Platypus globin genes and flanking loci suggest a new insertional model for beta-globin evolution in birds and mammals.
BACKGROUND
Vertebrate alpha (alpha)- and beta (beta)-globin gene families exemplify the way in which genomes evolve to produce functional complexity. From tandem duplication of a single globin locus, the alpha- and beta-globin clusters expanded, and then were separated onto different chromosomes. The previous finding of a fossil beta-globin gene (omega) in the marsupial alpha-cluster, however, suggested that duplication of the alpha-beta cluster onto two chromosomes, followed by lineage-specific gene loss and duplication, produced paralogous alpha- and beta-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 alpha-globin cluster (chromosome 21) contains embryonic and adult alpha- globin genes, a beta-like omega-globin gene, and the GBY globin gene with homology to cytoglobin, arranged as 5'-zeta-zeta'-alphaD-alpha3-alpha2-alpha1-omega-GBY-3'. The platypus beta-globin cluster (chromosome 2) contains single embryonic and adult globin genes arranged as 5'-epsilon-beta-3'. Surprisingly, all of these globin genes were expressed in some adult tissues. Comparison of flanking sequences revealed that all jawed vertebrate alpha-globin clusters are flanked by MPG-C16orf35 and LUC7L, whereas all bird and mammal beta-globin clusters are embedded in olfactory genes. Thus, the mammalian alpha- and beta-globin clusters are orthologous to the bird alpha- and beta-globin clusters respectively.


CONCLUSIONS
We propose that alpha- and beta-globin clusters evolved from an ancient MPG-C16orf35-alpha-beta-GBY-LUC7L arrangement 410 million years ago. A copy of the original beta (represented by omega 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 beta-globin genes with different expression profiles in different lineages.
Patel
Vidushi S
VS
The ARC Centre for Kangaroo Genomics, Research School of Biological Sciences, The Australian National University, Canberra, ACT 2601, Australia. vidushi.patel@anu.edu.au
Cooper
Steven J B
SJ
Deakin
Janine E
JE
Fulton
Bob
B
Graves
Tina
T
Warren
Wesley C
WC
Wilson
Richard K
RK
Graves
Jennifer A M
JA
D000818:Animals; D001717:Birds / Q000235:genetics*; D015139:Blotting, Southern; D022202:Chromosomes, Artificial, Bacterial; D003001:Cloning, Molecular; D004251:DNA Transposable Elements / Q000235:genetics*; D019143:Evolution, Molecular; D005914:Globins / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D008322:Mammals / Q000235:genetics*; D008957:Models, Genetic; D005810:Multigene Family; D010802:Phylogeny; D010987:Platypus / Q000235:genetics*; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D017422:Sequence Analysis, DNA
D004251:DNA Transposable Elements; D005914:Globins