While the latitudinal diversity gradient is one of the rare patterns that might be considered general and widely accepted in ecology, the latitudinal specialisation gradient remains controversial. Ecological specialisation varies greatly across the tree of life, with clades that depend on a host organism for survival being among the most specialised. Understanding the processes that produce the global pattern of increasing host specialisation with decreasing latitude would be a breakthrough in the field of evolutionary ecology. This project will investigate the causes and consequences of higher host specialisation in the tropics, focusing on fundamental repertoires (expressed and non-expressed host use abilities). First, we will test if tropical species are genetically more specialised to their hosts by identifying host-associated gene modules and reconstructing their evolution across tropical and temperate species in the tribe Nymphalini. Then, we will test if higher specialisation is an artifact of data scarcity in the tropics. For that, we will develop an efficient approach for data collection based on interaction prediction, and produce a comprehensive dataset of fundamental and realised host repertoires for tropical Melitaeini butterflies. Then, including the entire Nymphalidae family, we will test if evolution of species interactions in the tropics favours specialisation. We will apply a combination of phylogenetic and network analyses to existing global datasets (augmented with all data produced in this project) to unravel how fundamental host repertoires evolve and whether current theory is able to explain tropical interactions equally well as the better-studied temperate interactions. Finally, we will quantify the consequences of host specialisation in a changing world in terms of risk of coextinction. The frameworks developed here can be easily expanded to other symbiont-host systems, increasing the significance of this research far beyond butterflies.