Landscape and water characteristics correlate with immune defense trait differences across Blanchard’s Cricket frog (Acris blanchardi) populations

Background/Question/Methods

Amphibians are protected from pathogens by two key skin-associated immune defense traits: the microbial communities which inhabit their skin and the antimicrobial peptides (AMPs) produced within the skin.  It is unknown the extent to which the environment may influence these traits.  Specific aspects of the environment including water and land-use characteristics may alter these traits, potentially making amphibians more susceptible to disease.  The Blanchard’s Cricket Frog (Acris blanchardi) is a species suspected to be in decline due to a variety of anthropogenic environmental changes including habitat degradation, chemical contamination, and the introduction of disease. To determine the influence water and landscape characteristics have on A. blanchardi skin-associated immune defense traits, we conducted a field survey across the species’ declining range in Ohio and Michigan.  We surveyed the skin-associated microbial communities and AMPs of multiple individuals at each site.  We hypothesized that environmental variation may influence immune defense trait phenotypes across populations.  We predicted that pond site would explain differences in microbial community structure, AMP production, and AMP bioactivity.  Furthermore, we predicted that these trait differences would correlate to differences between sites in terms of water and landscape characteristics.    

Results/Conclusions

Utilizing an AICc model selection and model averaging approach, we found the skin associated microbial community structure was predicted by multiple environmental factors including the ratio of natural to managed land, water conductivity, water surface area, and latitude, as well as interactions between the ratio of natural to managed land and water surface area, frog sex and latitude, and finally frog sex and water surface area.  AMP production was influenced by the interaction between water surface area and conductivity. Host characteristics (AMP production and AMP bioactivity) were not found to influence skin-associated microbial communities; however, AMP bioactivity against Bd in culture was positively associated with AMP production.  The more AMPs produced, the faster Bd grew in culture, indicating that AMPs in this species are not bioactive against this pathogen and may serve as a nutrient source or may stimulate Bd zoospore production. Further research is needed to determine if environmental variations alter the bioactivity of A. blanchardi AMPS against other common environmental pathogens. This study demonstrates that environmental characteristics can influence these amphibian immune defense traits and calls for research into the relative effects of other, potentially less benign, anthropogenic environmental changes on immune defense traits.