Phylogenetic and structural analysis of the HbA (αA/βA) and HbD (αD/βA) hemoglobin genes in two high-altitude waterfowl from the Himalayas and the Andes: Bar-headed goose (Anser indicus) and Andean goose (Chloephaga melanoptera)

Two species of waterfowl living at high altitude provide a prominent example of parallel adaptation at the molecular level. The bar-headed goose (Anser indicus) breeds at high elevations in central Asia and migrates across the Himalayas, where the partial pressure of oxygen (O2) is one-third of sea level. In South America, the distantly related Andean goose (Chloephaga melanoptera) is endemic to the high Andes. Both species exhibit increased blood–O2 affinity, which has been attributed to the effects of single amino acid substitutions in the major hemoglobin. Here we present phylogenetic analyses of the swans and geese (Anserinae) and South American sheldgeese (Anatinae) using the three genes that encode the major (HbA) and minor (HbD) hemoglobin isoforms. We sought to determine whether two amino acid substitutions that have been the focus of extensive biochemical analysis (Ala-αA119 and Ser-βA55) are uniquely derived in bar-headed goose and Andean goose, respectively, and to examine evidence of molecular adaptation at other positions in hemoglobin genes by comparing these two high-altitude taxa to their closest relatives. Bayesian analysis of the αA-, αD-, and βA-subunit genes produced well-resolved phylogenies, with high posterior probabilities and bootstrap values for most genera. The bar-headed goose is likely sister to all other Anser species. Andean goose, the sole highland representative of the South American sheldgeese is either sister to the other Chloephaga species or sister to Neochen. In the bar-headed goose, four derived substitutions were observed in HbA (αA12, 18, 63, 119) and two in HbD (αD2, 47). Four derived substitutions in Andean goose include three in HbA (αA8, 77; βA86) and two in HbD (αD9; βA86). Considering both highland species, four substitutions (Ala-αA8, Ala-αA12, Ser-αA18, Leu-αD9) were located at adjacent positions on the A helix (or AB corner) of the α-chains, three others (Thr-αA77, Ser-βA86, Ser-αD2) were in close proximity to inositolpentaphosphate (IP5) binding sites, and Ala-αA119 occurred at an αβ intersubunit contact. Ser-βA55, which is involved in the same αβ intersubunit contact and was previously shown to increase Hb–O2 affinity, is not unique to Andean goose, but is a synapomorphy of the South American sheldgeese, a clade of predominantly lowland waterfowl. Our findings illustrate the importance of understanding phylogenetic relationships and polarity of character-state changes when making inferences about adaptive evolution.