| Cytogenetical studies of B-chromosomes in a weedy rye,
Secale afghanicum Toru AKITA and Sadao SAKAMOTO Plant Germ-plasm Institute, Faculty of Agriculture, Kyoto University Secale afghanicum (Vavilov) Roschev. is a weedy species closely related to cultivated rye, Secale cereale. B-chromosomes (Bs) found in the original samples of S. afghanicum were investigated cytogenetically (AKITA & SAKAMOTO 1982). In this brief article two paticular characteristics, such as the pairing frequencies of Bs and the effects of Bs on the pariring of A-chromosomes (As) at MI of pollen mother cells (PMCs), are reported. The results were also compared with the cytogenetic characteristics of Bs found in S. cereale. The original samples of S. afghanicum used in this study were collected in Afghanistan in 1978. Of eight populations from five original samples, six included the plants with 1B-4Bs. The standard type of Bs had subterminal centromere. Though two other karyotypes of Bs, probably derived from standard ones, were also found, only the plants with the standards Bs were examined. The data on the B-chromosome pairing at MI of PMCs is shown in Table 1. In the plants with two Bs, two univalents of Bs were observed in more than 60% of PMCs examined. Most bivalents of Bs were rod ones, whereas ring bivalents were found to be much rare (less than 5% of bivalents). The pairing frequency of Bs in S. afghanicum was not different from that in several reports on Bs of S. cereale (MUNTZING 1945, SARVELLA 1959). In the plants with 4Bs or 6Bs, the pairing behavior of Bs was characterized by much rare formation of multivalents at MI of PMCs. Such multivalents were usually formed as a Y-shaped trivalent (in two 4B plants) or an X-shaped quadrivalent (in a 6B plant). To examine the effects of Bs on the pairing of As, total 71 plants in five strains derived from the same plant, were observed at MI of PMCs, as shown in Table 2. The plants without Bs usually showed seven bivalents. On the other hand, the plants with 2Bs in the same strain showed the increase of univalents, the decrease of ring bivalents and the increase of rod bivalents. Moreover, the number of paired arms per cell and the mean chiasma frequency per cell of As in the 2B plants decreased and became variable. For instance, in culture no. 1001 the mean chiasma frequency of each 0B plant ranged from 12.88 to 14.08 (13.51 on the average). On the other hand, that of each 2B plant ranged from 10.84 to 13.26 (12.24 on the average). In general, the number of ring bivalents and the mean chiasma frequency of As in the 2B plant were lower than those without Bs. Similar results were reported by TSUMOTO & SASAKI (1972), while, contradictory results were also found by ZECEVIC & PAUNOVIC (1969) in S. cereale. |