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Adaptations of skin peptide defences and possible response to the amphibian chytrid fungus in populations of Australian green-eyed treefrogs, Litoria genimaculata

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Abstract Aim 

Rapidly evolving pathogens may exert diversifying selection on genes involved in host immune defence including those encoding antimicrobial peptides (AMPs). Amphibian skin peptides are one important defence against chytridiomycosis, an emerging infectious disease caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd). We examined the population-level variation in this innate immune defence to understand its relationship with disease dynamics. Location 

Queensland, Australia. Methods 

We examined skin peptide defences in five geographically distinct populations of Australian green-eyed treefrogs, Litoria genimaculata. Skin peptide samples were collected from 52 frogs from three upland populations that previously declined as chytridiomycosis emerged, but subsequently recovered, and from 34 frogs in two lowland populations that did not decline. Historical samples of skin peptides preceding Bd emergence were not available from any population. Results 

In general, lowland populations had more effective peptide defences than upland populations. Peptide profiles were similar among populations, although relative amounts of peptides expressed differed significantly among populations and were more variable in the uplands. Infected frogs in upland populations carried a significantly higher infection burden compared to lowland populations. The presence of effective AMPs in the skin of L.¬†genimaculata does not eliminate infection; however, more effective peptide defences may limit infection intensity and the progression of disease. Main conclusions 

The population bottleneck in upland populations caused by chytridiomycosis emergence did not appear to produce responses to selection for more effective peptide defences against chytridiomycosis compared to lowland populations of L.¬†genimaculata. This does not exclude the possibility that current peptide defences have adapted in response to disease emergence. A suggestive (P <0.10) interaction between infection status and population indicates that in lowland populations, infected individuals tend to be those with lower relative intensities of AMPs, whereas in the upland populations, infected and uninfected individuals are similar. Thus, both the AMPs and the environment may act to mediate resistance to Bd infection.
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Keywords: Antimicrobial peptide; Batrachochytrium dendrobatidis; MALDI mass spectrometry; chytridiomycosis; innate immunity

Document Type: Research Article

Affiliations: 1: School of Marine and Tropical Biology, James Cook University, Townsville, Qld 4811, Australia 2: Biostatistics Shared Resources, Vanderbilt University School of Medicine, 571 Preston Building, Nashville, TN 37232, USA 3: Departments of Microbiology and Immunology, Pediatrics, and Biological Sciences, A-5301 Medical Center North, Vanderbilt University Medical Center, Nashville, TN 37232, USA

Publication date: 2010-07-01

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