Sammendrag
Piscidins are cationic antimicrobial peptides that play a major role in the innate immune response of bony fishes. To date, they have been found in several teleost species from the superorder Acanthopterygii and constitute a family of small peptides with an amphipathic _a-helicalstructure. We have recently identified a piscidin gene in Atlantic cod (Gadus morhua), thus showing for the first time that piscidins are not restricted to evolutionarily modern teleosts. Nucleotide diversity was higher in the regions of the piscidin gene that code for the mature peptide and its pro-domain. To learn more about the mechanisms underlying this sequence diversity we have performed a maximum likelihood-based codon substitution analyses on the piscidin genes found thus far in 10 teleost species. Comparison of various models with or without positive selection revealed that certain sites of the piscidin gene are indeed under positive Darwinian selection. The amino acid substitutions observed in positively selected sites within the mature peptide change its charge or hydrophobicity. These changes can alter the amphipathic structure of piscidin and therefore may have a profound effect on its function. The excess of nonsynonymous substitutions identified in piscidins is likely to be associated with adaption to new habitats or rapidly evolving pathogens.
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