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 Lemniscomys barbarus spermatocytes were made to investigate the meiotic behaviour of the peculiar sex chromosomes of this species. A typical sex vesicle is absent, as the X and Y chromosomes appear unfolded at zygotene and pachytene. In most cells, the sex chromosomes are associated at distal segments at metaphase I, probably as a consequence of a distal chiasma. The pairing segment is located in the heterochromatic regions of both sex chromosomes, which include silent ribosomal cistrons interspersed throughout the heterochromatin. This may suggest a possible involvement of ribosomal genes in both pairing and recombination processes. X-Y pairing proceeds beyond the pseudoautosomal region, thus involving heterologous segments of the differential regions, a fact that is clearly evident at the Y centromeric region.