抄録
The writer found an unexpected occurrence of univalent chromosomes in the microsporogenesis of rice-plant in the summer of 1934. Materials were collected from plants cultivated in the glass-house at the Plant Breeding Institute of the Hokkaido Imperial University, Sapporo. All of the three varieties examined showed the same abnormality, the frequency being approximately 40% in average. The fixation numbers given in the tables indicate the varieties used, and the dates on which the fixation was made, as follows :
Frequencies of asynaptic cells were counted in three following stages, diakinesis, prometaphase-I and the metaphase-I (Tables 1-4). It was found that the number of asynaptic cells in diakinesis suddenly decreased in prometaphase-I and again increased in metaphase-I.
At metaphase-I, the univalents scatter randomly on the spindle, but it was often observed that each two univalents lie in close proximity (Figs. 2, 3, 8, etc.).
Distribution of the univalents on the spindle was noted. Some of the univalents lie within the equatorial plate (Figs. 5, etc.), while the others scatter on the spindle. In the cells with two univalent chromosomes, the position of them was studied (Table 5), and it was found that the behaviour of the two univalents was not always at random. Data from Table 5 suggest that the univalents have a tendency to avoid entering the equatorial plate, and that when two univalents which are naturally homologous wander on the spindle, they are apt to take their position in the same side of the plate. These facts as the increase in the frequency of apparently normal cells at the first prometaphase where the repulsion force between chromosomes suddenly decreases, the tendency of each two univalents to lie in close proximity in the spindle and the tendency of two homologous univalents to be in the same side of the equatorial plate rather to lie in the different sides, all may be explained on one and the same hypothesis : the operation of an attractive force between the two homologous but unpaired univalents.
The reason why asynapsis is caused in this case is unknown. But it was suggested that it might have perhaps been caused by some environmental conditions. The genetic effect which sometimes brings about asynapsis seems not to be applicable in this case. It is the writer's great regret, however, to mention that his subsequent experiments failed to detect what the very cause may have been.
Most of the pollen grains were aborted. Fertility, too, was extremely low as shown below :
An extremely bad harvest of rice was recorded in 1934 all other the northern parts of Japan. Besides failure of micro- and macrosporogenetic cytokinesis under low temperatures which was found by the writer (Sakai 1937 a; b, 1939), the unexpected and the cause-unknown asynapsis described above might have played a certain role in the bad harvest of rice of that year.
