Local adaptation to temperature in populations and clonal lineages of the Irish potato famine pathogen Phytophthora infestans

Abstract

Environmental factors such as temperature strongly impact microbial communities. In the current context of global warming, it is therefore crucial to understand the effects of these factors on human, animal, or plant pathogens. Here, we used a common-garden experiment to analyze the thermal responses of three life-history traits (latent period, lesion growth, spore number) in isolates of the potato late blight pathogen Phytophthora infestans from different climatic zones. We also used a fitness index (FI) aggregating these traits into a single parameter. The experiments revealed patterns of local adaptation to temperature for several traits and for the FI, both between populations and within clonal lineages. Local adaptation to temperature could result from selection for increased survival between epidemics, when isolates are exposed to more extreme climatic conditions than during epidemics. We also showed different thermal responses among two clonal lineages sympatric in western Europe, with lower performances of lineage 13_A2 compared to 6_A1, especially at low temperatures. These data therefore stress the importance of thermal adaptation in a widespread, invasive pathogen, where adaptation is usually considered almost exclusively with respect to host plants. This must now be taken into account to explain, and possibly predict, the global distribution of specific lineages and their epidemic potential.

Keywords: Climate change; epidemic; late blight; life‐history traits; phenotypic plasticity; plant pathogen; temperature adaptation.

Figure 1

Sampling characteristics with the location of source Phytophthora infestans populations (A) and clonal lineage distribution among the populations (B). Populations are coded as follows: NE, northern Europe; WE, western Europe; MB, Mediterranean Basin. The seven points on the map represent the main location of potato fields sampled in each country. Unknown genotypes are represented in white in the graphics.

Figure 2

Temperature responses of 42 Phytophthora infestans isolates sampled in three geographic areas for the latent period (A), the lesion growth rate (B), the sporangia production (C), and the sporangia size (D) (mean ± SE). Significant differences between the geographical areas at a given temperature, as revealed by Wilcoxon rank‐sum tests or lsmeans: *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 3

Temperature responses of 14 Phytophthora infestans isolates belonging to the 13_A2 clonal lineage sampled in two geographical areas for latent period (A), lesion growth rate (B), sporangia production (C), and sporangia size (D). SE were omitted for clarity. Significant differences between the geographical areas at a given temperature, as revealed by Wilcoxon rank‐sum tests or lsmeans: *P < 0.05, **P < 0.01.

Figure 4

Temperature responses of 17 west European Phytophthora infestans isolates belonging to two clonal lineages (6_A1 and 13_A2) for latent period (A), lesion growth rate (B), sporangia production (C), and sporangia size (D). SE were omitted for clarity. Significant differences between the clonal lineages at a given temperature, as revealed by Wilcoxon rank‐sum tests or lsmeans: *P < 0.05, **P < 0.01, ***P < 0.001.