Evolution of an extreme hemoglobin phenotype contributed to the sub-Arctic specialization of extinct Steller’s sea cows

Abstract

The extinct Steller’s sea cow (Hydrodamalis gigas; †1768) was a whale-sized marine mammal that manifested profound morphological adjustments to exploit the harsh coastal climate of the North Pacific. Yet despite first-hand accounts of their biology, little is known regarding the physiological specializations underlying their evolution to this environment. Here, the adult-expressed hemoglobin (Hb) of this species is shown to harbor a fixed amino acid replacement at an otherwise invariant position (β/δ82Lys→Asn) that alters multiple aspects of Hb function. First, our functional characterization of recombinant sirenian Hb proteins demonstrate that the Hb–O₂ affinity of this sub-Arctic species was less affected by temperature than those of living (sub)tropical sea cows. This phenotype presumably safeguarded O₂ delivery to cool peripheral tissues and largely arises from a reduced intrinsic temperature sensitivity of the H. gigas protein owing to the β/δ82Lys→Asn substitution. Additional experiments on H. gigas β/δ82Asn→Lys mutant Hb further reveal this exchange renders Steller’s sea cow Hb unresponsive to the potent intraerythrocytic allosteric effector 2,3-diphosphoglycerate, a radical modification that is the first documented example of this phenotype among mammals. Notably, β/δ82Lys→Asn moreover underlies the secondary evolution of a reduced blood–O₂ affinity phenotype that would have promoted heightened tissue and maternal/fetal O₂ delivery. Finally, this replacement is shown to increase protein solubility, which is consistent with an increased O₂ carrying capacity within both the adult and pre-natal circulations and may have further contributed to meeting the elevated metabolic (thermoregulatory) requirements and fetal growth rates associated with this species cold adaptation.