Deconstructing Partitiviridae proteins reveals the evolutionary determinants behind their origin and ecological breadth

Viruses represent a primary source of biological diversity, yet the specific mechanisms by which viral proteins shape host range remain unclear. Members of the family Partitiviridae offer a minimalist model to address this question: despite their collectively broad host range, they typically encode only an RNA-dependent RNA polymerase (RdRP) and a capsid protein (CP). Here, we integrate sequence phylogenetics with structural analyses of solved and AlphaFold-predicted structures for both proteins to reconstruct their evolutionary trajectories. We show that CPs feature a conserved Reo-like shell core alongside a genus-specific, highly variable protrusion (P) domain. Similarly, the RdRP retains a conserved catalytic core, with terminal variations giving rise to additional elements within the finger domain. Evolutionary analysis indicates that these two proteins originated independently: an ancestral Betapartitivirus-like RdRP triggered the origin of partitiviruses, while a repurposed totivirus-like CP provided the original capsid—traces of which remain evident in Epsilonpartitivirus. Furthermore, structural and phylogenetic similarities reveal subclusters within established genera and suggest three independent insertion events shaping the P domain. Ultimately, this detailed dual-protein framework demonstrates how structural minimalism and modular adaptability facilitate viral survival and cross-kingdom spread, providing a structural explanation for the exceptional ecological breadth of partitiviruses.