Abstract
Fishes living in icy seawater are usually protected from freezing by endogenous antifreeze proteins (AFPs) that bind to ice crystals and stop them from growing. The scattered distribution of five highly diverse AFP types across phylogenetically disparate fish species is puzzling. The appearance of radically different AFPs in closely related species has been attributed to the rapid, independent evolution of these proteins in response to natural selection caused by sea level glaciations within the last 20 million years. In at least one instance the same type of simple repetitive AFP has independently originated in two distant species by convergent evolution. But, the isolated occurrence of three very similar type II AFPs in three distantly related species (herring, smelt and sea raven) cannot be explained by this mechanism. These globular, lectin-like AFPs have a unique disulfide-bonding pattern, and share up to 85% identity in their amino acid sequences, with regions of even higher identity in their genes. A thorough search of current databases failed to find a homolog in any other species with greater than 40% amino acid sequence identity. Consistent with this result, genomic Southern blots showed the lectin-like AFP gene was absent from all other fish species tested. The remarkable conservation of both intron and exon sequences, the lack of correlation between evolutionary distance and mutation rate, and the pattern of silent vs non-silent codon changes make it unlikely that the gene for this AFP pre-existed but was lost from most branches of the teleost radiation. We propose instead that lateral gene transfer has resulted in the occurrence of the type II AFPs in herring, smelt and sea raven and allowed these species to survive in an otherwise lethal niche.
Highlights
Acquisition of a new gene/trait typically arises from gene duplication and divergence [1]
The ten-Cys lectin-like antifreeze proteins (AFPs) have no close matches in the database
The highest amino acid sequence identity of mature type II AFP with fish lectin-like proteins is less than 40%
Summary
Acquisition of a new gene/trait typically arises from gene duplication and divergence [1]. A classic example of this gradual process is the evolution of a set of pancreatic serine proteases, trypsin, chymotrypsin and elastase, from a common precursor [2,3]. These paralogs have the same three-dimensional fold and operate by the same enzymatic mechanism, but cleave proteins after different amino acids. The opportunities to shortcircuit this process and pass a gene between species by horizontal or lateral gene transfer (LGT) would seem extremely limited, and are largely restricted to prokaryotes. LGT becomes evident where the acquisition of the transferred gene confers a selective advantage on the host, as for example in antibiotic resistance [4,5]. There is the opportunity to acquire a new gene type within one generation rather than by gradual evolution
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