How Do Fish in the Southern Ocean Keep From Freezing?
Evolution of Macromolecular Antifreezes: The Key Evolutionary Adaptation of Antarctic Notothenioids
The body fluids of teleost fishes are ~35-40% as concentrated as seawater. Due to the extreme cold of the Antarctic marine ecosystem, marine fishes living at the high latitudes of the Antarctic are confronted by the threat of freezing for most or all of the year. Although several mechanisms have evolved in various taxonomic groups to resist the harmful effects of intracellular ice formation, Antarctic notothenioids rely on the production of the macromolecular antifreeze glycopeptides (AFGPs; DeVries & Cheng, 1992). The AFGPs inhibit the lethal formation of intracellular ice by preventing the growth of ice nuclei that are present in the fluid compartments of the body. In the Antarctic fishes, they apparently function by preventing the propagation of ice from external, ice-containing tissues (e.g., the gills and intestinal tract) into other fluid compartments. The novel evolution of the AFGP genes of Antarctic notothenioids from the the gene for a protease (a protein that breaks down other proteins to their constituent amino acids) (Hsiao et al., 1990; Chen et al., 1997a; Cheng, 1998; Cheng & Chen, 1999) and the convergent evolution of AFGP genes in Antarctic and Arctic fishes (Chen et al., 1997b) provide a paradigm for understanding the acquisition of new protein functionality from preexisting genetic stock.
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