The evolution of haemoglobin gene loci in amniotes

Abstract

The genes in alpha ({u03B1})- and beta ({u03B2})- globin clusters constitute a functional haemoglobin molecule, crucial for oxygen transportation. In most fish and amphibians, {u03B1}- and {u03B2}-globin genes are located together, whereas in amniotes (birds and mammals), there are two distinct clusters. Several complex models have been proposed to explain the evolution of these gene clusters. However, there was a lack of data for key positions in amniote phylogeny to discern which one was most parsimonious. Therefore, the main aims of this project were to characterise {u03B1}- and {u03B2}-globin clusters and their regulatory regions in a monotreme Ornithorhynchus anatinus (Australian duck-billed platypus) and two reptilian species Pogona vitticeps (Australian bearded dragon) and Anolis carolinensis (green anole lizard), to gain insight into globin loci evolution. This thesis is presented as a collection of research papers covering each topic, and a review and discussion that summarises my research. The first paper (Chapter 2) reports a comprehensive study on the characterisation, expression and evolution of {u03B1}- and {u03B2}-globin gene clusters in the platypus, using a combination of molecular and bioinformatics approaches. The most important findings from this work leading to the development of a new and simple model for globin gene evolution concerned the discovery of a {u03B2}-like globin gene within the a-globin cluster and genomic context analysis of {u03B1}- and {u03B2}-globin clusters across vertebrates. I showed that the amniote a-globin cluster is in fact the same as the a-{u03B2} cluster found in fish and amphibians, and both clusters share common flanking genes (C16orf35 and LUC7L). I proposed a transposition model in which a copy of {u03B2}-globin gene was inserted into a cluster of olfactory receptors (flanked by RRMl, CCKBR and ILK) in the ancestor of amniotes, thus originating the amniote {u03B2}-globin cluster. To elaborate this model further, my second paper (Chapter 3) reviews some events that could have led to this transposition, and their effects on the current fate of regulation. Information on the organization of globin genes in reptiles was required to test this transpositional model. I looked into the globin gene organization in the green anole using a bioinformatics approach and in the bearded dragon using a molecular approach. The results are reported in Chapter 4 and my third paper, which describe how fragmentary data from the green anole genome sequence assembly and mapping data from bearded dragon provided further evidence to support my proposed model for the evolution of the {u03B2}-globin gene cluster in amniotes. I also studied the evolution of regulatory regions of the platypus {u03B1}- and {u03B2}-globin clusters to address the question whether the translocation of the {u03B2}-globin locus resulted in a transposition of its regulatory region, or whether a new regulatory region evolved as a result of this translocation (reported in the fourth paper, Chapter 5). By using some novel techniques, I showed that the platypus a-globin has a major regulatory element that is conserved with other jawed vertebrates, whereas the regulatory regions of their {u03B2}-globin cluster do not show any conservation at the sequence level to those of birds and therian mammals. This suggested that the regulatory regions of amniote {u03B2}-globin genes evolved either more rapidly (more substitutions) or more extensively (e.g. more rearrangements) from a common ancestral regulatory region. Alternatively, these regulatory regions may have independent origins in different amniote lineages. In my final chapter, I discuss the overall implications of my findings on this area of research. I highlight the special value of studying non-model species mammals and reptiles, by which researchers are able to gain novel information about globin evolution and regulation. Show more