Paleophysiology of oxygen delivery in the extinct Steller’s sea cow, Hydrodamalis gigas

• Main Author: Signore, Anthony
• Other Authors: Campbell, Kevin (Biological Sciences)
• Published: 2017
• Subjects: Physiology; Molecular Biology; Ancient DNA; Evolution; Hemoglobin
• Online Access: http://hdl.handle.net/1993/32108

The order Sirenia is one of only two mammalian groups to have completely forgone terrestrial life. While extant sirenians are confined to the tropical waters, the recently extinct Steller’s sea cow (Hydrodamalis gigas) evolved to exploit the frigid waters of the North Pacific Ocean. As limits on O2 availability during submergence and decreased tissue temperature impose severe constraints on oxygen delivery, the oxygen binding (globin) proteins of sirenians are expected to have evolved under strong selection pressures. My comparative molecular analyses indicate that selection pressures on two globin genes (HBA and HBZ-T1) increased in transitional sirenians. As these genes encode the α-chains of all Hb isoforms throughout sirenian development, the resulting functional consequences to adult sirenian Hbs were tested using recombinant Hbs of Steller’s sea cow, the dugong (Dugong dugon), their last common ancestor, and the Florida manatee (Trichechis manatus latirostris). These tests reveal that high affinity Hbs—exceeding those of other mammals examined to date—arose early in sirenian evolution, presumably to maximize O2 extraction from the lungs and limit premature O2 offloading during submergence. Moreover, I demonstrate that the Hb–O2 affinity of the extinct sub-Arctic Steller’s sea cow is less affected by temperature than other sirenians, safeguarding O2 delivery to cool peripheral tissues. However, while this phenotype has primarily been attributed to the binding of additional allosteric effectors to the Hb moiety, Steller’s sea cow Hb binds relatively few of these ligands. Instead, my results suggest the thermodynamic properties of discrete allosteric effector sites are altered by epistatic interactions, a phenomenon that appears to be a critical component to cold adaptation in mammalian Hbs. As the HBA and HBZ-T1 loci also encode sirenian prenatal Hbs, the functional properties these proteins were tested to reveal their O2 affinity increased in parallel to maternal Hb. Notably, Steller’s sea cow HbF has the highest reported O2 affinity of any mammalian Hb tested to date. As the HBA gene encodes the -subunit of both the prenatally expressed HbF and adult expressed HbA proteins, the molecular remodeling of this locus may have concurrently increased the affinity of each protein. === February 2017