Un défi important en Biologie est de comprendre comment les organismes s’adaptent à des environnements fluctuants et de déterminer l’importance relative de la plasticité phénotypique et des mutations dans cette adaptation. Nous avons examiné la réponse d’un pathogène du blé (Zymoseptoria tritici) aux fluctuations de température grâce à des approches de transcriptomique, de phénotypage (fitness relative et pathogénie) et de génomique. Pour cela, une évolution expérimentale a été menée in vitro à partir de deux clones ayant évolué dans trois régimes thermiques : à 17°C, à 23°C et en température fluctuante. Le niveau d’expression de 11% du génome a évolué de manière distincte entre les deux génotypes fondateurs en conditions de fluctuations. Nous avons également observé une plus forte densité de gènes différentiellement exprimés dans des régions connues pour être riches en éléments transposables. L’évolution en conditions fluctuantes a favorisé la robustesse du transcriptome. La fitness relative estimée dans les conditions d’évolution a augmenté uniquement pour les lignées fluctuantes issues d’un des deux génotypes fondateurs. La différence de croissance entre les deux ancêtres en conditions de fluctuations et leur différent niveau de plasticité d’expression pourraient expliquer ces résultats différents. Enfin, nous avons observé : i). des pertes de pathogénie in planta suite à l’évolution à 17°C et en fluctuations, ii). aucune perte de chromosomes accessoires, iii). de nombreuses mutations dans le génome, dont des mutations codantes dans des effecteurs. Ces travaux apportent de nombreux éléments de compréhension des mécanismes évolutifs et moléculaires sous-jacents à l’évolution de Z. tritici dans des environnements variables. / An important challenge in Biology is to understand how organisms adapt to fluctuating environments and to determine the relative significance of phenotypic plasticity and mutations in this adaptation. We examined the response of a wheat pathogen (Zymoseptoria tritici) to temperature fluctuations using transcriptomics, phenotyping (relative fitness and disease level) and genomics. With this goal, we conducted an in vitro experimental evolution from two Z. tritici clones that evolved in three thermal conditions: at 17°C, at 23°C and under temperature fluctuations. Expression level of 11% of the genome evolved in a different way between the two founder genotypes that evolved under fluctuating conditions. We also observed a higher density of differentially expressed genes in regions known to be enriched in transposable elements. Evolution under fluctuating selection promoted robustness of the transcriptome. The relative fitness estimated in the same conditions as for the experimental evolution did increase for fluctuating lineages only for one of the founder genotypes. The difference of growth between the two ancestors in fluctuating conditions and their distinct level of expression plasticity could explain these opposite results. Finally we observed: i). in planta pathogenicity losses for lineages evolved at 17°C or under fluctuations ii). no accessory chromosome loss, iii). many de novo mutations, including coding mutations in effector genes. This work contributes to shade light on the evolutionary and molecular mechanisms underlying the evolution of Z. tritici in variable environments.

Competition between plant individuals and how plants alter soil properties are key processes which drive changes in plant communities over time. Estimating the relative importance of these processes and how they affect plant growth in different ecological contexts and communities is an active area of research. Furthermore, interdependencies between the two processes have been suggested to occur in many cases, but research in this area is also lacking. In this study, soil conditioned by common yarrow (Achillea millefolium) was collected from field plots and was then used in a growth chamber competition experiment, which controlled for plant-soil feedbacks. Measured soil properties such as soil pH, soil nitrogen, and soil texture were primarily used as background data in the experiment. Field parameters such as light availability, plant density, and grass to forbs ratio were used to predict optimal A. millefolium habitat in relation to other vascular plant species. The results indicate that A. millefolium was a weaker competitor than cornflower (Centaurea cyanus), while a positive plant-soil feedback effect was observed by A. millefolium grown in field soil. Intraspecific competition had a strong negative effect on A. millefolium growth when grown in non-conditioned soil, but not when grown in A. millefolium conditioned soil. Finally, competition and plant-soil feedbacks appeared to be additively affecting A. millefolium growth, meaning the plant-soil feedback effect did not have a disproportionate effect on competitive outcomes, or vice versa. The findings of this study can be of interest to conservationists or farmers who wish to predict how plant communities respond to plant competition and plant-soil feedbacks as processes.