Low Structural Variation in the Host-Defense Peptide Repertoire of the Dwarf Clawed Frog Hymenochirus boettgeri (Pipidae)

Severine Matthijs,Franky Bossuyt,Pierre Cornelis,Kim Roelants,Benoit Stijlemans,Lumeng Ye,Colin Dale

doi:10.1371/journal.pone.0086339

Severine Matthijs, Franky Bossuyt + Show 5 more

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https://doi.org/10.1371/journal.pone.0086339

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Abstract

The skin secretion of many amphibians contains peptides that are able to kill a broad range of microorganisms (antimicrobial peptides: AMPs) and potentially play a role in innate immune defense. Similar to the toxin arsenals of various animals, amphibian AMP repertoires typically show major structural variation, and previous studies have suggested that this may be the result of diversifying selection in adaptation to a diverse spectrum of pathogens. Here we report on transcriptome analyses that indicate a very different pattern in the dwarf clawed frog H. boettgeri. Our analyses reveal a diverse set of transcripts containing two to six tandem repeats, together encoding 14 distinct peptides. Five of these have recently been identified as AMPs, while three more are shown here to potently inhibit the growth of gram-negative bacteria, including multi-drug resistant strains of the medically important Pseudomonas aeruginosa. Although the number of predicted peptides is similar to the numbers of related AMPs in Xenopus and Silurana frog species, they show significantly lower structural variation. Selection analyses confirm that, in contrast to the AMPs of other amphibians, the H. boettgeri peptides did not evolve under diversifying selection. Instead, the low sequence variation among tandem repeats resulted from purifying selection, recent duplication and/or concerted gene evolution. Our study demonstrates that defense peptide repertoires of closely related taxa, after diverging from each other, may evolve under differential selective regimes, leading to contrasting patterns of structural diversity.

Highlights

In situations of acute stress, like injury or a predator attack, many amphibians produce a skin secretion containing a complex mixture of peptides
Basic Local Alignment Search Tool (BLAST) searches indicate that the corresponding protein resembles antimicrobial peptides (AMPs) precursor proteins of X. laevis and S. tropicalis (Figure 1B)
The protein contains two tandem repeats of 138– 147 bp. Both repeats correspond to the AMP-encoding region of most X. laevis and S. tropicalis AMP genes which spans part of their exon 2 and exon 3

Summary

Introduction

In situations of acute stress, like injury or a predator attack, many amphibians produce a skin secretion containing a complex mixture of peptides Some of these peptides have been identified as structural analogues of evolutionary conserved vertebrate hormones, and have been shown to bind the same hormone receptors in a range of vertebrates. Other peptides show potent cytolytic activity against a broad range of microorganisms, including bacteria, fungi and protozoan parasites Most of these antimicrobial peptides (AMPs) are small to medium-sized (,30 amino acids), have an alternated sequence of hydrophobic and polar/cationic residues, and are able to adopt an alpha-helical structure upon contact with cell membranes. Regardless of the size of the peptide repertoire, AMPs of a single species typically show major variation in AA sequence As a result, they are often classified in different peptide ‘‘families’’ upon sequence comparison [7,11,12]. High sequence variation characterizes the AMP repertoires of amphibians across a broad phylogenetic diversity, the universality of this pattern is unknown

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