Secondary Association of Fragment Chromosomes in Generative Nucleus of Tradescantia and Its Bearing on Their Origin

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Abstract
Pollen tubes of a diploid (2n=12) clone of T. paludosa were studied, following treatment with colchi-cine. In this clone, 2 fragments are present in the sporo-phyte, one fragment in the gametophyte. In almost half of the 800 tubes examined, the fragment, which has a median centromere, showed secondary association with one of the major chromosomes in metaphase. Of the fragments exhibiting such association, half showed terminal association with the shorter arm of one of the heterobrachial chromosomes (possibly the same chromosome). One arm of the fragment is apparently homologous with the distal portion of one of the heterobrachial chromosomes, and the centric region is probably homologous with a similar region in the same or another heterobrachial chromosome. The fragment is thought to have arisen either through crossing over in a region of overlapping inversions or through unequal reciprocal translocations. The presence of fragments in a diploid does not support the hypothesis that fragments in Tradescantia arose in a polyploid species.