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Open Access Highly Accessed Research article

Genomic organization and gene expression of the multiple globins in Atlantic cod: conservation of globin-flanking genes in chordates infers the origin of the vertebrate globin clusters

Ola F Wetten12, Alexander J Nederbragt3, Robert C Wilson2, Kjetill S Jakobsen3, Rolf B Edvardsen4 and Øivind Andersen15*

Author Affiliations

1 Department of Animal and Aquacultural Sciences, University of Life Sciences, P.O. Box 5003, N-1430 Aas, Norway

2 Department of Natural Sciences and Technology, Hedmark University College, P.O. Box 4010 Bedriftsenteret, N-2306 Hamar, Norway

3 Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of Oslo, P.O. Box 1066 Blindern, N-0316 Oslo, Norway

4 Institute of Marine Research, P.O. Box 1870 Nordnes, N-5817 Bergen, Norway

5 Nofima Marine, P.O. Box 5010, N-1430 Aas, Norway

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BMC Evolutionary Biology 2010, 10:315  doi:10.1186/1471-2148-10-315

Published: 20 October 2010

Abstract

Background

The vertebrate globin genes encoding the α- and β-subunits of the tetrameric hemoglobins are clustered at two unlinked loci. The highly conserved linear order of the genes flanking the hemoglobins provides a strong anchor for inferring common ancestry of the globin clusters. In fish, the number of α-β-linked globin genes varies considerably between different sublineages and seems to be related to prevailing physico-chemical conditions. Draft sequences of the Atlantic cod genome enabled us to determine the genomic organization of the globin repertoire in this marine species that copes with fluctuating environments of the temperate and Arctic regions.

Results

The Atlantic cod genome was shown to contain 14 globin genes, including nine hemoglobin genes organized in two unlinked clusters designated β5-α1-β1-α4 and β3-β4-α2-α3-β2. The diverged cod hemoglobin genes displayed different expression levels in adult fish, and tetrameric hemoglobins with or without a Root effect were predicted. The novel finding of maternally inherited hemoglobin mRNAs is consistent with a potential role played by fish hemoglobins in the non-specific immune response. In silico analysis of the six teleost genomes available showed that the two α-β globin clusters are flanked by paralogs of five duplicated genes, in agreement with the proposed teleost-specific duplication of the ancestral vertebrate globin cluster. Screening the genome of extant urochordate and cephalochordate species for conserved globin-flanking genes revealed linkage of RHBDF1, MPG and ARHGAP17 to globin genes in the tunicate Ciona intestinalis, while these genes together with LCMT are closely positioned in amphioxus (Branchiostoma floridae), but seem to be unlinked to the multiple globin genes identified in this species.

Conclusion

The plasticity of Atlantic cod to variable environmental conditions probably involves the expression of multiple globins with potentially different properties. The interspecific difference in number of fish hemoglobin genes contrasts with the highly conserved synteny of the flanking genes. The proximity of globin-flanking genes in the tunicate and amphioxus genomes resembles the RHBDF1-MPG-α-globin-ARHGAP17-LCMT linked genes in man and chicken. We hypothesize that the fusion of the three chordate linkage groups 3, 15 and 17 more than 800 MYA led to the ancestral vertebrate globin cluster during a geological period of increased atmospheric oxygen content.