Study system and sampling
Our study focused on viruses infecting natural populations ofPlantago lanceolata in the Ă…land Island, SW of Finland. These populations have been monitored since 1993, and they form a highly fragmented network of ~4000 populations (Ojanen et al. 2013). We used a stratification process to select 20 focal populations that were sampled in early June 2017 for their virus communities. We collected leaf samples from 20 randomly selected plants in each population, resulting in altogether 400 samples for small RNA sequencing. For more details on the study system and sampling, see Supplementary Material and Methods.