Abstract
The reduction divisions in PMC's were studied in three hybrids, N. paniculata×N. benavidesii, N. debneyi×N. glutinosa and N. tabacum×N. forgetiana, and one amphidiploid of N. rustica×N. tabacum produced by colchicine treatment.
1) F1 of N. paniculata (n=12)×N. benavidesii (n=12).
At MI of PMC's of this hybrid, twelve bivalents were usually counted whose partners were of same size and shape, and very seldom eleven bivalents and two univalents were found. At 2nd division, a few irregularities were very rarely observed such as a chromosome bridge, and one to two chromosomes or a fragment strayed in the cytoplasm.
In the same hybrid, Goodspeed (1954) found that twelve bivalents were the norm, and univalents, trivalents and quadrivalents occurred very rarely. Although there are some differences between Goodspeed's observations and mine, it is thought that, the two species are closely related but the genomes of N. paniculata and of N. benavidesii are somewhat differentiated by inversions, translocations and other irregularities.
2) F1 of N. debneyi (n=24)×N. glutinosa (n=12).
So far as I know, this hybrid has not been previously reported. The nuclear plate in MI of PMC's of this hybrid consisted of bivalents and many univalents and it was somewhat difficult to count them clearly. But the number of univalents did not reach generally to one half of the total chromosome number of 36 and many bivalents were observed. Therefore it is assumed that N. debneyi is somewhat related to N. glutinosa on one hand but on the other hand intragenomic bivalents may be formed in the former, since it is probably of amphidiploid origin.
3) F1 of N. tabacum (n=24)×N. forgetiana (n=9).
So far as I know, no report on this hybrid had been publisched. At MI of PMC's of this plant, the number of bivalents was from 1 to 8, with the mode at 3. The frequency of PMC's with 4 and 5 bivalents followed that of PMC's with 3 bivalents. PMC's with 2, 6 and 7 bivalents were occasionally found but those with 1 and 8 bivalents. were very rare. Some of them may be due to autosyndetic affinities between the two subgenomes of N. tabacum which is of amphidiploid origin and others may be caused by allosyndetic affinities between the genomes of the parents. Accordingly N. tabacum is assumed to be slightly related to N. forgetiana.
4) Amphidiploid hybrid of N. rustica (n=24)×N. tabacum (n=24).
This amphidiploid was produced by colchicine treatment of F1 of N. rustica×N. tabacum with the purpose of breeding a new strain containing the genomes of N. tabacum and N. rustica. But this plant was highly sterile and the offspring showed segregation for various characters.
At MI of PMC's of this amphidiploid, the number of univalents ranged from 0 to 12, with the mode at 4. The frequency of PMC's with 2, 3, 5, 6, 7, 8 and 9 univalents followed that of PMC's with 4 univalents, and PMC's with 0, 1, 10, 11 and 12 univalents were rare.
F1 of N. tabacum×N. rustica showed the number of ivalents to be from 0 to 10 with the mode at 4 (Takenaka, 1955). Accordingly, it is assumed that in this amphidiploid allosyndetic affinity between the genomes of the parents leads to the conjugation of some chromosomes.
