Evolution of Factors Affecting Placental Oxygen Transfer

Evolution of the beta-globin gene in vertebrates. In this hypothesis an ancestral gene was duplicated after amniotes diverged from mammals. One of the products (HBW, ω-globin) is found in extant monotremes and marsupials but was lost in other lineages. A further duplication in the common ancestor of marsupials and mammals resulted in embryonic (HBE, ɛ-globin) and adult (HBB, β-globin) forms. Further duplications in placental mammals gave rise, respectively, to globins expressed in the fetus (HBG, γ-globin) and adult (HBD, δ-globin). This resulted in a cassette of four genes. From ref. [22]. Copyright (2008) National Academy of Sciences, U.S.A.

Evolution of the interhaemal barrier in eutherian mammals. Red branches indicate clades with epitheliochorial placentation, yellow branches those with endotheliochorial placentation, and blue branches those with haemochorial placentation. The black line indicates an unresolved situation. In this interpretation, which is supported by other recent studies, epitheliochorial placentation is found to be a derived state. Tree adapted from Douady et al. [71] with character coding from Mess and Carter [60].