Sex-chromosome pairing through heterochromatin in the African rodent Lemniscomys barbarus (Rodentia, Muridae). A synaptonemal complex study

Giemsa-stained spread preparations and microspread preparations of Lemnisco mys barbarus spermatocytes were made to investigate the meiotic behaviour o f the peculiar sex chromosomes of this species. A typical sex vesicle is ab sent, as the X and Y chromosomes appear unfolded at zygotene and pachytene. In most cells, the sex chromosomes are associated at distal segments at me taphase I, probably as a consequence of a distal chiasma. The pairing segme nt is located in the heterochromatic regions of both sex chromosomes, which include silent ribosomal cistrons interspersed throughout the heterochroma tin. This may suggest a possible involvement of ribosomal genes in both pai ring and recombination processes. X-Y pairing proceeds beyond the pseudoaut osomal region, thus involving heterologous segments of the differential reg ions, a fact that is clearly evident at the Y centromeric region.