It will be seen that for the first time it has been possible to obtain
plants that were monosomic or disomic for the short telocentric of 5B and
at the same time deficient for the long arm. One objective of this exercise
was to ascertain whether the sterility of the nullisomic could be circumvented
by including the short arm alone, and this proved to be so since these plants
were weakly fertile on selfing. Moreover the plants, monosomic and disomic
for the short arm, were also multivalent - forming - presumably due to homoeologous
pairing. Their employment may consequently facilitate the use of homoeologous
recombination in breeding investigations. Multivalent frequency was essentially
similar to that in plants nullisomic for 5B, with trivalents or quadrivalents
and rarely higher associations between 20 per cent and 40 per cent of cells.
No plant carrying the long arm showed similar multivalents.
While it is clear from this that the short arm produces no effect in restraining
homoeologous pairing, its lack of influence was most clearly displayed in
the behaviour of the 21 - chromosome haploid that carried the short telocentric.
This plant had many bivalents and trivalents, like 20 - chromosome nullisomic
- 5B haploids, and the telocentric was paired in eight per cent of cells.
The mean meiotic pairing behaviour of this plant has been compared with
earlier haploid data in Table 2. From this the
close similarity of its meiosis to that of haploids deficient for chomosome
5B can be clearly seen. This evidence must, therefore, be regarded as unequivocal
confirmation that the effect of chromosome 5B in suppressing homoeologous
pairing is wholly due to the activity of the long arm and that the short
arm produces neither a direct effect nor any influence stemming from interaction
with the long arm. |