Atlantic Cod Piscidin and Its Diversification through Positive Selection

Jorge M O Fernandes,Viswanath Kiron,Jareeporn Ruangsri,Art F Y Poon

doi:10.1371/journal.pone.0009501

Abstract

Piscidins constitute a family of cationic antimicrobial peptides that are thought to play an important role in the innate immune response of teleosts. On the one hand they show a remarkable diversity, which indicates that they are shaped by positive selection, but on the other hand they are ancient and have specific targets, suggesting that they are constrained by purifying selection. Until now piscidins had only been found in fish species from the superorder Acanthopterygii but we have recently identified a piscidin gene in Atlantic cod (Gadus morhua), thus showing that these antimicrobial peptides are not restricted to evolutionarily modern teleosts. Nucleotide diversity was much higher in the regions of the piscidin gene that code for the mature peptide and its pro domain than in the signal peptide. Maximum likelihood analyses with different evolution models revealed that the piscidin gene is under positive selection. Charge or hydrophobicity-changing amino acid substitutions observed in positively selected sites within the mature peptide influence its amphipathic structure and can have a marked effect on its function. This diversification might be associated with adaptation to new habitats or rapidly evolving pathogens.

Highlights

Innate immunity is considered to be important in teleosts, since their adaptive immune response has poor immunological memory and a short-lived secondary response [1]
We have identified piscidin in Atlantic cod (Gadus morhua), a more basal teleost of the Paracanthopterygii superorder, and examined nucleotide divergence in all piscidin genes reported to date and used likelihood methods with various models of evolution to investigate patterns of positive selection
Using specific primers based on the contig of these sequences, we have isolated a 278 bp cDNA that contained the full length coding sequences (CDS) of a piscidin orthologue in Atlantic cod (FJ917596).The putative mature cod piscidin is rich in isoleucine (27.3%) and histidine (22.7%) and it contains the two conserved histidines that define the piscidin group of antimicrobial peptides (Figure 1)

Summary

Introduction

Innate immunity is considered to be important in teleosts, since their adaptive immune response has poor immunological memory and a short-lived secondary response [1] It is limited by environmental constraints such as temperature [2]. AMPs are a group of small peptides with broadspectrum activity that show remarkable diversity in sequence, secondary structure and function. They are generally cationic and amphipathic, which allows them to interact with cell membranes and form pores by barrel-stave, carpet or toroidal pore mechanisms [4]. We have identified piscidin in Atlantic cod (Gadus morhua), a more basal teleost of the Paracanthopterygii superorder, and examined nucleotide divergence in all piscidin genes reported to date and used likelihood methods with various models of evolution to investigate patterns of positive selection

Materials and Methods

Results and Discussion

Method Model