Origin and Mechanism of Evolution of Antifreeze Glycoproteins in Polar Fishes

Abstract

The frigid waters of the polar oceans delimit the cold extreme for marine life. This is particularly true in the case of the thermally isolated Antarctic Ocean which is perpetually near or at freezing (−1.9 °C) due to the thermal barrier imposed by the Antarctic Circumpolar Current [1]. The most fundamental survival challenge faced by teleost fishes in these waters is a physical one — the threat of being frozen. The body fluids of marine teleosts including polar species are hyposmotic to sea water, 300–600 mOsM [2,3] versus 1000 mOsM, and thus have a higher colligative freezing point than the latter, −0.56 °C to −1.1 °C versus −1.86 °C. By these simple physical considerations alone, freezing death would be unavoidable especially in the presence of ice. Unlike some reptiles and amphibians, fish cannot survive even partial freezing of their body fluids. A number of polar and subpolar fishes had overcome this environmental challenge with a biological solution — they evolved ice-binding antifreeze proteins which enabled them to successfully colonize icy habitats that were otherwise out of their reach. The impact of the evolution of these unique antifreezing proteins on organismal and ecological success is manifested most strikingly in the case of the Antarctic notothenioid fishes — a single teleost suborder (Notothenioidei) that has come to dominate today’s Antarctic fish fauna in terms of species number (∼50%) and biomass (≥90%) [2,4–6].