PROJECT SUMMARY The evolution of pathogen traits such as virulence and transmission poses an increasingly formidable challenge to basic and applied biology. Virulence and transmission fundamentally shape the severity and spread of disease and the evolution of these traits frequently undermines strategies to mitigate disease (e.g., vaccines, drugs, diet). Predicting virulence evolution remains challenging, in large part, because one of the most important drivers of pathogen evolution, host defense, remains poorly understood. Multiple host defense mechanisms strongly determine pathogen production (within hosts) and thus transmission between hosts at the population-level. Hence, both within-host and between-host processes ultimately govern pathogen evolution. Yet, host defense mechanisms and their effects on transmission are also difficult to unravel. Host defenses are energetically costly, interfere with one another and with other aspects of host physiology (e.g., growth, reproduction), vary across host genotypes, and are sensitive to environmental conditions (e.g., resource availability). Better understanding of within-host processes and their effects on transmission will substantially improve our ability to link these different scales of biological organization to pathogen evolution. Recent theory suggests that such cross-scale links can provide key insight into pathogen evolution. To date, however, this theory has not been tested empirically. We propose to identify how fundamental interactions between host energetics and multiple host defense mechanisms shape the evolution of virulence and transmission. This project integrates (i) the development of novel energetic and evolutionary theory with (ii) individual and population-level experiments using a model host-pathogen system, Daphnia magna and Pasteuria ramosa (to leverage this system’s well-known genetic and environmental variation in host defense and rigorously test theoretical predictions). This integration will determine (1) the unique and composite effects of different host defense mechanisms on pathogen production; (2) how environmental variation in resource availability affects host defense strategies, pathogen production, virulence, and transmission; and (3) whether understanding mechanistic connections within and between hosts improves our ability to accurately predict pathogen evolution across different environmental and genetic backgrounds. We will develop mathematical models that explicitly integrate resources (host diet) and within-host and between- host dynamics to predict epidemiological and evolutionary dynamics (virulence and transmission evolution).