Fish antifreeze proteins: physiology and evolutionary biology

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Abstract
Many marine teleosts have adapted to ice-laden seawater by evolving antifreeze proteins and glycoproteins. These proteins are synthesized in the liver for export to the blood where they circulate at levels of up to 20 mg/mL. There are at least four distinct antifreeze protein classes differing in carbohydrate content, amino acid composition, protein sequence, and secondary structure. In addition to antifreeze structural diversity, fish species differ considerably with respect to mechanisms controlling seasonal regulation of plasma antifreeze concentrations. Some species synthesize antifreeze proteins immediately before the onset of freezing conditions, some synthesize them in response to such conditions, whereas others possess high concentrations all year. Endogenous rhythms, water temperature, photoperiod, and pituitary hormones have all been implicated as regulators of plasma antifreeze protein levels. The structural diversity of antifreeze proteins and their occurrence in a wide range of fish species suggest that they evolved separately and recently during Cenozoic glaciation. Invariably, the genes coding for these antifreeze proteins are amplified, sometimes as long tandem arrays, suggesting intense selective pressure to produce large amounts of protein. The distribution of antifreeze gene types among fish species suggests that they could serve as important tools for studying phylogenetic relationships.

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