Nonrandom association between a specific autosome and the X chromosome in meiosis of the male mouse: possible consequence of the homologous centromeres’ separation

A nonrandom association is described between chromosome No. 15, which is involved in the T(14;15)6Ca translocation, and the X chromosome in diakinesis/ metaphase I plates of the male mouse. The tight attachment of centromeric heterochromatin (CH) regions of chromosomes X and 15 is predominantly found in those primary spermatocytes in which the distance between the homologous CH regions of the No. 15s exceeds some critical value. The rule holds true for both T6 homozygotes (i.e., for those of their primary spermatocytes displaying two 15t univalents) and T6 heterozygotes (i.e., for translocation configurations “III + I” and certain types of “chain-of-four” quadrivalents). The tight X-15 CH association is highly specific, since out of 1540 primary spermatocytes of the T6 heterozygotes, only 3 showed the tight X-14 CH association. In contrast to the tight CH attachment, the frequency of loose X-15 chromosome association does not show dependence upon the centromere separation of the No. 15s. The nature of the described CH association remains to be clarified; for the present, it seems that the occurrence of a nucleolus organizer on the translocated chromosome 15t presumably plays a role in the causal mechanism of the loose X-15t CH association. Some circumstantial evidence indicates the adverse effect of the observed tight CH attachment on the course of spermatogenesis.