Evolution of an extreme hemoglobin phenotype contributed to the sub-Arctic specialization of extinct Steller's sea cows
The extinct Stellers sea cow (Hydrodamalis gigas; {dagger}1768) was a whale-sized marine mammal that manifested profound morphological specializations to exploit the harsh coastal climate of the North Pacific. Yet despite first-hand accounts of their biology, little is known regarding the physiological adjustments underlying their evolution to this environment. Here, the adult-expressed hemoglobin (Hb; 2{beta}/{delta}2) of this sirenian is shown to harbor a fixed amino acid replacement at an otherwise invariant position ({beta}/{delta}82Lys[->]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. Additional experiments on H. gigas {beta}/{delta}82Asn[->]Lys mutant Hb further reveal this exchange renders Stellers 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, {beta}/{delta}82Lys[->]Asn moreover underlies the secondary evolution of a reduced blood-O2 affinity phenotype that would have promoted heightened tissue and maternal/fetal O2 delivery. This conclusion is bolstered by analyses of two Stellers sea cow prenatal Hb proteins (Hb Gower I; {zeta}2{varepsilon}2 and HbF; 2{gamma}2) that suggest an exclusive embryonic stage expression pattern, and reveal uncommon replacements in H. gigas HbF ({gamma}38Thr[->]Ile and {gamma}101Glu[->]Asp) that increased Hb-O2 affinity relative to dugong HbF. Finally, the {beta}/{delta}82Lys[->]Asn replacement of the adult/fetal protein is shown to increase protein solubility, which may have elevated red blood cell Hb content within both the adult and fetal circulations and contributed to meeting the elevated metabolic (thermoregulatory) requirements and fetal growth rates associated with this species cold adaptation.