EVOLUTION OF ENHANCED OXYGEN UNLOADING IN TELEOST FISHES

My doctoral work has focussed on expanding our understanding of the unique oxygen unloading system found in teleost fishes. Teleosts make up around half of all vertebrates found on Earth today and have invaded almost every aquatic habitat, including waters low in oxygen, high in carbon dioxide, and with varying salinity. They owe their success, at least in part to their unique oxygen unloading system. This system is comprise of three major components; highly pH sensitive hemoglobin, RBC intracellular pH (pHi) regulation, and a heterogeneous distribution of the plasma accessible isoform of carbonic anhydrase. Highly pH sensitive hemoglobins are advantageous as they allow oxygen uptake and unloading to be strictly controlled through the circulation; at the gills pH is relatively high which promotes oxygen uptake and at the respiring tissues pH is lower which promotes oxygen unloading where it is required. The second component, RBC pHi protection is important to protect oxygen uptake by such pH sensitive hemoglobin. Under stressful conditions, RBC pH can drop throughout the circulation and compromise oxygen uptake at the gills. By protecting RBC pHi, this danger is averted and teleosts can continue to uptake environmental oxygen even during stressful situations, e.g. exhaustive exercise resulting from predator avoidance. The third component of this system is a heterogeneous distribution of plasma accessible carbonic anhydrase (paCA). This enzyme is responsible for the reversible hydration of carbon dioxide and as such is essential for gas exchange. In its absence, e.g. at the gills, the RBC pHi protection system functions properly. However, where it is present, it can short circuit this pHi protection (causing pH to drop) and cause large amounts of oxygen to be unloaded (low pH reduces the affinity for oxygen of highly pH sensitive hemoglobin). 

The focus of my doctoral work was to improve our collective understanding of the less studied aspects of this system. I firstly confirmed the assumption that paCA is absent from all four gill arches in the model teleost, the rainbow trout. Secondly, I investigated the presence of this system in developing fishes and found it to be absent in pre-hatch rainbow trout. Finally, I investigated the evolution of this system and found that gill paCA was lost much earlier than originally expected.

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Together this work adds critical basic knowledge to our understanding of a respiratory system that is likely present in half of all vertebrates.