Seasonal Changes Drive Short-Term Selection for Fitness Traits in the Wheat Pathogen Zymoseptoria tritici

In a cross-infection experiment, we investigated how seasonal changes can affect adaptation patterns in a Zymoseptoria tritici population. The fitness of isolates sampled on wheat leaves at the beginning and at the end of a field epidemic was assessed under environmental conditions (temperature and host stage) to which the local pathogen population was successively exposed. Isolates of the final population were more aggressive, and showed greater sporulation intensity under winter conditions and a shorter latency period (earlier sporulation) under spring conditions, than isolates of the initial population. These differences, complemented by lower between-genotype variability in the final population, exhibited an adaptation pattern with three striking features: (i) the pathogen responded synchronously to temperature and host stage conditions; (ii) the adaptation concerned two key fitness traits; (iii) adaptation to one trait (greater sporulation intensity) was expressed under winter conditions while, subsequently, adaptation to the other trait (shorter latency period) was expressed under spring conditions. This can be interpreted as the result of short-term selection, driven by abiotic and biotic factors. This case study cannot yet be generalized but suggests that seasonality may play an important role in shaping the variability of fitness traits. These results further raise the question of possible counterselection during the interepidemic period. While we did not find any trade-off between clonal multiplication on leaves during the epidemic period and clonal spore production on debris, we suggest that final populations could be counterselected by an Allee effect, mitigating the potential impact of seasonal selection on long-term dynamics.