Cytogenetic studies on the artificially raised trigenomic hexaploid hybrid forms in the genus Brassica
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1. The mode of meiosis, the grade of fertility, and certain cytogenetical features were observed in an artificially synthesized trigenomic hexaploid form, having all the genomes, a, b, and c, belonging to the genus Brassica, and in its progeny. 2. Synthesis of this trigenomic form was only possible by a cross between B. carhata and B. pekinensis, and not by a cross between any monogenomic and digenomic species, as shown in Table 2. The crosses were made on the diploid and on the tetraploid levels. Some seeds obtained by these crosses were small-sized and the rest otherwise, showing clearly that the former were true F_1 and the latter matroclinous seeds. 3. The F₁ hybrid obtained, growing more vigorously than its parent species, were morphologically somewhat intermediate between its parent species. 4. In the trigenomic triploid F_1 hybrid, several bivalents were usually found at metaphase-I and their number per PMC ranged from 2 to 9, and various irregularities occurred continuously through the later meiotic stages. 5. With its meiotic irregularities, the trigenomic triploid F_1 hybrid was remarkably low in fertility, its pollen-fertility being 0.4 per cent in an average and the number of viable seeds attaining 27.8 per individual. The F_1 plants of this triploid hybrid showed by their morphological and chromosomal features that most of the fertilizable gametes produced in the triploid F_1 hybrid were each possessed of 27, 28, or 26 chromosomes. 6. The following were the main cytological aspects observed in the PMCs of the trigenomic hexaploid F₁ hybrid at metaphase-I, -II, and at the pollen-tetrad stage of their meioses: Rather frequent formation of multivalents and univalents at metaphase-I; appearance of the configuration 27Ⅱ in 36.4 per cent of the cells; occurrence of metaphase-II daughter nuclear plates, of which 48.8 per cent were normal containing 27 chromosomes, and the remaining 51.2 per cent abnormal, each plate containing 25, 26, 28, or 29 chromosomes in it; and the appearance of various kinds of sporads in addition to normal tetrads in less than 16 per cent of the cells examined. 7. The trigenomic hexaploid was not very fertile, provided with its stamens degenerating in different degree from individual to individual, and also in different periods of their growing. Such degeneration of the stamens was likely to develop from some genie interaction among the three different genomes, a, 6, and constituting a hexaploid hybrid form, because the stamens in any of the aneuploids educed among the hexaploid progeny were quite normal in appearance. The average pollen- and the average seed-fertility of the hexaploid F_1 plants were 38.8 per cent and 36.3 per cent, respectively. 8. In the selfed F_2-F_5 progeny of the hexaploid hybrid, the frequency appearance of the configuration 27Ⅱ at metaphase-I was only 25 per cent or less and that of multivalents ca. 60 per cent ofthe cells examined; the number of univalents appearing simultaneously was 0 to 6 per cell and the frequency of their appearance increased gradually with the advance of generations in the progeny, so that only ca. 40 per cent of the metaphase-II daughter plates examined were each possessed of 27 chromosomes, and these normal plates were likely to decrease in number in the successive generations of the hexaploid strain. The frequency appearance of aneuploid individuals was 23.1 per cent, being a relatively small in proportion, in the selfed F_₂ progeny of the hexaploid and 50 per cent or more in the F_3-F_5 generations, and the variation in the number of chromosomes composing the aneuploids showed an increasing tendency towards the later generations. The continuous selective breeding was undertaken to obtain any one hexaploid strain, which may be highly fertile, among the selfed F_2-F_5 generations has remained ineffective at all. And a comparative examination in the fertility of eu-hexaploid and aneuploid plants showed that the latter were far higher than the former in the pollenfertility and also that the former were slightly higher or nearly equal to the latter in the seed-fertility. 9. Morphological observations of the F_2-F_4 progenies obtained by the successive open-pollinations from the hexaploid F_1 plant showed that the hexaploid F_1 plant could be easily fertilized by the pollengrains of the tetraploid B. cernua, and that the hybrid-type F₂ plants thus produced, which had probably been aaabbbc genomes, seemed easy to collapse into the descendant plants having nabb or neighborhood in their genome constitutions. 10. A follow-up examination of the selfed F_3-F_6 progenies of a 6x + 1 and a 6x- 1 plants, each educed among the F_2 plants of the hexaploid F_ hybrid, led to the following findings: Of 5 F_1 descendants from a 6x--t- 1 F_2 plant, 2 were 6x and the remaining 3 were 6x+ 1, 6x--- 1, or 6x-3, whereas 6 F_3 descendants from a 6x- 1 F_2 plant were all 6x-(1-6) and not 6x. The number of chromosomes showed marked decreasing tendency towards the F_3-F_6 descendants raised by the successive selfing of 6x-- 1 F_2 plant. In those aneuploidal F_3-F_6 descendants the meiotic divisions became gradually stabilized with those plants in good correspondence to the decreasing of the chromosome number, but such decrease in the number of chromosomes was not generally followed by any marked deterioration in the fertility with those plants. 11. As has been just stated above, the progeny of the trigenomic hexaploid was highly intolerant of the pedigree culture of sustaining its original hexaploid genotypic structure. But these individuals may serve as the promising breeding materials to be used in obtaining the intergenomic gene recombination and the substitution or addition of allochromosomes, if their conspicuous meiotic irregularity is to be turned to good account.
- Journal of the Faculty of Agriculture, Kyushu University
Journal of the Faculty of Agriculture, Kyushu University 13(2), 309-349, 1964-10-30