Prophase Association in Triploid Nicotiana tabacum

抄録

The origin of four modified triploid plants of Nicotiana tabacum var. purpurea is described.
In each rase, approximately diploid female gametes (46) from the monosomic female parent, when uniting with the normal male gamete (24), produced (3n-2) plants.
The triploids were found to conform to the generally described behaviour of other triploid plants, in that there were a variable number of trivalents at first metaphase.
From a detailed study of prophase pairing, it is concluded that synapsis may occur in either one of three ways:
1. Pairing at zygotene, once commenced between any two chromonemata, proceeds along the length of the chromosomes in a progressive manner. Pairing between any two homologues interferes with association of the third, so that at early pachytene two chromonenata are completely synapsed, the third later coming in contact with the couple at one or a few points.
2. Pairing at zygotene may involve all three threads simultaneously. In such Gases, there is usually an exchange of partner, one thread pairing with a second and farther an with a third. More complex associations may occur.
3. In rare Gases there may be failure of orientation of the three homologues, so that one thread is left out of association entirely.
Triple synapsis may occur for skort regions of the chromosome, but triple threads are never found in which synapsis is complete thruout the whole length of the chromosome. The reason suggested is that linear contraction of paired chromonemata is more rapid than that of a single chromonema. This also applies to portions of chromonemata, so that there is a failure of chromomere alignment which hinders further triple pairing. Although complete triple synapsis may be potentially possible, it is prevented by the onset of diplotene.
Association between all three chromosomes is more intimate at late pachytene than at any previous stage of prophase.