Abstract
BackgroundWhile there is evidence of both purifying and balancing selection in immune defense genes, large-scale genetic diversity in antimicrobial peptides (AMPs), an important part of the innate immune system released from dermal glands in the skin, has remained uninvestigated. Here we describe genetic diversity at three AMP loci (Temporin, Brevinin and Palustrin) in two ranid frogs (Rana arvalis and R. temporaria) along a 2000 km latitudinal gradient. We amplified and sequenced part of the Acidic Propiece domain and the hypervariable Mature Peptide domain (~ 150-200 bp) in the three genes using Illumina Miseq and expected to find decreased AMP genetic variation towards the northern distribution limit of the species similarly to studies on MHC genetic patterns.ResultsWe found multiple loci for each AMP and relatively high gene diversity, but no clear pattern of geographic genetic structure along the latitudinal gradient. We found evidence of trans-specific polymorphism in the two species, indicating a common evolutionary origin of the alleles. Temporin and Brevinin did not form monophyletic clades suggesting that they belong to the same gene family. By implementing codon evolution models we found evidence of strong positive selection acting on the Mature Peptide. We also found evidence of diversifying selection as indicated by divergent allele frequencies among populations and high Theta k values.ConclusionOur results suggest that AMPs are an important source of adaptive diversity, minimizing the chance of microorganisms developing resistance to individual peptides.
Highlights
While there is evidence of both purifying and balancing selection in immune defense genes, largescale genetic diversity in antimicrobial peptides (AMPs), an important part of the innate immune system released from dermal glands in the skin, has remained uninvestigated
While there is evidence of geographic variation in effectiveness of antimicrobial defenses in various organism groups [22, 25,26,27,28,29], populationlevel variation in peptide profiles [27, 29] and positive selection acting on the Mature Peptide domain [6, 7, 16], little attention has been directed towards understanding the evolutionary forces acting on the genes coding for AMPs [17, 22, 30, 31]
Adaptive evolution/ positive selection in AMPs multilocus gene family We examined patterns of nucleotide diversity (π) and average pairwise nucleotide differences (Theta k) within each of the 46 Mature Peptide and Acidic Propiece domains among four set of clades (Temporin-Brevinin) and 12 Mature Peptide and Acidic Propiece domains among two set of clades (Palustrin) (Table 2)
Summary
While there is evidence of both purifying and balancing selection in immune defense genes, largescale genetic diversity in antimicrobial peptides (AMPs), an important part of the innate immune system released from dermal glands in the skin, has remained uninvestigated. While there is evidence of geographic variation in effectiveness of antimicrobial defenses in various organism groups [22, 25,26,27,28,29], populationlevel variation in peptide profiles [27, 29] and positive selection acting on the Mature Peptide domain [6, 7, 16], little attention has been directed towards understanding the evolutionary forces acting on the genes coding for AMPs [17, 22, 30, 31]. This is especially the case for the role of selection acting upon sequence variation in AMP genes, and how this variation is distributed within populations as well as at larger geographical scales
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