A new type of nonhomologous synapsis in T(X;4)1R1 translocation male mice.

The synaptonemal complexes of T(X;4)1R1 (abbreviated R1) translocation heterozygotes have been examined by electron microscopy and compared with those of two X-7 translocations: R5 and R6. The X chromosome breakpoint of R1 is estimated to lie between 78 and 82% from the proximal end of the X, in the same general region as the R5 and R6 breakpoints. The position of the autosomal breakpoint of R1, like that of R6, is about 30% from the proximal end of the respective autosome. R1 is also similar to R6 in that there is extensive nonhomologous synapsis both in quadrivalents and heteromorphic bivalents. We have recently found that the location of breakpoints with respect to the position of the G-bands appears to be related to the synaptic behavior seen in translocation heterozygotes. If both breaks of a reciprocal translocation lie in G-light bands, as was the case with R5, synapsis is confined to homology. However, if one break lies in or immediately adjacent to a G-dark band, there is nonhomologous synapsis, as occurs with R1 and R6. Comparison of the synaptic behavior of R1 with R5 and R6 leads to the conclusion that this G-band-related nonhomologous synapsis is of a different type than the "synaptic adjustment" phenomenon that has been described by Moses (1977a). This G-band-related nonhomologous synapsis is not substage-specific, but competes with homologous synapsis during zygotene-early pachytene.