タバコ属植物の細胞遺伝学的研究 XIII

タバコの半数体

抄録

A variety of Nicotiana tabacum, “Bright Yellow” was pollinated with pollen of N. alata treated by X-rays (4, 800γ). From the seed obtained, 8, 730 were sown, 150 germinated but only two plants reached maturity. One of them was a hybrid, and the other was a haploid tobacco plant which was a little smaller than the parental diploid “Bright Yellow.” In studying MI in PMC's of the haploid plant, most frequently 24 univalents were found; in about 30% of PMC's one to three bivalents were Found; in about 30% of PMC's one to three bivalents were observed. On the average 0.39 bivalents occurred per PMC. An early study on meiotic behavior in haploid N. tabacum “purpurea” was reported by Clausen and Mann (1924). They indicated complete lack of pairing at MI, while in the same material Chipman and Goodspeed (1927) saw an occasional bivalent and interpreted it as a product of adherence of two chromosomes closely associated on the MI spindle rather than a reflection of pachytene pairing, since association was not observed at diakinesis or earlier. Kostoff found in a haplont of N. “triplex”.N. tabacum×(N.sylvestris×N. tomentosiformis)-and in a haplont of amphidiploid N. sylvestris×N. tomentosiformis, both of which he considered equivalent to haploid N. tabacum, a range of zero to three pairs. The former had 0.35 and the latter 0.34 bivalents per PMC. The above mentioned observations in the haploid tobacco plant agree with these results. On the cotrary, in the three hybrids, N. otophora×N. sylvestris, N. sylvestris× N. Setchellii, and N. sylvestris×N. tomentosa, Goodspeed (1934, 1954) found a range of zero to seven bivalents with the mode at 2-3, and also Takenaka (1954, 1955) counted, in the hybrids N. sylvestris×N. tomentosiformis, N. sylvestris×N. tomentosa and N. sylvestris×N. otophora, 1-9 bivalents with the mode at 4, 0-7 with the mode at 3 and 0-5 with the mode at 2 respectively, in the above cited order. Takenaka's observations generally agree with Goodspeed's findings concerning chromosome affinity between the sylvestris genome and those of the tomentosa group. Accordingly, bivalent number at MI of the hybrid between N. sylvestris and the species of tomentosa group is always higher than that of haploid tobacco. Concerning the decrease of intragenomatic affinity in haploid tobacco, Clausen (1941) advocated the following assumption : “the alterations which have diminished its duplicational completeness must have arisen largely since it became established as an amphidiploid.” On the contrary, in the hybrid N. sylvestris×N. tomentosa, Kostoff found usually univalent chromosomes or 1 to 4 bivalents, rarely more than 4, and in F₁ N. sylvestris ×N. tomentosiformis somewhat less bivalents than in the former. He stated that meiosis of the N. “triplex” haploid and the haploid of amphidiploid N. sylvestris× N. tomentosiformis could not be distinguished from the meiosis in F₁ N. sylvestris× N. tomentosiformis. The difference between Kostoff's and Goodspeed's as well as Takenaka's findings in the hybrids between N. sylvestris and tomentosa group are ascribed to the different methods of cultivation. Nevertheless, it is certain that the bivalent number in haploid tobacco is less than that of the hybrids between N. sylvestris and species of the tomentosa group.