Assignment of the thymosin beta 4 X/Y chromosome (TMSB4X/Y) gene to tammar wallaby chromosome 5p by fluorescence in situ hybridisation.

All mammals have an XX female:XY male sex chromosome system. The X and Y are thought to have evolved from a homologous pair of autosomes by a process of Y degradation. This process left a small, heterochromatic and gene-poor Y, and a relatively unchanged X chromosome (Ohno, 1967). The origin of the human sex chromosomes has been explored by comparing their gene content with that of sex chromosomes from distantly related mammals. Previous work has defined conserved (XCR, shared with the X of marsupials and monotremes) and added regions of the human X (XAR, homologous to marsupial and monotreme autosomal regions, represented by chromosome 5p in the tammar wallaby) (Watson et al., 1991; Spencer et al., 1991). As a degraded X, the Y also consists of conserved (YCR) and added (YAR) regions (Graves, 1995; Waters et al., 2001). We cloned and characterized the tammar wallaby homologue of TMSB4X (GenBank accession number AY341342), a gene with a copy on both the X and Y in human, and mapped it by fluorescent in situ hybridisation. No signals were found over the X or Y chromosome, but clear signals were apparent on chromosome 5p. This location is consistent with the hypothesis that the region of human Xp that includes TMSB4X was recently added (XAR), and that the human Y is largely derived from the same added region.