Evolution of Thermal Plasticity in Hymenoscyphus fraxineus During Ash Dieback Expansion in Europe

Abstract

The plasticity of adaptive traits may be critical for population persistence in heterogeneous environments. However, its evolution is rarely investigated in forest pathogens, potentially limiting the accuracy of epidemic risk predictions. Ash dieback is an emblematic example of a forest epidemic caused by an invasive fungal pathogen-Hymenoscyphus fraxineus, which has likely been introduced to Eastern Europe from East Asia. We investigated the plasticity and thermal niche evolution of H. fraxineus during its spread across Europe. We characterized the reaction norms of in vitro mycelial growth and viability of H. fraxineus isolates from five European populations sampled along a latitudinal gradient spanning from Lithuania to Italy. While all populations responded uniformly to temperature decrease, their responses to temperature increase diverged markedly. The growth of H. fraxineus isolates from the northernmost population (Lithuania) was most negatively affected by high temperatures, whereas the southernmost isolates (Italy) showed optimal growth at a higher temperature compared to the other populations. Additionally, the viability of Lithuanian isolates was significantly reduced by higher temperatures compared to that of the other populations. These findings suggest that both growth plasticity and thermal niche have evolved during the pathogen's expansion in Europe, with potentially important implications for predicting and managing future epidemic risks. We further discuss how evolutionary processes may have shaped these phenotypic differences.

Keywords: Chalara fraxinea; Hymenoscyphus fraxineus; Hymenoscyphus pseudoalbidus; ash dieback; invasive fungal pathogen; microbial evolution; plasticity evolution; thermal plasticity.

FIGURE 1

Mycelial growth of Hymenoscyphus fraxineus isolates in response to different temperature treatments. Each data point represents the mean fungal culture diameter of 15 isolates within a specific population. For each isolate at each temperature, the diameter value corresponds to the average of three replicates. (a) Results of two different experiments focusing on low temperatures (3°C and 6°C), each with 22°C as a control condition. For clarity, data from the two experiments are not aligned with this value. The 3°C experiment is represented by triangles and dashed lines. The 6°C experiment is represented by circles and solid lines. (b) Results of a single experiment focusing on high temperatures. Error bars indicate the 95% confidence interval of the mean. Colors and letters correspond to the origin of the isolates: Dark blue represents Lithuania (LT), blue represents Denmark (DK), purple represents Switzerland (CH), orange represents France (FR), and red represents Italy (IT).

FIGURE 2

Estimated intercepts (mm) and slopes (mm/°C) of growth reaction norms in five European Hymenoscyphus fraxineus populations exposed to different temperatures for two weeks. Panels (a) to (e) correspond to the different temperature treatments. The reaction norms were estimated based on individual diameter measurements at 22°C (control temperature) and at the corresponding treatment temperature. Each point represents the mean estimate of 15 fungal isolates within a specific population, with three replicates per isolate and per temperature. Error bars indicate the 95% confidence intervals of the estimates. Colors and letters correspond to the origin of the isolates: Dark blue represents Lithuania (LT), blue represents Denmark (DK), purple represents Switzerland (CH), orange represents France (FR), and red represents Italy (IT).

FIGURE 3

Viability of Hymenoscyphus fraxineus isolates following different temperature treatments in two separate experiments at lower (a) and higher (b) temperatures. The viability rate of each isolate ranges from 0 (no agar plugs resumed growth) to 1 (all agar plugs resumed growth). Each data point represents the mean viability rate of 15 fungal isolates within a specific population, with nine replicates per isolate and per temperature. Colors and letters correspond to the origin of the isolates: Dark blue represents Lithuania (LT), blue represents Denmark (DK), purple represents Switzerland (CH), orange represents France (FR), and red represents Italy (IT).

FIGURE 4

Estimated intercepts and slopes (unit/°C) of viability reaction norms in five European Hymenoscyphus fraxineus populations exposed to different temperatures for 21 and 24 days, respectively. Viability ranges from 0 (no agar plugs resumed growth) to 1 (all agar plugs resumed growth). Panels (a) to (d) correspond to the different temperature treatments. The reaction norms were estimated based on individual diameter measurements at 22°C (control temperature) and at the corresponding treatment temperature. Each data point represents the mean of the estimates of viability of 15 fungal isolates within a specific population, with nine replicates per isolate and per temperature. Error bars indicate the 95% confidence interval of the estimates. The colors and letters indicate the origin of the isolate: Dark blue for Lithuania (LT), blue for Denmark (DK), purple for Switzerland (CH), orange for France (FR), and red for Italy (IT).