Pachytene chromosomes in male and female mice heterozygous for the Is(7;1)40H insertion.

Pairing of pachytene chromosomes was studied in oocytes and spermatocytes of mice heterozygous for the male-sterile Is(7;1)40H insertion using light and electron microscopy for synaptonemal complex analysis in surface-spread, silver-stained preparations. The data comprised four males and four female embryos. The insertion/deletion configurations appeared as either two bivalents or one quadrivalent in both sexes, but the proportion of bivalents was higher in oocytes. Some insertion and deletion bivalents showed synaptic adjustment. The insertion/deletion configurations were associated with, or adjacent to, the XY bivalent in the majority of spermatocytes. End-to-end association of different bivalents was more frequent in oocytes than in spermatocytes. It is suggested that physiological differences between male and female gametocytes may lead to the difference in their reproductive potential.