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A Cytological Investigation of a Tetraploid Rhoeo discolor

Marta Sherman Walters and D. U. Gerstel
American Journal of Botany
Vol. 35, No. 3 (Mar., 1948), pp. 141-150
Stable URL: http://www.jstor.org/stable/2438237
Page Count: 10
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
A Cytological Investigation of a Tetraploid Rhoeo discolor
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Abstract

Diploid Rhoeo discolor is a complete translocation heterozygote which generally forms a ring or chain of 12 chromosomes in meiosis. The autotetraploid exhibits univalents, chains of from 2-12 chromosomes, rings containing from 2-8 chromosomes, and occasionally more complex configurations involving multiple chiasmata. The chain of two and ring of two, or ring pair, are the most commonly observed configurations. Although the tetraploid contains 12 pairs of homologous chromosomes, the expected frequency of ring pairs is only 0.78. The difference between this and the observed frequency of 2.0 ring pairs per cell is highly significant and it has been conjectured that there is not complete independence in the pairing of any four homologous arms. There is a decrease in chiasma frequency in the tetraploid, expressed by a reduction factor of 0.84 half-chiasmata per chromosome. The rates of some physiological processes are often slowed down in tetraploids. It is thus suggested that the decrease in chiasma frequency in the tetraploid Rhoeo may be due to a reduction in the rate of pairing, without a compensating change in the time during which pairing or chiasma formation takes place. No interstitial chiasmata have ever been reported at metaphase in the diploid Rhoeo. In spite of the presence of 12 pairs of completely homologous chromosomes in the tetraploid, only 2 interstitial chiasmata were observed in an estimated 400-600 cells. Several explanations for such marked chiasma localization are discussed. The solution of this problem, however, depends upon a study of prophase in the tetraploid, which has not yet been possible. Pollen fertility, judged by the ability of the pollen grains to take up stain, is much higher in the tetraploid than in the diploid. This is ascribed to a greater tolerance of the diploid pollen grain for changes in chromosome number or balance. Whereas no aneuploids have ever been reported in diploid Rhoeo, the majority of the progeny of the tetraploid had aneuploid chromosome numbers. Thus the tetraploid progeny provides evidence that numerically and structurally unbalanced gametes are able to function. One of the aneuploid offspring of the tetraploid contained a telocentric chromosome.

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