Adaptive evolution of endothelial nitric oxide synthase (NOS3) may reduce cetacean susceptibility to decompression sickness.

Cetaceans tolerate repeated diving bouts. While the extent to which cetaceans experience decompression sickness remains debated, this group of fully aquatic mammals must have evolved a tolerance to damaging nitrogen (N2) gas bubbles that can form during overly rapid ascents or after prolonged dives. Here, we present the first in-depth molecular evolutionary analysis of the nitric oxide synthase gene family across cetaceans and identify cetacean-specific amino acid substitutions in NOS3 (encoding endothelial nitric oxide synthase) that likely arose in a stem cetacean ancestor, coincident with the transition to obligate aquatic life. Using in vitro assays, we demonstrate that cetacean endothelial nitric oxide synthase exhibits enhanced enzymatic activity and function, potentially mediated by strengthened binding to its molecular chaperone Hsp90. Our findings provide a molecular foundation for an evolved, more resilient vascular system in cetaceans, offering new insights into their adaptations to a hyperbaric environment.