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Evolution of the Mammalian X Chromosome
Paul D Waters, The Australian National University, Canberra, Australia Terence J Robinson, University of Stellenbosch, Matieland, South Africa
The mammalian X chromosome has a fascinating evolutionary history which is intimately linked to that of the Y chromosome, since both evolved in concert from identical chromosomes. Teasing apart the events of thisevolution (and their timing) has been helped greatly by identifying similarities and differences between the X and Y, and by comparing the sex chromosomes of different species.

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Article Contents
. Introduction . Once an Autosomal Pair . Comparisons of Sex Chromosomes between Species . X Chromosome Inactivation . Evolution of X Gene Content . Conclusion

Online posting date:14th March 2008

Introduction
In most mammals (with a few notable exceptions) females have two X chromosomes, whereas males have a single X and a small, heterochromatic and gene poor Y that bears the dominant male sex-determining gene SRY that sets off a cascade of events leading to the development of a testis. Because females have two X chromosomes and males just one, one X in female somaticcells is switched off in a process called X chromosome inactivation. See also: X-chromosome Inactivation The human X chromosome is 155 Mb and represents approximately 5% of the haploid genome containing just less than 1100 genes (Ross et al., 2005). These genes have a mixture of housekeeping and specialized functions and, although the X is a sex chromosome, most do not have sex-specific functions.There is, however, an increased frequency of sexand reproduction-related (SRR) genes in comparison with the remainder of the genome. This chromosome also bears an increased frequency of genes involved in X-linked mental retardation (XLMR) syndromes (reviewed in Graves and Delbridge, 2001). Over and above its unusual gene content, the X also has an elevated level of repeated deoxyribonucleic acid (DNA)elements called Long Interspersed Nuclear Element 1 (LINE1) (Ross et al., 2005) which are thought to play a role in spreading the inactivation signal along this chromosome (Lyon, 1998). See also: Chromosome X In contrast to the X, the male-specific Y (MSY) chromosome is much smaller (approximately 65 Mb, representing

approximately 2% of the haploid genome in humans) with more than half (40 Mb)of its length comprising heterochromatin that is devoid of genes. The other 25 Mb of the human Y is euchromatic and contains 172 transcriptional units, many of which are not translated. In fact there are only 78 protein-coding units that code for just 27 discrete proteins, of which 20 have a partner on the X chromosome (Skaletsky et al., 2003). Along with this preponderance of shared genes, the Xand Y share a small region of homology called the pseudoautosomal region (PAR) within which there is an obligatory recombination event during male meiosis that mediates X and Y segregation. See also: Chromosome Y

Once an Autosomal Pair
The homology shared by the X and Y chromosomes supports the hypothesis that they evolved from the same ordinary pair of autosomes (Charlesworth, 1991). Thisoccurred via a process of Y degradation after the proto-Y obtained a testis-determining factor (Charlesworth and Charlesworth, 2000) (Figure 1). Genes beneficial to males accumulated near this new testis-determining factor, across which recombination was suppressed (probably by an inversion on the Y) making it male specific. The lack of recombination between the X and Y resulted in progressive Ydegradation because selection did not act on a single gene, but rather the entire MSY. Therefore, even quite severe mutations (i.e. inversions and deletions that result in loss of gene function) on the MSY were not eliminated from the population (reviewed in Graves, 2004). Lahn and Page (1999) suggested that suppression of recombination between the X and Y chromosome was the result of inversions on...
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