Yang, C.*; Cheng, C.-H.C.: Antifreeze Glycoprotein Genes in Arctic Cod: Gene Dosage, Structure, Organization and Evolution
Antarctic notothenioids and northern cods are phylogenetically distant, but have convergently evolved near-identical antifreeze glycoproteins (AFGPs) to survive in their respective freezing marine environment. We have previously shown that Notothenioid AFGP gene evolved from a pancreatic trypsinogen-like protease gene, and extant AFGPs are encoded by a large family of polyprotein genes. The evolutionary ancestry and the organization of the northern cod AFGP genes remain to be determined. Our current studies on the AFGP gene family of the Arctic cod, Boreogadus saida, show that its AFGPs are also encoded by polyprotein genes, i.e. each gene encodes a polyprotein precursor of multiple AFGP molecules connected in a series by conserved cleavable spacers (Arg). Genomic library screening statistics and southern analyses of nine genomic clones together indicate an AFGP gene family of as many as 128 genes. Many of these genes are closely linked in the genome, some of which are tandemly repeated. This high coding capacity commensurates with the high AFGP protein level necessary for survival in the very northerly geographic ranges of the Arctic cod which includes the Arctic polar basin. The evolutionary origin of the cod AFGP gene remains elusive. We have isolated two clones from a partial library of a non-AFGP bearing freshwater cod, Lota lota, which shows sequence similarity to the upstream and downstream sequence respectively of Arctic cod AFGP gene. Isolating and characterizing longer sequences of these two clones may eventually shed light on the evolutionary mechanism that gives rise to the first cod AFGP gene.