Meiotic chromosome association in diploid and tetraploid Avena strigosa Schrel.

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Abstract
In diploid Avena strigosa the frequency and distribution of open (rod) bivalents suggest that a limited number of chromosomes, possibly two, fail to have a chiasma in one (short) arm in over 60% of the cells. In the tetraploid these are expected to form bivalents instead of quadrivalents at metaphase I of meiosis in a high frequency. In addition, interstitial chiasmata are expected in the long arms of these chromosomes, when involved in quadrivalents. In the tetraploid the frequency of quadrivalents is indeed low, and the configurations with interstitial chiasmata are distributed as expected. However, application of Sybenga's models suggests that the pairing system rather than the chiasma system would be responsible for the low quadrivalent frequency. It is suggested that apparent negative chiasma interference across the point of pairing partner exchange, resulting from variation in its location, invalidates application of these models to the present material. Negative interference results in an even higher than expected open bivalent frequency, in combination with a relatively high ring quadrivalent frequency. In addition, not only the subacrocentric chromosomes but also the more metacentric chromsomes, by this same mechanism, tend to form open bivalents more often than in the diploid.Key words: meiosis, bivalent, quadrivalent, diploid, tetraploid, Avena strigosa

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