Variation in Thermal Performance of a Widespread Pathogen, the Amphibian Chytrid Fungus Batrachochytrium dendrobatidis

Rates of growth and reproduction of the pathogens that cause emerging infectious diseases can be affected by local environmental conditions; these conditions can thus influence the strength and nature of disease outbreaks. An understanding of these relationships is important for understanding disease ecology and developing mitigation strategies. Widespread emergence of the fungal disease chytridiomycosis has had devastating effects on amphibian populations. The causative pathogen, Batrachochytriumdendrobatidis (Bd), is sensitive to temperature, but its thermal tolerances are not well studied. We examined the thermal responses of three Bd isolates collected across a latitudinal gradient in eastern Australia. Temperature affected all aspects of Bd growth and reproduction that we measured, in ways that often differed among Bd isolates. Aspects of growth, reproduction, and their relationships to temperature that differed among isolates included upper thermal maxima for growth (26, 27, or 28°C, depending on the isolate), relationships between zoospore production and temperature, and zoospore activity and temperature. Two isolates decreased zoospore production as temperature increased, whereas the third isolate was less fecund overall, but did not show a strong response to temperature until reaching the upper limit of its thermal tolerance. Our results show differentiation in life-history traits among isolates within Australia, suggesting that the pathogen may exhibit local adaptation. An understanding of how environmental temperatures can limit pathogens by constraining fitness will enhance our ability to assess pathogen dynamics in the field, model pathogen spread, and conduct realistic experiments on host susceptibility and disease transmission.引发新发传染病的病原体，其生长与繁殖速率可受局地环境条件调控；此类环境条件进而能够影响疾病暴发的强度与特征。厘清此类关联，对于解析疾病生态学特征及制定疾病防控策略均具有重要意义。真菌性疾病壶菌病（chytridiomycosis）的大范围暴发，已对两栖动物种群造成毁灭性打击。其致病病原体为蛙壶菌（Batrachochytrium dendrobatidis, Bd），该病原体对温度敏感，但当前学界对其温度耐受特性的研究尚不充分。本研究以澳大利亚东部沿纬度梯度采集的3株Bd分离株为材料，探究了其温度响应特征。温度可对本研究测定的所有Bd生长与繁殖相关指标产生影响，且此类影响在不同Bd分离株间往往存在差异。不同分离株间存在差异的生长、繁殖相关指标及其与温度的关联，包括生长的最高温度阈值（因分离株不同分别为26、27或28℃）、游动孢子（zoospore）产生量与温度的关系，以及游动孢子活性与温度的关系。其中2株Bd分离株的游动孢子产生量随温度升高而降低，而第三株分离株整体繁殖力较弱，但仅在达到其温度耐受上限时才对温度表现出显著响应。本研究结果显示澳大利亚境内不同Bd分离株的生活史特征存在分化，提示该病原体可能存在局地适应性。厘清环境温度如何通过限制病原体的适合度来调控其种群动态，将有助于提升我们野外评估病原体动态、构建病原体传播模型，以及开展宿主易感性与疾病传播相关真实实验的能力。