Synaptonemal complex analysis of X-7 translocations in male mice. I. R3 and R5 quadrivalents

The synaptonemal complexes of surface-spread spermatocytes of mice heterozygous for one of two reciprocal translations (R3 and R5) between the X and chromosome 7 have been examined by light and electron microscopy (EM). The break points of R3 were determined to be at 70% of chromosome 7, as measured from the centromere, and at 22% of the X. Translocation quadrivalents were formed almost exclusively. The break points of R5 were at 21% of chromosome 7 as measured from the centromere, and at 83% of the X. There was little indication that the break in the X interfered with sex-chromosome synapsis between the 7X and Y. Univalent Y's were not observed in R3, and only seldom observed (8-14%) in R5. However, in contrast to R3, R5 formed quadrivalents relatively rarely (20% in the EM study of 100 nuclei), and heteromorphic bivalents of 7X-Y and X7-7 quite frequently (72%). Possible causes of this high bivalent frequency are discussed. Light-microscope (LM) analysis alone was found to be inadequate for interpreting synaptic configurations (quadrivalents vs. bivalents) in R5. The LM analysis was further complicated by the occurrence of nonhomologous synapsis in the heteromorphic bivalents of R5, a phenomenon easily recognized and interpreted in the EM portion of the study.