An orphaned mammalian β-globin gene of ancient evolutionary origin

Mammals possess multiple, closely linked β-globin genes that differ in the timing of their expression during development. These genes have been thought to be derived from a single ancestral gene, by duplication events that occurred after the separation of the mammals and birds. We report the isolation and characterization of an atypical β-like globin gene (ω-globin) in marsupials that appears to be more closely related to avian β-globin genes than to other mammalian β-globin genes, including those previously identified in marsupials. Phylogenetic analyses indicate that ω-globin evolved from an ancient gene duplication event that occurred before the divergence of mammals and birds. Furthermore, we show that ω-globin is unlinked to the previously characterized β-globin gene cluster of marsupials, making this the first report of an orphaned β-like globin gene expressed in a vertebrate.