Abstract:
This work studies the relationship between the univalents seen at metaphase I and the distribution of dyads at anaphase I in the pollen mother cells (PMCs) of triploid Allium triquetrum. This work was supplemented with living cell studies of the behaviour of the X chromosome in .Acheta domesticus spermatocytes.
It was established that the frequency of univalents at metaphase I does not change with increasing developmental age of the PMCs or the plants. All but two collections studied had "low" metaphase I total univalent frequencies of approximately 0.3 univalents/cell, and had polar univalent frequencies of approximately 0.2 univalents/cell and equatorial univalent frequencies of approximately 0.1 univalents/cell. Two other collections had "high" frequencies of total univalents per cell (approximately 0.5) contributed to by "high" frequencies per cell of polar univalents (0.3) and of equatorial univalents (O.2)
A study was made of the orientation of the chromosome centromeres within the trivalents and bivalents at metaphase I; the centromeres are oriented nonpreferentially towards the two poles.
The distribution of polar univalents at the two poles at metaphase I was random for cells containing two and three polar univalents.
At late anaphase I the frequency of laggard chromosomes is the same for all collections (approximately 0.1/cell), (both low and high univalent frequency collections). This laggard frequency does not change with increasing developmental age of the PMCs, the inflorescences, or the plants.
In low univalent frequency collections the distribution of dyads at anaphase I is random. However, in one high univalent frequency collection the distribution of dyads at anaphase I is nonrandom in that there is an excess of cells with the most equal dyad distribution.
It is postulated that in the low univalent frequency collections, the equatorial univalents remain in the equatorial region during anaphase I and are seen as laggards at late anaphase I, and the remaining chromosomes move according to the metaphase I orientation of their centromeres, giving a random distribution of dyads at anaphase I. In the high univalent frequency collections, the nonrandom dyad distribution seen at anaphase I is the result of nonrandom movement of some of the equatorial univalents away from the equatorial region during anaphase I. The remaining equatorial univalents remain in the equatorial region and are seen as laggards at late anaphase I.
From a study of living spermatocytes of Acheta domesticus it was shown that at metaphase I the univalent X chromosome at the equator is amphitelically oriented and can give rise to a laggard chromosome at anaphase I. The amphitelic orientation of the stationary X chromosome at metaphase I is maintained during anaphase I. In some cells the univalent X chromosome moves away from the equator during anaphase I at approximately the same time and velocity as the dyads. During this poleward movement the X chromosome maintains its amphitelic orientation.
Thus the movements and orientation of the univalent X chromosome of Acheta domesticus are precisely those predicted for the equatorial univalents of triploid Allium triquetrum.
