Architectural trade-offs between environmental stability and genomic redundancy reveal divergent pneumoviral entry strategies

Human respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) exhibit distinct seasonal epidemiology, with RSV circulating in early autumn and hMPV peaking in midwinter, yet the structural basis for this niche partitioning remains undefined. Here, we integrate in situ cryo-electron tomography and functional virology to decode the architectural logic governing their entry dynamics. RSV employs a matrix (M)-regulated prefusion F (pre-F) organization, partitioning trimers into stabilizing hexagonal superlattices and fusion-competent pools to maintain superior thermotolerance. By contrast, hMPV compensates for its intrinsically unstable, monomeric pre-F with extreme ribonucleoprotein polyploidy, packaging [~]4-fold more genome equivalents to ensure productive infection. Fusion events localize exclusively to M-depleted, non-arrayed membrane regions, establishing a spatial checkpoint for activation. These findings reveal a conserved trade-off between environmental resilience and genomic redundancy that dictates divergent pneumoviral entry strategies, explaining their distinct seasonal ecological niches.