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 (O(2)) 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-O(2) 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-alpha(A)119 and Ser-beta(A)55) 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 alpha(A)-,
alpha(D)-, and beta(A)-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 (alpha(A)12, 18, 63, 119) and two in HbD (alpha(D)2, 47). Four derived substitutions
in Andean goose include three in HbA (alpha(A)8, 77; beta(A)86) and two in HbD (alpha(D)9;
beta(A)86). Considering both highland species, four substitutions (Ala-alpha(A)8,
Ala-alpha(A)12, Ser-alpha(A)18, Leu-alpha(D)9) were located at adjacent positions
on the A helix (or AB corner) of the alpha-chains, three others (Thr-alpha(A)77, Ser-beta(A)86,
Ser-alpha(D)2) were in close proximity to inositolpentaphosphate (IP(5)) binding sites,
and Ala-alpha(A)119 occurred at an alphabeta intersubunit contact. Ser-beta(A)55,
which is involved in the same alphabeta intersubunit contact and was previously shown
to increase Hb-O(2) 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.