Abstract
At MI in F1 tabacum×alata, 1-6 bivalents were observed, and the most frequent configurations found were 3II+17I, and 4II+15I, although rarely trivalents occurred. According to Kostoff's description (1943), the hybrids, tabacum×alata and tabacum×Sanderae contained 5-9 bivalents and also some polyvalents.
The writer (1953) reported that F1 tabacum×sylvestris had 0-9 trivalents with mode 3. In F1 tomentosa×sylvestris, Goodspeed (1934) counted 0-7 bivalents with mode 2-3, and Kostoff (1943) found a similar meiotic pairing behaviour. In F1 tomentosiformis×sylvestris, closely related to the former, Kostoff (1943) observed somewhat fewer bivalents. The writer also observed that three hybrids sylvestris×tomentosiformis, sylvestris×tomentosa and sylvestris× otophora had each 1-9, 0-9 and 0-5 bivalents with each mode 4, 3 and 2 in the above cited order. These facts suggest that there are 2-4 pairs of homologous or partially homologous chromosomes between tomentosa group and sylvestris genomes. Accordingly, the bivalents found in F1 tabacum×alata cannot be attributed simply to homologous relation between two subgenomes of N. tabacum and alata genome.
At MI in F1 tabacum×tomentosiformis, most frequent configuration found was 12II+12I and the next, 11II+1III+11I. Besides these configurations, a few deviations from them were found.
This meiotic behaviour agrees with Goodspeed's findings (1934) in the same hybrid. Since the studies on F1 tabacum×tomentosa by Goodspeed and Clausen (1928), N. tomentosa or N. tomentosiformis is recognized by many investigators to be one of the ancestors of N. tabacum.
