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The Relation of Chromocenters to the Differential Segments in Rhoeo discolor Hance

L. C. Coleman
American Journal of Botany
Vol. 28, No. 9 (Nov., 1941), pp. 742-748
Stable URL: http://www.jstor.org/stable/2436659
Page Count: 7
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The Relation of Chromocenters to the Differential Segments in Rhoeo discolor Hance
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Abstract

The presence of chromocenters in premeiotic resting nuclei from the pollen sacs of Rhoeo discolor has been established. The structure of the nuclei resembles greatly that already described for Veltheimia viridifolia. These chromocenters have been traced through the early stages of meiosis and have been shown in pachytene to congregate into one, two or three clumps in a manner to be predicted from the single ring or the single, double or triple chain observed at metaphase. While it has not been possible to associate them definitely with the primary constriction regions at pachytene, as has been done in the case of Veltheimia, there is a presumption amounting almost to certainty that this is their real relationship. This view is greatly strengthened by the fact that, in pollen grains where from four to six chromocenters are found in the resting nucleus, in some preparations markedly condensed regions have been found on either side of the primary constriction in the prophase stages. These are interpreted as chromocentric regions that have remained condensed. In late pachytene the chromocentric regions elongate and arrange themselves in a circle around a common center. This is in agreement with the accepted conception of the development of a ring by pairing and chiasma formation in the homologous end segments of chromosomes belonging to two complexes, one of which has undergone general reciprocal translocation of such segments. The chromocenters occupy the central differential segments which contain the balanced lethal factors upon which ring formation depends. The coiled nature of the chromocenters prevents pairing and crossing-over in these regions and so preserves the two complexes forming the ring. It is suggested that the widespread distribution of chromocenters in the plant kingdom is generally associated with the protection of the genes they contain from crossing-over and possibly from many of the influences which induce mutations. They are thus presumed to function as a cytological isolating mechanism along with other mechanisms, such as inversions, which are known to reduce crossing-over.

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