Daily fluctuating temperatures decrease growth and reproduction rate of a lethal amphibian fungal pathogen in culture

Abstract

Background: Emerging infectious diseases (EIDs) are contributing to species die-offs worldwide. We can better understand EIDs by using ecological approaches to study pathogen biology. For example, pathogens are exposed to variable temperatures across daily, seasonal, and annual scales. Exposure to temperature fluctuations may reduce pathogen growth and reproduction, which could affect pathogen virulence, transmission, and environmental persistence with implications for disease. We examined the effect of a variable thermal environment on reproductive life history traits of the fungal pathogen Batrachochytrium dendrobatidis (Bd). Bd causes chytridiomycosis, an emerging infectious disease of amphibians. As a pathogen of ectothermic hosts, Bd can be exposed to large temperature fluctuations in nature. To determine the effect of fluctuating temperatures on Bd growth and reproduction, we collected temperature data from breeding pools of the Yosemite toad (Anaxyrus canorus), a federally threatened species that is susceptible to chytridiomycosis. We cultured Bd under a daily fluctuating temperature regime that simulated Yosemite toad breeding pool temperatures and measured Bd growth, reproduction, fecundity, and viability.

Results: We observed decreased Bd growth and reproduction in a diurnally fluctuating thermal environment as compared to cultures grown at constant temperatures within the optimal Bd thermal range. We also found that Bd exhibits temperature-induced trade-offs under constant low and constant high temperature conditions.

Conclusions: Our results provide novel insights on variable responses of Bd to dynamic thermal conditions and highlight the importance of incorporating realistic temperature fluctuations into investigations of pathogen ecology and EIDs.

Keywords: Batrachochytrium dendrobatidis; Disease ecology; Sierra Nevada; Temperature; Yosemite toad.

Fig. 1

Observed and experimental diurnal temperature fluctuations. a Water temperature over a 24-h period of 10 different breeding pools containing Yosemite toad tadpoles (grey lines; yellow line represents pool fluctuating within 27.5 and 7.5 °C). b Incubator temperature profiles over a 24-h period. Fluctuating temperature = black; constant temperature at daily thermal maximum (27.5 °C) = red; constant temperature at daily thermal minimum (7.5 °C) = blue; constant temperature at daily thermal mean (17.5 °C) = green. Bd thermal optimum (green shaded band) and Bd thermal tolerance (grey shaded band) shown for reference

Fig. 2

Daily fluctuating temperature reduces Bd growth, zoospore production, and zoosporangia viability but not culture fecundity compared to Bd grown at an optimal temperature. a Daily optical density measurements of Bd cultures suggest differences in maximum growth and logistic growth rate among cultures incubated at 17.5 °C (green), 27.5 °C (red), and fluctuating temperatures (black). OD readings of cultures grown at 7.5 °C (blue) did not follow a pattern of logistic growth. Points represent means (n = 16 treatment⁻¹ day⁻¹). b Zoospore production differed among Bd cultures grown at constant (7.5 °C, 17.5 °C, or 27.5 °C) and fluctuating temperatures. Cultures grown at 17.5 °C had the highest maximum zoospore production (ANOVA, P < 0.001). Cultures grown at 7.5 °C had similar maximum zoospore production as cultures grown under fluctuating temperatures (Tukey HSD, P = 0.99) but reached peak production later. Cultures grown at 27.5 °C did not produce zoospores. Points represent mean counts of motile zoospores (n = 6 treatment⁻¹ day⁻¹). c Fluctuating and optimal temperature cultures did not differ in fecundity. Culture fecundity was measured as the ratio of motile zoospores to total culture growth (OD) day⁻¹. Despite reduced total growth (OD), cultures grown at 7.5 °C had higher fecundity than cultures grown at 17.5 °C or fluctuating temperatures (ANOVA, P < 0.001). Cultures grown at 27.5 °C did not produce zoospores. Dashed vertical lines correspond to time of peak zoospore production in fluctuating (day 8), 17.5 °C (day 10), and 7.5 °C (day 14) cultures. Points represent means (n = 6 treatment⁻¹ day⁻¹). d Zoosporangia viability differed among Bd cultures grown at fluctuating and constant temperatures. Cultures grown under fluctuating temperatures had lower zoosporangia viability than cultures grown at 17.5 °C but higher viability than cultures grown at 7.5 °C or at 27.5 °C (Kruskal–Wallis, P < 0.001). Points represent means (n = 8 treatment⁻¹ day⁻¹). In all panels, error bars represent standard error and letters indicate significant differences