EM investigations of surface spread synaptonemal complexes in a human male carrier of a pericentric inversion inv(13)(p12q14): the role of heterosynapsis for spermatocyte survival.

EM investigations of surface spread synaptonemal complexes of pachytene spermatocytes were performed on a human male carrier of a pericentric inv(13), ascertained through his daughter with a duplication-deficiency of the same chromosome. The inv(13) bivalent could be unambiguously identified by either its asymmetrical kinetochores or its nucleolar association or both. There was scarcity of reversed homosynapsis of the inverted segments with inversion loops, possibly related to the small size of the inversion. The majority of pachytene spermatocytes showed the inverted segment to be either asynapsed or heterosynapsed. Generally initial homosynapsis is highly efficient with high fidelity, while heterosynapsis at the secondary pachytene pairing phase may take place with any available partner remaining unsynapsed and including the differential segments of the XY. It is suggested that survival of spermatocytes to later stages of meiosis may be ensured by heterosynapsis, and the recombinant chromosome in the offspring might be the result of a U-type exchange.