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.
Paracentric Chromosome Inversion: effect on crossover products
A paracentric ("away from the center") inversion does not include the centromere (o). In the example, the BCD region of the standard chromosome oABCDEFGH has been inverted, to give the locus order oA(DCB)EFGH. After chromatid duplication in meiosis, the paired inverted chromosome forms a loop so that the loci pair with their homologous on the uninverted chromosome.
Occurence of a crossover event inside the loop produces four types of products: a dicentric brdige, an acentric fragment, and two chromosomes with the standard and inverted gene orders. Because it lacks a centromere and cannot be drawn to either cell pole, the acentric fragment will be lost during meiosis. Because the two centromeres of the dicentric bridge are drawn to opposite poles, the bridge will be broken mechanically at a random position, with resultant production of two duplication / deletion products in which one or more loci have been duplicated or deleted, according to where the breakage of the bridge occured with respect to the inverted region. Inheritance of one of these products will result in segmental aneuploidy.
Note that, unlike a pericentric inversion, not all gene loci are represented in the final products, because those on the acentric fragment are lost. In the final set of fragments, count the number of A versus B C D versus E F G H loci.
Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2014 by Steven M. Carr