| Nuclear and interphase chromosome volumes of four Triticum
species and of eight species from related genera Sadao ICHIKAWA1) and Arnold H. SPARROW Biology Department, Brookhaven National Laboratory, Upton, New York 11973, U. S. A. Interphase chromosome volume has been shown by us to be the primary factor determining radiosensitivity not only of higher plants (SPARROW et al. 1965, Radiation Botany, Suppl. 1: 101), but also of other organisms ranging from unicellular systems to higher animals (UNDERBRINK and SPARROW 1966, J. Cell Biol. 31: 119A). Recently, we have also shown that this volume is a useful tool in cytotaxonomical research (ICHIKAWA et al., in preparation). This note reports the nuclear and interphase chromosome volumes of some Triticum, Aegilops, Secale, Hynaldia, Agropyron and Hordeum speciess and briefly indicates how these values can be applied by a cytotaxonomist. For determining the nuclear volumes, vegetative apical shoots were removed from young seedlings (2- or 3-leaf stage), fixed with Craf III, sectioned at 8 or l0micro, and stained with safranin and fast green. The nuclear volumes were obtained from these preparations by the method described elsewhere (SPARROW et al. 1965, see above). When the nuclear volumes are divided by the somatic chromosome numbers, the average interphase chromosome volumes characteristic of each species can be calculated. The volumes determined are presented in Table 1. The average nuclear volumes for di-, tetra- and hexaploid Triticum species are 248, 461 and 642micro3, respectively, giving a ratio of 1: 1.86: 2.59. The volumes of the polyploids are smaller than those expected from a proportional relationship between the ploidy levels. As a consequence, the average interphase chromosome volumes for these three levels show a decrease with increasing ploidy, i. e., 17.7, 16.5 and 15.3micro3, respectively. The above ratio of nuclear volumes does not agree with the ratio of total length of mitotic metaphase chromosomes, 2x: 4x: 6x= 1: 1.7: 2.1 by MARSHAK and BRADLEY (1944. Proc, Nat. Acad. Sci., U. S. 30: 231) nor 1: 1.6: 2.0 by BHASKARAN and SWAMINATHAN (1960, Exptl. Cell Res. 20: 598). It has been widely accepted that tetraploid wheat was derived from hybridization and amphiploid formation between diploid wheat and probably Aegilops speltoides, and that the hexaploid wheat came from the tetraploid and Ae. squarrosa. A "synthesized" value for the average nuclear volume of tetraploid wheat can be obtained by adding the average value calculated for the three diploid wheats measured to the Ae. speltoides value. The result is 482micro3 which is close to 461micro3, the average nuclear volume for the tetraploid wheat. Similarly, adding the values obtained for the tetraploid wheat and Ae. squarrosa results in 683micro3 which is also close to the average volume for the hexaploid, 642micro3. Although there is only a little difference (5 to 6 %) between the "synthesized" and measured volumes, we should not ignore the fact that the measured volumes are smaller than the "synthesized" volumes in both cases. Another "synthesized" volume of 704micro3 determined from the values for the diploid wheat species, Ae. speltoides and Ae. squarrosa, is about 10 % larger than the measured volume of the hexaploid wheat. Similar decreases of interphase chromosome volumes in polyploids have been observed in Aveua (unpub.) and more strikingly in Rumex (ICHIKAWA et al., in preparation), Chrysanthemum, Sedum, Tradescantia and several other genera (SPARROW 1965, In, Cellular Radiation Biology, p. 199). Agropyron cristatum, Hynaldia villosa and Secale cereale (all 2 x) have larger average interphase chromosome volumes than any of the Triticum species studied, while Agropyron subsecundum (4 x), Ag. intermedium (6 x) and Hordeum vulgare (2x) have smaller interphase chromosome volumes. The research was carried out at Brookhaven National Laboratory under the auspices of the U. S. Atomic Energy Commission. (Received Feb. 4, 1967) |
| 1) Present address: Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Kyoto, Japan. |