Abstract
Antimicrobial peptides (AMPs) are evolutionarily ancient molecules that play an essential role in innate immunity across taxa from invertebrates to vertebrates. The evolution system of AMP system has not been well explained in the literature. In this study, we cloned and sequenced AMP transcriptomes of three frog species, namely Rana dybowskii, Rana amurensis, and Pelophylax nigromaculatus, which are partially sympatric in northeast Asia, but show different habitat preferences. We found that each species contained 7 to 14 families of AMPs and the diversity was higher in species with a large geographic range and greater habitat variation. All AMPs are phylogenetically related but not associated with the speciation process. Most AMP genes were under negative selection. We propose that the diversification and addition of novel functions and improvement of antimicrobial efficiency are facilitated by the expansion of family members and numbers. We also documented significant negative correlation of net charges and numbers of amino acid residues between the propiece and mature peptide segments. This supports the Net Charge Balance Hypothesis. We propose the Cut Point Sliding Hypothesis as a novel diversification mechanism to explain the correlation in lengths of the two segments.
Highlights
Amphibians live in humid or aquatic environments that support the growth of complex microbial communities, including pathogenic microbes
We examined three frog species of northeast Asia, viz. Amur brown frog (Rana amurensis), Dybowski’s frog (Rana dybowskii), and black-spotted frog (Pelophylax nigromaculatus)
A total of 2718 positive clones were sequenced from the three frog species, 915 for R. amurensis, 879 for R. dybowskii, and 924 for P. nigromaculatus
Summary
Amphibians live in humid or aquatic environments that support the growth of complex microbial communities, including pathogenic microbes. Unlike higher vertebrates whose bodies are well protected by a thick and hard keratin layer, this class of organisms has only one or two layers of mild keratinocyte cells on the body surface, making the skin highly permeable and vulnerable [1,2]. Such a weak physical barrier cannot provide guaranteed protection from pathogenic attacks. These are synthesized, matured, and stored in granular glands, and dissolved in the mucosal layer from which they attack microbes that invade the skin [5,6]. Amphibians have a powerful antiseptic barrier in the skin that is made up of antimicrobial peptides (AMPs), small cationic and amphipathic peptides highly efficient at destroying microbial pathogens [3,4].
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