Monosomics in a diploid organism are ideal for characterizing the behavior of univalent chromosomes because each meiotic cell contains a univalent chromosome. We have isolated microsporocyte samples from all monosomic types except monosomics 3 and 5 and have carried out extensive analyses of the meiotic behavior in each of the different available monosomic types.

From these studies, it is clear that univalent chromosomes can undergo equational division at the first anaphase and the resultant monads are not lost during the remainder of meiosis.

Prophase II cells were analyzed to determine the frequency of cells containing 9 dyads and 1 monad in each of the resultant prophase II cells from a single microspore mother cell. Four-hundred-fifty-three prophase II cell pairs were analyzed from six different monosomic-seven plants and 53.1 ± 5.4% of the cell pairs contained 9 dyads in one cell and 10 in the other (9-10 cell pairs). Nine dyads and 1 monad were found in each of the two sister prophase II cell's in 19.1 ± 1.7% of the pairs (9+U-9+U cell pairs), and nine dyads were found in each of the two sister prophase II cells in 27.8 ± 5.9% of the prophase II cell pairs (9-9). The first class is produced when the univalent passes undivided to one of the poles at anaphase I, the second when the univalent divides equationally and a half-univalent (monad) passes to each pole, and the third when the univalent is not incorporated into either cell.

If the monads were lost before the quartet matured, only the 9-10 prophase II cell pairs could produce viable mature pollen. Because the single chromosome 7 would be incorporated into only two of the four members of a quartet, the pollen fertility of such a plant would be predicted to be no higher than 53.1%/2 or 26.6%.

If, on the other hand, monads were not lost but were incorporated before the quartet matured, both the 9-10 prophase II cell pairs and the 9+U-9+U types would produce viable pollen. In this case, the predicted pollen viability would be no higher than (53.1% + 19.1%)/2 or 36.1%.

The actual pollen fertility determined for four monosomic 7 plants (14,295 pollen grains scored) is 36.9%. This value is remarkably close to the estimated value of 36.1% if the equationally divided univalents were not lost. Clearly, the univalents which underwent equational division at anaphase I were not lost during later stages of meiosis. Similar results were obtained for other monosomic types.

David Weber

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