Mammalian sex chromosomes. I. Cytological changes in the chiasmatic sex chromosomes of the male musk shrew, Suncus murinus.
The X and Y chromosomes of the musk shrew are the two largest in the complement and they regularly form a single chiasma during meiosis. This chiasma is located in the short arms of the X and Y, both of which show partial C-banding at meiosis. The in vitro incorporation of 5-bromodeoxyuridine/tritiated thymidine during late S reveals that the non-C-band region of the Y finishes replication later than the C-band positive heterochromatin. During meiosis, the sex bivalent opens out early in pachytene to reveal a single chiasma which persists until late metaphase-I. In surface-spread, silver-stained meiocytes, the sex bivalent morphology changes from a phase of extensive pairing to one which includes a visible chiasma through a brief diffuse stage. Observations on C-banded meiocytes show a shift in the sex pair from a C-band positive to a negative state as compared to their corresponding somatic pattern. Comparable changes are also observed in the sex bivalents of other mammals which undergo a chiasmatic exchange. This suggests that in addition to pairing homology, an alteration in the chromatin configuration may be necessary for crossing over to occur between the sex chromosomes.
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D000818:Animals; D001854:Bone Marrow Cells; D002478:Cells, Cultured; D002871:Chromosome Banding; D004171:Diploidy; D007621:Karyotyping; D008214:Lymphocytes / Q000166:cytology; D008297:Male; D008540:Meiosis; D008677:Metaphase; D012730:Sex Chromosomes / Q000648:ultrastructure*; D012788:Shrews / Q000235:genetics*; D013045:Species Specificity; D013090:Spermatocytes / Q000166:cytology; D013154:Spleen / Q000166:cytology