Is within-host viral community assembly shaped by local adaptation?

Host-parasite coevolution describes the continuous reciprocal selection driving host defense and parasite infectivity, with direct consequences for disease dynamics. While abundant evidence exists for coevolution shaping host-parasite dynamics within the ‘one host-one parasite’ framework, hosts are typically infected by multiple parasites and the extent to which coevolutionary processes shape within-host parasite communities remains poorly understood. Investigating these interactions is essential for understanding how coevolution drives parasite diversity, competition, and coexistence within hosts. Here, we conducted a local adaptation experiment to investigate the effects of coevolution on within-host viral community assembly in Plantago lanceolata. Greenhouse-grown individuals were reciprocally transplanted into wild populations during natural viral epidemics. We combined small-RNA sequencing to identify the viral communities and joint species distribution modelling to quantify the effects of local adaptation, population and host characteristics on viral community assembly. Our results show that host populations vary in the extent to which local adaptation influences within-host viral diversity. Across all populations, host maternal line and origin population were the main determinants of viral community composition and infection status. The effects varied across virus families, suggesting virus-specific assembly processes and variation in the potential for coevolution to shape these interactions.