AN X-Y HOMOLOGOUS PAIRING SEGMENT IN TREE SHREWS (TUPAIA)

High-resolution early replication banding of tupaia metaphase chromoso mes revealed a synchronous early replicating segment in the short-arm telomeric regions of the active and inactive X chromosomes and in the long-arm telomeric region of the Y chromosome. Hybridization with the human-derived pseudoautosomal probe 113F (STIR) showed that this repea t is conserved and specifically localized within these synchronously e arly replicating segments of the X short arm and the Y long arm of all three tupaia species (Tupaia belangeri, T. chinensis, and T. glis) in vestigated. Moreover, meiotic studies demonstrated that a synaptonemal complex is formed at one telomeric end of the XY bivalent during the pachytene stage of meiosis in a male T. glis specimen. Thus, apart fro m the mouse, the tupaias are the first nonprimate mammals for which cy togenetic and molecular evidence is provided that their highly heterom orphic X and Y chromosomes share a conserved homologous segment in the telomeric position, a location that is compatible with pairing and cr ossing-over in male meiosis. Taken together, these observations strong ly, albeit indirectly, suggest that this chromosome segment at the tip of a sex-chromosome arm might behave pseudoautosomally.