Antifreeze glycoproteins of two unrelated polar fishes: Gene structure, organization and evolution

Antifreeze glycoproteins (AFGPs) are synthesized by two phyletically unrelated fish taxa, the Antarctic notothenioids and the northern cods. In this study, AFGPs of the Antarctic notothenioids are found to be encoded by a large gene family. Each gene contains multiple distinctly-sized AFGP coding sequences linked in tandem by a conserved tripeptide spacer, Leu-Ile/Asn-Phe, which post-translationally cleaved to yield the mature AFGPs. The AFGP gene of the Antarctic fishes is found to have originated from a trypsinogen gene through a combined process of partial gene recruitment and de novo minisatellite-like DNA amplification. The trypsinogen to AFGP transformation is estimated to occur about 5-14 million years ago, which coincides with the mid-Miocene cooling and freezing of the Antarctic Ocean. The primordial AFGP gene was then expanded to a large polyprotein gene family through at least 3 hierarchical levels of DNA duplication, which generated size heterogeneity, polyprotein gene structure and the large gene number. AFGP genes are also found to be highly expressed in pancreas and less in liver. The extent of the hierarchical levels of DNA duplication and the AFGP gene integrity correlate well with the severity of the environmental selective pressure as shown from the comparative study of the AFGP genes in the Antarctic and sub-Antarctic notothenioids.;The AFGPs of the northern cods are also encoded by a family of polyprotein genes in which multiple AFGP coding sequences linked in tandem by small cleavable spacers Args or Arg-Ala-Ala-Arg's. However, the northern cod AFGP genes show no sequence identity with trypsinogen gene, indicating trypsinogen gene is not the progenitor of this group of AFGP genes. The southern and northern AFGP genes also have different signal peptides, different intron-exon organizations, different spacer sequences thus are the processing of the polyprotein precursors, and drastically different codon patterns for the repetitive AFGP coding sequence. This molecular evidence in conjunction with paleontological and paleoclimatic data indicate the near-identical AFGPs of Antarctic notothenioids and northern cods evolved independently.