Recombination and synaptic adjustment in oocytes of mice heterozygous for a large paracentric inversion Anna A. Torgasheva & Nikolai B. Rubtsov & Pavel M. Borodin Received: 26 October 2012 / Revised: 26 December 2012 / Accepted: 28 December 2012 / Published online: 25 January 2013 # Springer Science+Business Media Dordrecht 2013 Abstract Homologous chromosome synapsis in inver- sion heterozygotes results in the formation of inversion loops. These loops might be transformed into straight, non-homologously paired bivalents via synaptic adjust- ment. Synaptic adjustment was discovered 30 years ago; however, its relationship with recombination has remained unclear. We analysed this relationship in female mouse embryos heterozygous for large paracen- tric inversion In(1)1Rk using immunolocalisation of the synaptonemal complex (SYCP3) and mature recombi- nation nodules (MLH1) proteins. The frequency of cells containing bivalents with inversion loops decreased from 69 % to 28 % during pachytene. If an MLH1 focus was present in the non-homologously paired inverted region of the straight bivalent, it was always located in the middle of the inversion. Most of the small, incom- pletely adjusted loops contained MLH1 foci near the points at which pairing partners were switched. This observation indicates that the degree of synaptic adjust- ment depended on the crossover position. Complete synaptic adjustment was only possible if a crossover (CO) was located exactly in the middle of the inversion. If a CO was located at any other site, this interrupted synaptic adjustment and resulted in inversion loops of different sizes with an MLH1 focus at or near the edge of the remaining loop. Keywords Meiosis . Recombination . Synapsis . Synaptic adjustment . Inversion . MLH1 Abbreviations CO Crossover Cy3 Orange fluorescing cyanine DAPI 4′-6-Diamidino-2-phenylindole DSB Double-strand break DOP-PCR Degenerate oligonucleotide-primed polymerase chain reaction dpc Days post-conception FISH Fluorescent in situ hybridization FITC Fluorescein isothiocyanate MLH1 MutL Homolog 1 NCO Non-crossover SC Synaptonemal complex SYCP3 Synaptonemal complex protein 3 TAMRA Carboxytetramethylrhodamine Introduction Synapsis and recombination are the key processes of meiosis. In mammals, these events are initiated at the very beginning of meiotic prophase (leptotene stage) Chromosome Res (2013) 21:37–48 DOI 10.1007/s10577-012-9336-6 Responsible Editor: Beth A. Sullivan. A. A. Torgasheva : N. B. Rubtsov : P. M. Borodin (*) Siberian Department, Institute of Cytology and Genetics, Russian Academy of Sciences, Novosibirsk 630090, Russia e-mail: borodin@bionet.nsc.ru N. B. Rubtsov : P. M. Borodin Department of Cytology and Genetics, Novosibirsk State University, Novosibirsk, Russia