Sample records for dicentric chromosomes

Sample records 1 - 1 shown.


Observações citológicas em Dysdercus cadeias de cromossômios em tecido somático de Dysdercus mendesi Bloete (Hemiptera-Pyrrhocoridae)

Mendes, Luiz O. T.

Resumo em inglês The male of Dysdercus mendesi Bloete (1937) has 2n = 16 chromosomes : 14 autosomes and 2 sex-chromosomes. Details are presented on the morphology of these chromosomes of the somatic cells of embryonic tissue. The long pair of rod shaped chromosomes is the longest of the set, and they show a conspicuous sub-terminal constriction (supposed to be the centromere) and five other smaller constrictions. In most of the other chromosomes sub-terminally localized constriction (supp (mais) osed also to be the centromere) can also be observed. Based on these observations it has been concluded that the chromosomes of D. mendesi are morphologically normal. Dicentric chromosomes in the somatic tissue of this species are absent and the hypothesis of existence of diffuse centromere is also excluded. All of the chromosomes appear to have a definitive localized centromere. Metaphase spermatogonial plates have been observed where all the chromosomes appear as short and thick rods, apparently attached end-to-end, by definite connections, and forming a ring-shaped chain. Sometimes one or two chromosomes were found inside a ring formed by the others, but always connected to one or two of the chromosomes of the ring. Chromosomes chains of varied shapes were also found in the metaphase plates. In anaphase the longitudinally split chromosomes of the rings attain a curved shape as they move to the poles, giving the impression that both their extremities are pulled to the poles. A new hypothesis is presented to explain why the normal chromosomes of D. mendesi, provided with one localized centromere, can attain the curved shape observed in the anaphase configurations. It is based on the existence of connections attaching the chromosomes by their extremities, from metaphase until anaphase, and giving origin to the chain configuration already mentioned. These chains of chromosomes behave in anaphase as one rather long chromosomes with several centromeres. The centromeres are pulled to the poles, but as the extremities of the chromosomes of the chain are attached to each other, they give the false impression that both extremities of the chromosomes are being pulled to the poles, as if the chromosomes were provided with two centromeres, or as if there were diffuse centromeres.

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