Research article

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

Vidushi S Patel¹ , Steven JB Cooper^2,3 , Janine E Deakin¹ , Bob Fulton⁴ , Tina Graves⁴ , Wesley C Warren⁴ , Richard K Wilson⁴ and Jennifer AM Graves¹

¹The ARC Centre for Kangaroo Genomics, Research School of Biological Sciences, The Australian National University, Canberra, ACT 2601, Australia

²Australian Centre for Evolutionary Biology and Biodiversity, The University of Adelaide, Adelaide, SA 5005, Australia

³Evolutionary Biology Unit, South Australian Museum, Adelaide, SA 5000, Australia

⁴Genome Sequencing Center, Washington University School of Medicine, St Louis, Missouri 63108, USA

author email corresponding author email

BMC Biology 2008, 6:34doi:10.1186/1741-7007-6-34

Published:	25 July 2008

Abstract

Background

Vertebrate alpha (α)- and beta (β)-globin gene families exemplify the way in which genomes evolve to produce functional complexity. From tandem duplication of a single globin locus, the α- and β-globin clusters expanded, and then were separated onto different chromosomes. The previous finding of a fossil β-globin gene (ω) in the marsupial α-cluster, however, suggested that duplication of the α-β cluster onto two chromosomes, followed by lineage-specific gene loss and duplication, produced paralogous α- and β-globin clusters in birds and mammals. Here we analyse genomic data from an egg-laying monotreme mammal, the platypus (Ornithorhynchus anatinus), to explore haemoglobin evolution at the stem of the mammalian radiation.

Results

The platypus α-globin cluster (chromosome 21) contains embryonic and adult α- globin genes, a β-like ω-globin gene, and the GBY globin gene with homology to cytoglobin, arranged as 5'-ζ-ζ'-α^D-α³-α²-α¹-ω-GBY-3'. The platypus β-globin cluster (chromosome 2) contains single embryonic and adult globin genes arranged as 5'-ε-β-3'. Surprisingly, all of these globin genes were expressed in some adult tissues. Comparison of flanking sequences revealed that all jawed vertebrate α-globin clusters are flanked by MPG-C16orf35 and LUC7L, whereas all bird and mammal β-globin clusters are embedded in olfactory genes. Thus, the mammalian α- and β-globin clusters are orthologous to the bird α- and β-globin clusters respectively.

Conclusion

We propose that α- and β-globin clusters evolved from an ancient MPG-C16orf35-α-β-GBY-LUC7L arrangement 410 million years ago. A copy of the original β (represented by ω in marsupials and monotremes) was inserted into an array of olfactory genes before the amniote radiation (>315 million years ago), then duplicated and diverged to form orthologous clusters of β-globin genes with different expression profiles in different lineages.