Dissertation - Open Access
Master of Science (MS)
The appearance of true-breeding diploid after colchicine treatment of seedlings of certain diploid sorghum varieties has been attributed to a mutational effect made homozygous by somatic reduction followed by doubling to restore the original chromosome number. It was proposed that a mutant cell originating in the fashion described above could, though competitive advantage, form a new stem apex. After treatment of seedlings of two different but related tetraploid lines of sorghum, diploid plants have been obtained which were also mutants. To investigate the origin of a diploid step apex, which would later from the diploid plant, tetraploid seedlings were treated with colchicine under controlled environment and then fixed and sectioned. Cytohistological studies were made at 12- and 24-hour intervals over a 14-day period after application of colchicine. Diploid cells were identified by measurements of nuclear volume and by chromosome counts where possible. Of 121 treated seedlings, eight had stem apices entirely or almost entirely taken over by new diploid cells, and six showed the presence of few to many diploid cells in different locations of the apices. Two of the four survivors in 40 treated seedlings grown to maturity were diploids. In untreated tetraploid seedlings, no diploid cells were found. The occurrence of diploid cells in the treated plants must, therefore, be due to the colchicine treatment. Diploidization appeared to originate anywhere in the meristem tissue very soon after application of colchicine. Thirty-six hours after the application of colchicine, a stem apex was observed in which seven apparently diploid cells occurred in a leaf primordium. In one seedling fixed five days after colchicine application a completely diploid apical dome had been formed. Detailed observations were made on stem apices containing both tetraploid and diploid tissues to determine the means by which the transformation occurred. From these studies, it appeared that a new diploid stem apex could be formed either though replacement of cells in the tunica layer followed by periclinal division of some of these cells to form a diploid corpus, or by the intrusive growth of diploid tissue within the corpus which eventually replaced all the tetraploid cells and formed a new diploid tunica layer. The origin of diploid cells could not be discerned. However, the presence of cells with lobated nuclei and multinuclei in the apical dome of the colchicine treated stem apices might represent part of the events leading to somatic reduction. Neither chimeral sectors nor meiotic irregularity has been observed in diploid mutants arising after colchicine treatment of tetraploid seedlings. Therefore, it would seem that the new diploid stem apex is formed from a single cell which arose as a result of gene mutation followed by somatic reduction to a completely haploid set of chromosomes. These subsequently became diploid by endomitosis. From such a cell possessing a genetical and environmental competitive advantage, a new stem apex could develop which in turn would give rise to a diploid plant with an entirely different genotype than the original tetraploid tissue.
Library of Congress Subject Headings
Includes bibliographical references
Number of Pages
South Dakota State University
Chen, Chen Ho, "Cytohistological Studies on the Origin of Diploid Stem Apices in Colchicine-treated Tetraploid Sorghum Seedlings" (1964). Electronic Theses and Dissertations. 2985.