Primary trisomy in tall fescue

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Abstract
Chromosome pairing and chiasma frequency were studied in a tall fescue primary trisomic (2n = 6x + 1 = 43) and three of its sister disomics (2n = 6x = 42) that were morphologically indistinguishable from it. The extra chromosome involved is one of the shorter chromosomes of the complement with a submedian centromere. It remained unpaired in 62 percent of the PMC's scored at metaphase I, whereas in 36 percent of the cells it formed different types of trivalents. Factors contributing to the failure of pairing of the trisome are considered, and ways of identifying the extra chromosome are discussed. The trisome divided either at anaphase I or anaphase II so that about 50 percent of the microspores in the resultant tetrad received one of the chromatids. Misdivision of the chromatids was not observed. Total elimination of the trisome or one or both of its division products was rare. Chiasma frequency in the trisomic was significantly higher than that in its sister disomics. Although higher chiasma frequency in the trisomic plant cannot be conclusively attributed to the trisomic condition, it seems likely that the extra copies of the chiasma-controlling-genes on the trisome may have enhanced chiasma frequency thus showing dosage effect similar to that of polygenes.

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