Abstract
BackgroundThe X and Y sex chromosomes are conspicuous features of placental mammal genomes. Mammalian sex chromosomes arose from an ordinary pair of autosomes after the proto-Y acquired a male-determining gene and degenerated due to suppression of X-Y recombination. Analysis of earlier steps in X chromosome evolution has been hampered by the long interval between the origins of teleost and amniote lineages as well as scarcity of X chromosome orthologs in incomplete avian genome assemblies.ResultsThis study clarifies the genesis and remodelling of the Eutherian X chromosome by using a combination of sequence analysis, meiotic map information, and cytogenetic localization to compare amniote genome organization with that of the amphibian Xenopus tropicalis. Nearly all orthologs of human X genes localize to X. tropicalis chromosomes 2 and 8, consistent with an ancestral X-conserved region and a single X-added region precursor. This finding contradicts a previous hypothesis of three evolutionary strata in this region. Homologies between human, opossum, chicken and frog chromosomes suggest a single X-added region predecessor in therian mammals, corresponding to opossum chromosomes 4 and 7. A more ancient X-added ancestral region, currently extant as a major part of chicken chromosome 1, is likely to have been present in the progenitor of synapsids and sauropsids. Analysis of X chromosome gene content emphasizes conservation of single protein coding genes and the role of tandem arrays in formation of novel genes.ConclusionsChromosomal regions orthologous to Therian X chromosomes have been located in the genome of the frog X. tropicalis. These X chromosome ancestral components experienced a series of fusion and breakage events to give rise to avian autosomes and mammalian sex chromosomes. The early branching tetrapod X. tropicalis’ simple diploid genome and robust synteny to amniotes greatly enhances studies of vertebrate chromosome evolution.
Highlights
The X and Y sex chromosomes are conspicuous features of placental mammal genomes
Platypus autosome 6 shares synteny with the entire therian X-conserved region (XCR) [7,8] including the SOX3 gene from which the testis-determining gene SRY evolved, consistent with this part of the genome being the progenitor of X and Y [9]
Homologies between human X and X. tropicalis chromosomes We identified putative orthologs of human X chromosome genes in the X. tropicalis genome assembly, and obtained the chromosomal locations of 454 of these in two ways
Summary
Mammalian sex chromosomes arose from an ordinary pair of autosomes after the proto-Y acquired a male-determining gene and degenerated due to suppression of X-Y recombination. Our ability to identify the chromosomal components that gave rise to the X chromosome prior to the mammalian radiation has been limited both by the Within Theria, the human X chromosome long arm and a proximal portion of the short arm correspond to genes on the marsupial X chromosome. This domain, the X-conserved region (XCR), is shared by sex chromosomes of all live-bearing mammals.
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