Proposed origin of X-added region of human X chromosome. A protochromosome (blue, left), present in the progenitor of Synapsida and Sauropsida, fused with several chromosomes (pink) to form the precursor of the eutherian X-Added Region (‘Pre-XAR’), now extant as most of opossum chromosomes 4 and 7. The Pre-XAR subsequently fragmented and fused with the therian X-Conserved Region (XAR and opossum X, yellow) to produce the eutherian X chromosome as well as with other partners (unshaded) contributing to autosomes. Fusion partners of the proto-XAR in amphibian (green) and avian (orange) lineages are also shown.
Mácha et al. BMC Genomics 2012 13:315 doi:10.1186/1471-2164-13-315