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Figure 9 | BMC Biology

Figure 9

From: Platypus globin genes and flanking loci suggest a new insertional model for beta-globin evolution in birds and mammals

Figure 9

Proposed model for the evolution of the α- and β-globin clusters in vertebrate lineages. (A) A region containing MPG-C16orf35-α-β-GBY-LUC7L represented the ancient α-β globin cluster of jawed vertebrates (>450 MYA), which is seen in the amphibian lineage. This region further duplicated and underwent some gene silencing in teleost fish. In an amniote ancestor of reptiles, birds and mammals (>315 MYA), a copy of an ancestral β-globin gene from this region was inserted into a different chromosome within a region replete with multiple copies of ORG genes. The original amniote β-globin gene survives as the ω-globin gene (β1) in the α-globin cluster of marsupials and monotremes, whereas the transposed β-globin gene (β2) duplicated several times to form different clusters in the different lineages. (B) Tandem duplications of the ancestral amniote α-globin gene produced a three-gene (π-αDA) cluster in the avian lineage. In the mammalian lineage, further duplications gave rise to a six-gene (ζ-ζ'-αD321) cluster with ongoing gene conversion events homogenising the embryonic and adult genes. In monotremes, the ancestral ω (β1) and GBY are retained. After the divergence of monotreme and therian mammals, there was an additional duplication of α2 to form θ, giving rise to the seven-gene cluster (ζ-ζ'-αD321-θ) in marsupials and eutherians. Marsupials also retain the ancestral ω but may have lost GBY gene; eutherians retain no identifiable remnant of either gene. Furthermore, the ancestral transposed β2-globin gene duplicated independently in birds and mammals. Before the mammalian radiation, we propose that the ancestral β2 gene duplicated to form a two-gene β-globin cluster (ε-β) as seen in monotremes and marsupials, except that ongoing gene conversion events homogenised platypus ε to group with monotreme β genes. After the divergence of marsupial and eutherian mammals, there were further tandem duplications of these two genes to produce complex β-globin cluster (ε-γ-η-δ-β) in eutherians.

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