Abstract
Bends in midmetaphase chromosomes are not distributed randomly throughout the karyotype. A specific bend at Xq13.3 to Xq21.1 is evident in the lyonized X, clearly associated with the center for Barr body condensation (Flejter et al, Am J Hum Genet, in press). The relationship of the X chromosome bend with the X inactivation center makes it a useful marker for identifying the lyonized X and for locating the inactivation center in X chromosome rearrangements, including the X of other species.
In high-resolution G- and R-banded cells the bend site includes the entire region from Xq11.2 to Xq13.3. In some cells the X exhibits a 360° loop involving Xq11.2 to q13.3, or multiple loops which may extend further down Xq, but not into Xp. In many cells the X exhibits an apparent "synapsis" of sub-band Xq11.2 and Xq13.3, forming a distinctive omega-shaped curve beginning just below the centromere. We believe these behaviors represent visible manifestions of Barr body decondensation which will be useful in studies of X inactivation and chromosome structure.
Metaphase cells from a female gorilla fibroblast culture (provided by TC Hsu) had the bend in the same region as in the human X. Metaphase cells from a female chimpanzee (provided by DH Ledbetter) had the bend in the region Xq22 to Xq24. This suggests that the X inactivation center is located more distally in the chimpanzee than in the human or gorilla. Additional gorilla and chimpanzee subjects need to be examined, including midmetaphase and high-resolution karyotype studies. For the present, we postulate a pericentric inversion during primate evolution to explain our observations.
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Van Dyke, D., Weiss, L., Worsham, M. et al. THE X CHROMOSOME BENDING SITE IN THE HUMAN, GORILLA, AND CHIMPANZEE: LOCATION AND EVOLUTION OF THE X INACTIVATION CENTER. Pediatr Res 18 (Suppl 4), 227 (1984). https://doi.org/10.1203/00006450-198404001-00802
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DOI: https://doi.org/10.1203/00006450-198404001-00802