What This project aims to understand how fungal colonisations of new host species has influenced the diversity of species interactions. Fungi interact both positively and negatively with plants and animals, and some fungi become highly specialized and completely dependent on their host for survival. However, host-shifts to new host species do occur, but how, when, and why such host shifts occur is not always straightforward to explain. Why In recent years, we have greatly advanced our understanding of how colonisation of new host species drive pathogen evolution and diversity, due to new insights from host-pathogen molecular mechanisms and population genomic techniques. However, what I call "wide colonisations" between hosts of the plant and animal kingdoms are not easily explained by current evolutionary theory. This is despite the fact that such wide colonisations often are key events in organismal evolution, because they lead to subsequent diversification. This project aims to understand such wide host-shifts, ultimately with the goal of helping us prevent the emergence of new fungal diseases in plants and animals. How This project will focus on a specific group of fungi, all from the same genus, that occur in Brazilian citrus plantations. Within these very closely related fungal species there are beneficial plant endophytes, pathogenic fungi that cause plant diseases and insect-pathogenic species. By combining data of the characteristics of "source" populations and populations following recent wide colonisation events, it will be possible to study recent natural wide host-shifts. SSR Preventing loss to fungal diseases in our food production systems and maintaining biodiversity is of great societal interest. Fungal host shifts is often the initial spark that initiate the emergence of new fungal diseases in agricultural crops. Natural populations of plants and animals may also suffer from fungal diseases that emerged following a host shift. Only by understanding the evolutionary and ecological mechanisms at play during pathogenic host shifts will we be able to predict and prevent unwanted host-shifts in the future.