Synaptonemal complex analysis of mouse chromosomal rearrangements. IV. Synapsis and synaptic adjustment in two paracentric inversions.

Two paracentric inversions in the mouse, In (1) 1 Rk and In (2) 5 Rk, have been studied in surface microspreads of spermatocytes from heterozygotes. At zytogene, synaptic initiation occurs independently in three regions: within the inversion, and without, on either side. Synaptonemal complex (SC) formation is restricted to homologous regions, resulting in inversion loops in all early pachytene spermatocytes. An adjusting phase then occurs during pachytene in which the inversion loop is reduced by desynapsis of homologously synapsed SC, followed immediately by non-homologous synapsis with the alternate pairing partner, progressing from the ends toward the middle. Adjustment occurs during the first half of pachytene, but is not closely synchronized with sub-stage. It is complete by late pachytene, the loop having been eliminated in all cases and replaced by "straight" SCs in which the inverted region is heterosynapsis. Synapsis in the adjustment phase is evidently permitted only after the homosynaptic phase, and is indifferent to homology. It may lead to hetersynapsis, as in the inversion region, or to synapsis of homologous regions not synapsed at zytogene. The anaphase bridge frequency, a measure of crossing over within the inversion, is about 34% for both inversions studied, indicating that such crossovers do not block adjustment, that crossing over probably occurs before or during the adjustment period, and that there is some crossover suppression. The last could be the consequence of blocking by desynapsis/heterosynapsis. Synaptic adjustment appears to be a general phenomenon that occurs to varying extents in different forms. A hypothetical scheme for two phases of synapsis is proposed: at zytogene, a basic propensity for indifferent SC formation is limited by a restricting condition to synapsis between homologous regions, Subsequently, the restriction is lifted, whereupon synaptic instability is resolved by desynapsis, followed by resynapsis that is indifferent to homology, but that results in a topologically more stable structure.