Gene content of the mammalian X chromosome

The human and mouse X chromosomes have an atypical gene content. These X chromosomes contain more genes involved in reproduction and brain function than would be expected by chance alone. The accumulation of these types of genes may be facilitated by the structure of the X chromosome, which may be conducive to the amplification of genes in particular regions. In addition, as the X chromosome is hemizygous in males and is silenced during male meiosis, this affects both the fixation of male-beneficial and female-beneficial genes, and the expression of X-linked genes during meiosis. A requirement for the expression of X-linked genes during spermatogenesis may have driven retrotransposition of genes both on and off the X, and this has had a role in shaping the gene content of the mammalian X chromosome. Characterisation of the structure and gene content of the marsupial X chromosome is essential to understand whether the features of structure and expression observed on the human and mouse X chromosomes are fundamental to all therian X chromosomes. This will indicate whether the therian X chromosome plays a special role in reproduction and brain functions, and possibly speciation.