Variable susceptibility to an emerging infectious disease, chytridiomycosis, in anurans

Chytridiomycosis is an emerging infectious amphibian disease, caused by the fungus Batrachochytrium dendrobatidis (Bd), and has caused numerous amphibian declines around the globe. Chytridiomycosis can be lethal in many amphibian species but not in others, leading to three different responses to exposure, 1) the amphibian becomes infected with Bd and dies, 2) the amphibian becomes infected with Bd and survives and 3) the amphibian does not become infected even though it occurs in a habitat where Bd exists. My project aimed to increase our understanding of the causes of these interspecific differences. I investigated the hypotheses that they could be caused by innate immune defences (antimicrobial peptides) against Bd, innate or adaptive responses of individuals through microenvironment selection, or behavioural avoidance of infective water. I found evidence for all three mechanisms. In vitro, antimicrobial peptides (AMPs) of Litoria genimaculata (vulnerable to infection with Bd with highland population declines followed by recovery) and L. rheocola (vulnerable to infection with Bd with severe declines at higher elevations with little or slow recolonisation) can completely inhibit Bd growth. I also found large seasonal variation in antimicrobial peptide defences in both species. This may result from physiological shifts driven by temperature, or may reflect adaptation to seasonal fluctuations in the risk of infection. The proportion of L. genimaculata from high elevation populations, which have experienced strong viability selection pressure imposed by chytridiomycosis outbreaks, that produced AMPs that effectively inhibited Bd in vitro, was significantly higher than in low elevation populations, which have been protected from chytridiomycosis by environmental factors. There was also evidence that high elevation populations produced AMPs that differed slightly in chemical composition from those produced by low elevation populations. However, when individuals of either frog species produced AMPs that inhibited the growth of Bd, the effectiveness of AMPs from high and low elevation populations did not differ significantly. This suggests that any responses to selection may have occurred through an increase in the proportion of individuals producing effective AMPs, with no change in the types of AMPs produced. Antimicrobial peptide defences did not differ significantly between high and low elevation population of L. rheocola, suggesting that this species may have recolonised upland areas. On the other hand, L. rheocola had more effective antimicrobial peptide defences against Bd than L. genimaculata and may have experienced stronger selection pressure after the appearance of chytridiomycosis. Thermal microenvironments selected in the laboratory corresponded to those expected from decline patterns observed in the wild. Litoria caerulea (vulnerable to infection with Bd but no population declines due to chytridiomycosis have been detected) selected warm and hot environments significantly more often than L. genimaculata. Additionally, although not significant, there was a trend that intensity of Bd infection in all three species was more likely to decrease over time in individuals that had a choice of hydric and thermal microenvironments than in frogs that were housed under standard environmental conditions. There was also evidence of disease avoidance behaviour; some L. caerulea and L. genimaculata chose uncontaminated water significantly more often than water that contained Bd zoospores. None of the frog species were able to completely avoid water containing Bd zoospores, possibly in part because their pond selection was also influenced by side fidelity. My study demonstrates the complexity of host-pathogen interactions and that multiple factors, including innate immune defence, microenvironment selection and disease avoidance behaviour, can influence the progress of chytridiomycosis and should be considered when establishing species specific management plans.