Evolution of globin genes of the medaka Oryzias latipes (Euteleostei; Beloniformes; Oryziinae)

Abstract

Recently we cloned two globin gene clusters from the genome of medaka ( Oryzias latipes): one designated the embryonic globin gene cluster (E1; 5′α0 3′– 3′β1 5′– 5′α1 3′– 5′β2 3′– 5′α2 3′– 3′α3 5′– 5′β3 3′– 3′β4 5′– 5′α4 3′– 3′ψα 5′– 5′ψβ 3′) and the other the adult globin gene cluster (A1; 3′ad.α1 5′- 5′ad.β1 3′– 3′ad.α2 5′). The E1 and A1 clusters map to linkage groups 8 and 19, respectively. The genes β1/α1, α3/β3, β4/α4, ψα/ψβ and ad.α1/ad.β1 are organized in head-to-head orientation with respect to transcriptional polarity. The genes α0, α1 and α2 are arranged in tandem with the same orientation. The results suggest that a variety of events occurred in globin gene evolution such as chromosomal translocation, duplication of α/β-paired genes, tandem duplication of single α genes and the transformation of one pair of α/β-paired genes into pseudogenes (ψα/ψβ). Amino acid sequences predicted from the genes were compared with those of 42 α and 55 β teleostean globins using the neighbor-joining or maximum likelihood methods. The phylogenetic trees that were generated classified the teleostean globins into at least four groups, tentatively named ‘Embryonic Hb Group (I)’, ‘Notothenioid Major Adult Hb Group (II)’, ‘Anodic Adult Hb Group (III)’ and ‘Cathodic Adult Hb Group (IV)’. The medaka genes α0, β1, α1, α2, α3, β3, β4 and α4 belong to group I, and ad.α1 and ad.β1 to group II. Further analysis suggests that ψα/ψβ and β2/ad.α2 belong to groups III and IV, respectively. Thus, globin genes in the medaka probably were diversified from four ancestral genes, one for each group. On the basis of the gene comparisons, we present a hypothetical pathway for globin gene evolution in the medaka.