Synaptonemal complex analysis of X-7 translocations in male mice: R2 and R6 quadrivalents

Synaptonemal complexes of surface-spread spermatocytes of mice heterozygous for reciprocal translocations R2 or R6 between the X-chromosome and chromosome 7 were examined by light and electron microscopy (EM). Measurements of the lengths of all chromosome axes involved in the translocation configurations and of the extent of synapsis were used to calculate the position of the break points of the two translocations. The breaks for R2 were determined to be at 62% of the 7 as measured from the centromere, and at 27% of the X. Quadrivalents were formed almost exclusively. The break points for R6 were calculated to be at 30% of the 7 as measured from the centromere, and at 75% of the X. Although in R6 the break in the X lies within the potential pairing region of the sex chromosomes, univalent Ys were rarely observed (6%). The EM sample of 76 nuclei contained: 42% quadrivalents, 52% heteromorphic bivalents, 4% trivalent plus Y univalent, and 2% X7-7 bivalent plus two univalents (7X and Y). Nonhomologous synapsis occurred in the quadrivalents of both R2 and R6. In R6 nonhomologous synapsis of the X portion of the 7X with the 7 involved up to 14% of the length of the 7. Methods are discussed for determining the position of the break points in the presence of nonhomologous synapsis. It is proposed that the high percentage of bivalents is due to premature desynapsis of the 7X from the 7 and that the X portion of the 7X axis confers its property of premature desynapsis on that portion of the 7 to which it is attached.