Abstract
In Tradescantia paludosa a progeny was produced by crossing a plant showing an increased frequency of spontaneous chromosome breakage with a normal plant. One of the individuals, called Ma16, in this progeny was found to have two supernumerary chromosomes. Its parents did not have any supernumeraries. One supernumerary is a minute centric fragment, virtually a free centromere with only a small part of the proximal region of one arm attached. The other supernumerary is of nearly the same size and shape as the normal chromosomes, but it pairs only rarely with these to form a trivalent or within itself to form a loop univalent. This large extra chromosome appears to differ from the corresponding normal chromosomes by several structural changes. The plant Ma16 has a slightly reduced pairing of the chromosomes in its normal complement. For comparison, the metaphase-I pairing was investigated in an autotriploid individual of T. paludosa. The triploid shows a very high frequency of trivalent formation. During various stages of meiosis, the minute centric fragment of Ma16 demonstrates some structural details of the centromere region with greater clarity than ordinary chromosomes.
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