The gigantic chromosomes (“megachromosomes”) described previously as occurring spontaneously in hybrid combinations between N. tabacum and species of the Tomentosae section of Nicotiana were due to an enlargement of heterochromatic segments introduced from the latter into a N. tabacum background. Only chromosomes with large heterochromatic segments became megachromosomes and the enlarged parts themselves showed at interphase and prophase the intense staining characteristic of heterochromatin. Euchromatic arms of the same chromosomes did not undergo enlargement.
In contrast, megachromosomes described here for N. tabacum x N. plumbaginifolia hybrids originate from chromosomes which have no heterochromatic blocks. These megachromosomes are not recognizable at interphase and when distinguished at prophase are found to be stained lightly like the rest of the euchromatin.
The mode of origin of megachromosomes is still unknown. Spontaneous chromosome breakage is frequent in all hybrids in which megachromosomes are found and is probably associated in some way with their formation, but an origin of megachromosomes by breakage and end-to-end fusion of broken strands is unlikely. This leaves as a possibility an origin by repeated replication from the same template.
Other examples of very large chromosomes with characteristics of megachromosomes found in the literature are briefly discussed. They all arose in atypical situations of interspecific hydribization, exposure to mutagens or in tumors and cell cultures.
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References
Ar-Rushdi, A. H. (1957). The cytogenetics of variegation in a species hybrid in Nicotiana. Genetics 42: 312–325.
Britten, R. J. & D. E. Kohne (1968). Repeated sequences in DNA. Science 161: 529–540.
Burns, J. A. (1964). A technique for making preparations of mitotic chromosomes from Nicotiana flowers. Tobacco, N. Y. 158: 22–23.
Burns, J. A. & D. U. Gerstel (1969). Consequences of spontaneous breakage of heterochromatic chromosome segments in Nicotiana hybrids. Genetics 63: 427–439.
Burns, J. A. & D. U. Gerstel (1972). Inhibition of chromosome breakage and of megachromosomes by intact genomes of Nicotiana. Genetics 69: 211–220.
Collins, G. B. (1967a). Long chromosomes in a Nicotiana species hybrid, Tobacco, N.Y. 165: 173–174.
Collins, G. B. (1967b). DNA replication of the chromosomes of Nicotiana tabacum, N. plumbaginifolia and their F1 hybrid. Ph.D. thesis, North Carolina State University, Raleigh, N. C.
Collins, G. B., P. D. Legg, & M. K. Anderson (1970). Cytophotometric determination of DNA content in Nicotiana megachromosomes. Can. J. Genet. Cytol. 12: 769–778.
Ephrussi, B. (1972). Hybridization of somatic cells. Princeton University Press, Princeton, New Jersey.
Gerstel, D. U. & J. A. Burns (1966). Chromosomes of unusual length in hybrids between two species of Nicotiana. Chromosomes Today 1: 41–56.
Gerstel, D. U. & J. A. Burns (1967). Phenotypic and chromosomal abnormalities associated with the introduction of heterochromatin from Nicotiana otophora into N. tabacum. Genetics 56: 483–502.
Gerstel, D. U. & J. A. Burns (1969). Effect of dosage of a heterochromatic block on megachromosome formation in Nicotiana. J. Hered. 60: 107–110.
Gupta, S. B. (1969). Duration of mitotic cycle and regulation of DNA replication in Nicotiana plumbaginifolia and a hybrid derivative of N. tabacum showing chromosome instability. Can. J. Genet. Cytol. 11: 133–142.
Kao, K. N., R. A. Miller, O. L. Gamborg, & B. L. Harvey (1970). Variations in chromosome number and structure in plant cells grown in suspension cultures. Can. J. Genet. Cytol. 12: 297–301.
Kasha, K. J. (1974). Haploids from somatic cells. Proc. Inter. Symp. on Haploids in Higher Plants, Univ. Guelph. K. J. Kasha, ed. pp. 67–87.
Keyl, H.-G. (1966). Lokale DNS-Replikationen in Riesenchromosomen. 3. wiss. Konf. dtsch. Ges. Naturforscher and Ärzte, pp. 55–69. Springer, Berlin-Heidelberg-New York.
Keyl, H.-G. & K. Hägele (1966). Heterochromatin-Proliferation an den Speicheldrüsen-Chromosomen von Chironomus melanotus. Chromosoma 19: 223–230.
McClintock, B. (1951). Chromosome organization and genic expression. Cold Spring Harb. Symp. Quant. Biol. 16: 13–47.
Merritt, J. F. (1974). The distribution of heterochromatin in the genus Nicotiana (Solanaceae). Am. J. Bot. 61: 982–994.
Merritt, J. F. & J. A. Burns (1974). Chromosome banding in Nicotiana otophora without denaturation and renaturation. J. Hered. 65: 101–103.
Moav, R. (1961). Genetic instability in Nicotiana hybrids. II. Studies of the Ws(pbg) locus of N. plumbaginifolia in N. tabacum nuclei. Genetics 46: 1069–1087.
Moav, J., R. Moav & D. Zohary (1968). Spontaneous morphological alterations of chromosomes in Nicotiana hybrids. Genetics 59: 57–63.
Moav, R. & D. R. Cameron (1960). Genetic instability in Nicotiana hybrids. I. The expression of instability in N. tabacum x N. plumbaginifolia. Am. J. Bot. 47: 87–93.
O'Neill, F. J. (1974). Complex chromosome aberrations in continuous mammalian cell lines. Chromosoma 47: 453–462.
Pavan, C. & A. B.Da Cunha (1969a). Chromosomal activities in Rhynchosciara and other Sciaridae. A. Rev. Genetics 3: 425–450.
Pavan, C. & A. B.Da Cunha (1969b) Gene amplification in ontogeny and phylogeny of animals. Genetics (Suppl.) 61: 289–304.
Rudkin, G. T. (1964). The structure and function of heterochromatin. Genetics Today; Proc. XI Int. Congr. Genet. pp. 359–374.
Sager, R. & R. Kitchin (1975). Selective silencing of eukaryotic DNA. Science 189: 426–433.
Swift, H. (1973). The organization of genetic material in eukaryotes: Progress and Prospects. Cold Spring Harb. Symp. Quant. Biol. 38: 963–979.
Tjio, J. H. & A. Levan (1950). The use of oxyquinoline in chromosome analysis. An. Estac. exp. Aula Dei 2: 21–64.
Webber, J. M. (1930). Interspecific hybridization in Nicotiana. XI. The cytology of a sesquidiploid hybrid between tabacum and sylvestris. Univ. Calif. Publs Bot. 11: 319–354.
Wilson, G. B. & A. H. Sparrow (1960). Configurations resulting from iso-chromatid and iso-subchromatid unions after meiotic and mitotic prophase irradiation. Chromosoma 11: 229–244.
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Paper number 4748 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, North Carolina
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Gerstel, D.U., Burns, J.A. Enlarged euchromatic chromosomes (“megachromosomes”) in hybrids between Nicotiana tabacum and N. plumbaginifolia . Genetica 46, 139–153 (1976). https://doi.org/10.1007/BF00121030
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DOI: https://doi.org/10.1007/BF00121030