Comparative Genomics of Hypoxia and Barotrauma Tolerance in Diving Mammals

Hypoxia and barotrauma (pressure) tolerance are two main challenges for marine mammal evolution. Marine mammals have evolved many changes in their anatomy and physiology to combat hypoxia and barotrauma risks they are exposed to with their diving lifestyle. Changes in structures across vascular, pulmonary, musculoskeletal, and several other systems work together to prevent and delay injuries that are typically fatal to land mammals. Genomics studies on hypoxia tolerance in marine mammals have focused so far on a limited number of species or consider a limited number of genes. At the same time, genomic studies of barotrauma tolerance are almost absent due to limited physiological understanding of the relevant marine mammal phenotypes. This comparative genomic analysis explored the possibility of convergent evolution of members of pinnipeds, cetaceans and sirenians based on their diving patterns. Shared genes were extracted from complete genomes of 25 marine mammal species and used to infer a phylogenetic tree. I then assessed the evidence for positive selection between the deeper-diving members and their shallow-diving counterparts. Evidence of selection was detected in 315 genes in deeper-diving species, and these genes were significantly enriched for functions relating to hematopoietic (blood cell production) and immunological pathways. This result agrees with existing physiological studies of hypoxia and barotrauma tolerance. It supports existing working theories, such as preventing embolism caused by free-floating fibrin fragments in blood vessels and the intense, prolonged onset of the inflammatory response during dives and surfacing.