The Gene Content of Mammalian and Avian Sex Chromosomes

Abstract

Sex chromosomes evolve specialised collections of genes, making them distinct from autosomes. Recent, high‐quality, genomic sequencing of sex chromosomes allows us to trace the evolutionary history of the mammalian X and Y chromosomes and avian Z and W chromosomes, and examine how evolution has remodelled their gene content. The XY and ZW sex chromosomes arose from ordinary pairs of autosomes. Differentiation into specialised sex chromosomes initiated with the degeneration of gene content on the Y and W chromosomes and the preservation of gene content on the X and Z chromosomes. As these degeneration and preservation processes unfolded, the sex chromosomes also acquired new genes, many of which are located in large, nearly identical, segmental duplications called amplicons. Ampliconic sequences, independently evolving across multiple lineages, are a major driving force in sex chromosome evolution.

Key Concepts

  • Mammalian and avian sex chromosomes evolved independently from autosomes.
  • X and Z chromosomes retain the majority of the ancestral autosomal genes.
  • X chromosome ancestral autosomal genes are highly conserved across mammals.
  • The gene content of Y and W chromosomes has degenerated.
  • Ampliconic sequences have independently evolved on X, Y, Z and W chromosomes.

Keywords: sex chromosomes; gene content; ampliconic; DNA sequence; rapid evolution

Figure 1. A simplified example of stepwise formation of strata and degeneration of Y‐linked genes. This represents a generic mammalian X–Y chromosome pair. Sry evolves as the male sex‐determining gene on the Y chromosome. An inversion on the Y chromosome leads to suppression of recombination with the X chromosomes and thus the formation of the first stratum (genes B, C, D, E). The lack of recombination on the Y chromosomes leads to the degeneration of genes. Subsequent inversions encompassing X–Y recombining portions of the sex chromosomes lead to the formation of additional strata (gene F).
Figure 2. Mammalian and avian sex chromosomes generated with high‐quality sequencing approaches. Each chromosome is drawn to scale and colour‐coded, where the underlying sequence is ancestrally autosomal (yellow), ampliconic (blue), pseudoautosomal (green), heterochromatic (yellow with red lines) or other (grey). Centromeres are labelled as cen.
Figure 3. Mechanisms of new gene acquisition and the development of amplicons on the sex chromosomes. Newly acquired sex‐linked genes have evolved via duplication of a preexisting sex‐linked gene (red vertical line indicates gene) or gene movement from an autosome to the sex chromosomes. (a) Large genomic regions surrounding preexisting sex‐linked genes are duplicated (amplicons, shown as blue arrows) and maintained with a high level of sequence identity between the duplicated ampliconic sequences. (b) Autosomal genes are retrotransposed or transposed onto the sex chromosomes and subsequently duplicated.
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Further Reading

Bachtrog D (2013) Y‐chromosome evolution: emerging insights into processes of Y‐chromosome degeneration. Nature Reviews Genetics 14 (2): 113–124.

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Bennett‐Baker, Pamela, and Mueller, Jacob L(Feb 2015) The Gene Content of Mammalian and Avian Sex Chromosomes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0025480]