Species-specific barriers constrict Orsay virus host range across the Caenorhabditis genus

Predicting the host range and spillover potential of RNA viruses requires understanding how ecological, immunological, and evolutionary factors jointly shape viral life cycles across related hosts. Here we integrate population-level viral load dynamics, single-animal heterogeneity, tissue-level progression, transmission competence, and evolutionary sustainability to map the eco evolutionary barriers that Orsay virus encounters across six Caenorhabditis species. We show that host species identity determines the timing and completeness of the viral life cycle, producing species specific combinations of susceptibility, replication kinetics, RNA2 to RNA1 stoichiometric balance, virion egress, and onward transmission. These phenotypes correspond to distinct host competence phenotypes, ranging from permissive (Caenorhabditis elegans) to restrictive or evolutionarily dead end host species. Alternative host species disrupt viral life cycle synchrony through delayed replication, truncated cycles, or failure to produce lumen localized virus, thereby reducing transmission and preventing viral adaptation upon serial passage. Our results demonstrate how temporal mismatches between viral replication and host physiology create a series of eco evolutionary barriers to emergence, offering a mechanistic framework for predicting viral host range.