Abstract
Based on the study of secondary association of meiotic chromosomes in diploid plants of rice, Sakai (1935), and Nandi (1936), have made the suggestion that the haploid set (12 chromosomes) of rice woule have been derived from an ancestral genome consisting of five chromosomes. In order to test the validity of this hypothesis, the writer investigated the mode of chromosome association in haploid rice. The principal findings are:
1. At diakinesis, though twelve univalent chromosomes appeared in a greater part of cells, one or two bivalents loosely associated were occasionally seen. In addition to this, secondary associations were frequently found. (Table 1)
2. The primary as well as secondary association were most apparent at the trasitional stage from late diakinesis to MI. The mode of association most frequently found was 1(3)+3(2)+3(1), and the maximum association was 2(3)+3(2). (Table 2)
3. At MI, not only bivalent but trivalent chromosomes were pointed out in a part of cells (Table 4). Having compared with the “Poisson” distribution which was to be expected if the associations occurred merely by chance, the frequency distribution of cells with different numbers of primary associations was found to differ significantly from the former. Thus, the chromosome association appearing in haploid plants cannot be said as chance occurrence.
4. The frequency of primary and secondary associations found in haploid plants was comparable with that of secondary associations reported by the former authors with diploid plants.
5. It was discussed that the primary and secondary associations in haploid rice might suggest the presence of some residual homology among the twelve chromosomes of the present genome of rice.
