Effects of early life environment on post-metamorphic immune function in wood frogs

Michael Thomas Porzio

The developmental environment of an individual can have profound impacts on adult survival and reproduction. This study investigated the effects of adverse environmental conditions experienced early in life on immune function at a subsequent life history stage; a physiological system inherent to survival. Amphibians respond to their surroundings by exhibiting environmentally-mediated plasticity in a number of larval and post-metamorphic morphological, behavioral, and physiological traits. We therefore examined the effects of varying tadpole conditions on post-metamorphic immune function in wood frog juveniles. In our first experiment, we reared tadpoles in mesoscosms in either a constant or reduced water levels, which simulated pond desiccation. Animals exposed to the drying event were expected to metamorphose faster and with poorer body condition compared to animals at constant water levels, delaying adult immune system maturation. We predicted that challenged animals would have increased disease susceptibility when exposed to Batrachocytrium dendrobatidis and weakened immune responsiveness via reduced skin anti-microbial peptide (AMP) production. As expected, animals in reduced water levels metamorphosed 3-4 days faster and secreted 25% fewer AMPs. Infection severity and mortality rates were similar between treatments, however average time to death from infection occurred 6 days faster for animals experiencing water v reduction. In a second experiment we investigated the effects of a gradient of water quality on similar measures of post-metamorphic immune system function in wild populations of wood frogs collected from 3 regions of southcentral Alaska: Anchorage, Kenai, and Talkeetna. Water quality was assessed from each region using measures of pH, alkalinity, conductivity, and contamination levels (metals, biphenyls, pesticides) and tadpoles were collected before reaching metamorphic climax. We hypothesized that immune function would decline as a function of water quality and predicted that animals in poorer quality starting environments would show increased disease susceptibility and decreased skin peptide loads. Animals across all populations exhibited comparable survival rates from infections of similar severity. Nevertheless, animals from the highest water quality produced an average of 78% larger peptide loads. Impaired immune system performance correlated with reduced water levels and declining water quality, suggesting the deleterious impacts of poor early environmental quality through delayed postmetamorphic immune expansion.

Effects of early life environment on post-metamorphic immune function in wood frogs

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