The frequency of pollen mother cells with univalent chromosomes at first metaphase was determined in five varieties and ten varietal hybrids of
vulgare wheats. The proportion of cells with univalents in the varieties varied from 2.9 per cent to 9.7 per cent and in the hybrids from 5.2 to 39.1 per cent. Hybrids between related varieties on the whole gave lower univalent frequencies than those between unrelated varieties, but exceptions occurred.
In any cell with univalents two was the most common number, and as high as eight were observed. Trivalents and tetravalents were seen only rarely.
Wide variation in percentage of cells with univalents was observed in some instances. Higher univalent frequency occurred in first metaphase cells from anthers in which most of the cells were well past metaphase.
No significant difference in univalent frequency was found in two groups of Marquis plants kept under fairly constant temperatures the respective means of which were 60° and 77°F.
The univalents in different cells were of different sizes and shapes. In certain hybrids with high univalent frequency, the univalents seemed to be longer than in certain varieties. Attenuated bivalents and bivalents with fewer chiasmata were more frequent in hybrids with frequent univalents.
Typical metaphase bivalents are drawn and the number of chiasmata estimated for each. Terminal or terminal and closely sub-ter-minal chiasmata are the rule but exceptions occur. Configurations resembling some of those interpreted by HUSKINS and by POWERS as trivalents are here interpreted as bivalents.
It is to be expected that the occurrence of univalents will give rise in the next generation to plants with abnormal chromosome numbers and to disturbed genetic ratios.
The relationship of these observations to the chiasma theory is discussed and it is concluded that they support DARLINGTON's theory which ascribes the occurrence of univalents to a reduction in frequency of chiasmata.
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