Root-tip meristems of the pollen (H1) plants of maize strain 592, Chun-Dan 105, King Huang 13, King Huang 9, Lai-Ping-Bai, Shwei-Bai, Ba-Tang-Bai, and two hybrids of 592 x Lai-Ping-Bai and 592 x Eh-Bai were used for studies. These root-tips were also fixed with aceto-alcohol (1:3) and slides were prepared by following standard squash technique and doubly stained with Feulgen and aceto-carmine.
Variations in chromosome number: Chromosome counts were made with the
root-tip cells of the H1 plants of maize strains 592, Chun-Dan 105, King
Huang 13, Lai-Ping-Bai, King Huang 9, Ba-Tang-Bai, Shwei-Bai and two F1
hybrids of 592 x Eh-Bai, 592 x Lai-Ping-Bai. A pooled total of 352 cells
at either metaphase or late prophase were examined:
The chromosome numbers ranged from three to 40 per cell. Among these numbers, haploid cells (n = 10) were counted to be 197 (56%), diploid cells (20) to be 96 (27%), hypohaploid (< 10) to be 21 (6%), between haploid and diploid (11-19) to be 30 (10%), hyperdiploid (> 20) to be 2 (0.5%). It was surprising to see a cell with three intact chromosomes undergoing active mitotic division. It appears agreeable to Torrey's (1967) report that in tissue culture, only a portion of the plant genome might be necessary for normal mitosis.
When consideration is made on the basis of individual plants of the above six varieties and two hybrids, it has a total number of 29 plants. Among them, plants with root-tip cells having exclusively 10 chromosomes were scored to be 9 (31%); plants having exclusively 20 chromosomes to be 2 (7%); plants having chromosome numbers other than 10 and 20 (hypohaploids, hypodiploids and hyperdiploids) to be 18 (62%). Among these 18 plants, eight had in most of their cells 10 chromosomes, five had in most of the cells 20 chromosomes; and the remaining five had neither 10 nor 20 chromosomes as a predominant number.
Changes in chromosome structure: In general, alterations in chromosome structure were not so frequently found as those in chromosome number. However, at metaphase and anaphase of mitosis, fragments, bridges, tripolar separations, and early parallel splitting of chromatids were occasionally observed. These irregularities may account for, at least partly, the wide range of variations in chromosome number. Even though little study has been done on the cause of these irregularities it might be hypothesized that chemical as well as physical components of the culture may play some role.
Y. C. Ting, Ming-kwang Ku,* Li-chuan Kuo,* Shu-lan Kuan*
*Institute of Genetics, Academia Sinica, Beijing, China
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