Synaptonemal complex analysis of mouse chromosomal rearrangements. III. Cytogenetic observations on two paracentric inversions.

Synaptonemal complex (SC) analysis by electron microscopy of spermatocytes in surface microspreads was carried out in mice heterozygous for two paracentric inversions: either In(1) 1 RK or In(2)5Rk. characteristic SC inversion loops are formed at synapsis in bivalents carrying the rearrangements. Although all loops were observed to be eliminated by late pachytene through synaptic adjustment, every spermatocyte at early pachytene contained a fully synapsed loop. Cells in the earliest stage of pachytene contained the longest loops and thus had undergone minimal adjustment. The SC estimates of inversion lengths and breakpoint positions in such cells corresponded well with those from mitotic chromosome banding and could be correlated with genetic maps of chromosomes #1 and #2, thus demonstrating the basis for the mapping of pachytene chromosomes. The regularity of loop formation and reproducibility of the SC analysis are reflected in the constant relative positions of the estimated breakpoints. The method is sensitive enough to reflect small, real, interstitial length differences between meiotic and mitotic chromosomes. The results demonstrate the feasibility and precision of detection and quantitative characterization of inversions at early meiotic prophase by SC analysis.