Nonhomologous synapsis of the sex chromosomes in the heteromorphic bivalents of two X-7 translocations in male mice: R5 and R6.

Electron microscopy of pachytene nuclei of mice heterozygous for either of two reciprocal X-7 translocations (R5 or R6) revealed a high frequency of heteromorphic bivalents involving the translocated chromosomes. In both translocations the break was in the proximal third of the 7 and the distal third of the X, but the R5 breaks were closer to the 7 centromere and X telomere than the R6 breaks. In both translocations the 7 frequently synapsed nonhomologously with the X7. In R5 the part of the X to which the 7 synapsed may include a region that synapses with the Y in normal mice. However, in R6 the 7 synapsed with a portion of the X that never synapses with the Y (Synapsis was clearly in the "differentiated" region). In both translocations the Y synapsed maximally with the X portion of the 7X in those nuclei in which there was nonhomologous synapsis of the 7 with the X7. The Y occasionally synapsed nonhomologously with the 7 portion of the 7X. The behavior of the bivalents suggests that the autosomal portions of the 7X and X7 may alter the behavior of the sex-chromosome portions. Both the nonhomologous synapsis of the Y with the 7X and the timing of events during pachytene have led us to question the "homology" between the X and Y in this species.