Phenotyping Thermal Responses of Yeasts and Yeast-like Microorganisms at the Individual and Population Levels: Proof-of-Concept, Development and Application of an Experimental Framework to a Plant Pathogen

Abstract

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Patterns of thermal adaptation in a worldwide plant pathogen: local diversity and plasticity reveal two-tier dynamics

“…By characterizing the TPCs of Z. tritici strains collected over different spatiotemporal scales, we were able to develop a fine description of the extensive interindividual variation in thermal plasticity: maximum performance (Pmax), thermal optimum (Topt), and thermal performance breadth (TPB80). This detailed characterisation was made possible by the large range of temperatures and the high resolution of this experimental study (12 temperatures, ranging from 6.5 to 33.5 °C), the extensive sampling strategy (350 strains from 12 populations collected within the Euro-Mediterranean region) and the use of a dedicated and previously validated experimental framework based on turbidity measurements (Boixel et al, 2019). It is important to bear in mind that these turbidity measurements may not reflect the sole growth multiplication rate via yeast-like budding but more precisely quantify the total fungal biomass that could be affected by the pleomorphic nature of some strains of Z. tritici under some environmental stimuli (e.g.…”

“…partial transition to pseudohyphae or induction of a few chlamydospores, a very recently highlighted form at high temperatures; Francisco et al, 2019). Precautions were taken to work under culture conditions limiting morphological transitions in the four-day time window of the experiments: very few hyphae were observed at 96 h when validating the method (see ESM1-3 in Boixel et al, 2019), and no chlamydospore has so far been described in the literature before 96 h in liquid medium (Francisco et al, 2019). Furthermore, effects of these potential -although undetected here -morphological transitions on the estimation of thermal parameters can be neglected as it results from a double integration based on kinetics, which mathematically limits the impact of the latest time point measurements that are the most likely to be affected by morphological transitions.…”

“…5°C (6.5, 9.5, 11.5, 14.5, 17.5, 20.0, 22.5, 24.5, 26.5, 28.5, 30.5 and 33.5°C). The growth rate μ of each strain at each temperature was calculated according to the standardised specific experimental framework developed by Boixel et al (2019) which have been validated to be representative of in planta responses with respect to discrimination between cold-and warm-adapted individuals. TPCs describing in vitro growth rate as a function of temperature were established by fitting a quadratic function to the temperature-growth rate (or performance P) estimates for each strain:…”

“…The geographic variation of TPCs provides evidence of thermal adaptation to local conditions in Z. tritici, with: (i) an increase in the mean thermal optimum of a given population with the annual mean temperature of its location of origin; (ii) a particularly marked adaptation to high temperatures of the population sampled in Israel, consistent with the results obtained for another Israeli population investigated by Zhan & McDonald (2011); (iii) differences in the level of specialisation of individuals between populations with higher proportions of specialist individuals in the Cfb (climatic zone with lower annual temperature range) than in the Dfb (climatic zone with higher annual temperature range) populations, consistent with the assumption that thermal generalists are favoured in more variable environments. By contrast, over a smaller geographic scale (France), using the same experimental method, we detected: (i) high levels of local diversity but no structuring of thermal responses between spring populations sampled along a gradient of increasing mean annual temperature; (ii) a marked difference between post-winter populations sampled along a gradient of increasing annual temperature range: presence of thermal generalists in the population exposed to the largest annual temperature range (19.9°C) vs. the complete absence of such generalists in the population exposed to the smallest annual temperature range (11.9°C; Boixel et al, 2019). The phenotypic differentiation of thermal responses at the population level probably results from local shortterm selection of the fittest strains over the course of an annual epidemic.…”

“…This pathogen, the causal agent of one of the most economically important wheat diseases (Septoria tritici blotch or STB; Dean et al, 2012;Fones & Gurr, 2015), is now considered a model species for both basic and applied research. Besides its agronomic relevance, we chose to study Z. tritici as its aggressiveness traits are known to be temperaturesensitive (Shaw, 1990;Lovell et al, 2004) and to display interindividual variation (Bernard et al, 2013;Boixel et al, 2019). Furthermore, its populations present signatures of adaptation to a wide range of contrasted environments over space (wheat-growing areas worldwide; Zhan & McDonald, 2011) and time (covering seasonal changes e.g.…”