giemsa banding has indeed great potential in both the study of variations of plant karyotypes and genetic research in general.
Y. C. Ting and M. G. Gu
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