Les Hétérochromosomes Dans L’ovogenèse des Mammifères
This cytological research on the heterochromosomes of male and female mammals leads to the following conclusions:
Not all the chromosomes of the brown rat are acrocentric; in the spermatogonial mitosis of young animals I have counted an average of seventeen metacentric elements. Their exact number is hard to determine, but there is no doubt of their existence.
TheX chromosome has a very distinct short arm representing about one-sixth of the total length of the chromosome. TheY was not identified with certainty in male diploid mitoses; it seems to be one of the small V’s whose number is always uneven.
In the female the tetrad formed by the X’s behaves like the autosomal bivalents. In the rat and mouse it is retarded in the first meiotic anaphase, and shows a constant configuration: two chiasmata in the long arm, one localized near the centromere, the other near the end of this arm.
TheX can be recognized in mitosis by the fact that it always lies wholly in the equatorial plane, in the first female meiosis by the separation of the two chiasmata of its long arm; this behaviour suggests a special rigidity.
The comparison between mitoses and meioses of males and females agrees with the hypothesis ofKoller made to explain negative heteropycnosis in mammals. As this is not found in the female it is not due to a special chemical constitution. In the male it is only seen at the first meiotic division, which also tells against the idea of a special structure. On the contrary this heterochromatism seems to be associated with the solitary condition of theX meiosis. Measurements show that the unpaired segment of theX forms no ‘major spiral’, thus accounting for the apparent negative heteropycnosis.
More generally, the ‘major spiral’ seems to be associated with the existence of an homologous chromosomal segment.
Volume 100, 2021
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