Haemoglobin polymorphisms affect the oxygen-binding properties in Atlantic cod populations

Abstract

A major challenge in evolutionary biology is to identify the genes underlying adaptation. The oxygen-transporting haemoglobins directly link external conditions with metabolic needs and therefore represent a unique system for studying environmental effects on molecular evolution. We have discovered two haemoglobin polymorphisms in Atlantic cod populations inhabiting varying temperature and oxygen regimes in the North Atlantic. Three-dimensional modelling of the tetrameric haemoglobin structure demonstrated that the two amino acid replacements Met55beta1Val and Lys62beta1Ala are located at crucial positions of the alpha1beta1 subunit interface and haem pocket, respectively. The replacements are proposed to affect the oxygen-binding properties by modifying the haemoglobin quaternary structure and electrostatic feature. Intriguingly, the same molecular mechanism for facilitating oxygen binding is found in avian species adapted to high altitudes, illustrating convergent evolution in water- and air-breathing vertebrates to reduction in environmental oxygen availability. Cod populations inhabiting the cold Arctic waters and the low-oxygen Baltic Sea seem well adapted to these conditions by possessing the high oxygen affinity Val55-Ala62 haplotype, while the temperature-insensitive Met55-Lys62 haplotype predominates in the southern populations. The distinct distributions of the functionally different haemoglobin variants indicate that the present biogeography of this ecologically and economically important species might be seriously affected by global warming.

Figure 1

Amino acid polymorphisms in globin subunits of Atlantic cod. (a) Deduced amino acid sequences of the six isolated globin genes ((i) α-globins, (ii) β-globins), including the alternative amino acids at the polymorphic sites. The sequences are available in GenBank under the accession nos FJ392681–FJ392686. The helices are conventionally designated in bold letters. Highly conserved positions are shaded. (b) Sequence chromatogram of the mutated region of Hb-β₁ encoding the non-recombinant allelic globin variants (i) Met55–Lys62 and (ii) Val55–Ala62. Downward arrows, non-synonymous mutation; asterisks, synonymous mutation.

Figure 2

Met55βVal substitution in the α₁β₁ subunit contact of Atlantic cod haemoglobin. The three-dimensional model shows the superimposition of Met55β (red) and Val55β (blue) at the interface of the α₁ (black) and β₁ (grey) subunits. The D helix of β₁ and the GH corner and initial H helix of α₁ chain are shown in ribbon representation (yellow). The inset image depicts the distances (Å) from the CG atom of Pro120α to the CE atom of Met55β and to the CG2 atom of Val55β.

Figure 3

Lys62βAla substitution and water interaction in the haem pocket, showing the superimposition of GRID contour maps at −11.0 kcal mol⁻¹ for the water probe calculated for the distal haem pocket containing Lys62β (red) or Ala62β (blue). His63β, Lys59β and Lys62β were identified as the key residues contributing to the hydrophilic contour maps.

Figure 4

Relative allele frequencies of the Met55Val (left pie) and Lys62Ala (right pie) polymorphisms in the cod populations examined. The Met55 and Lys62 alleles are depicted in white, whereas the Val55 and Ala62 alleles are shown in black. Latitudes (ordinates) and longitudes (abscissas) are indicated.