Experimental and evolutionary evidence for horizontal transfer of an envelope fusion protein gene between thogotoviruses and baculoviruses

This study investigated the evolutionary relationship between thogotoviruses and baculoviruses by examining whether thogotovirus envelope fusion proteins (EFPs) can functionally substitute the baculovirus GP64 protein. The authors combined bioinformatic analyses, phylogenetics, recombinant virus construction, and cell culture experiments to test the hypothesis that baculoviral gp64 originated from a horizontal gene transfer (HGT) event involving thogotoviruses. Through RNA-seq data mining of lepidopteran transcriptomes, the researchers identified a novel virus named Melitaea didyma thogotovirus 1 (MediTHOV-1). Phylogenetic and phylodynamic analyses of glycoproteins from baculoviruses and thogotoviruses suggested that the HGT event giving rise to gp64 likely occurred during the Mesozoic era. To experimentally test functional compatibility, recombinant Autographa californica multiple nucleopolyhedrovirus (AcMNPV) lacking its native gp64 gene was engineered to express EFPs from MediTHOV-1 or Apis thogotovirus 1 (ATHOV-1). Infection assays showed that the MediTHOV-1 glycoprotein could not restore viral infectivity. In contrast, the ATHOV-1 EFP partially rescued infectivity, allowing viral replication and cell-to-cell spread in insect cells, although with delayed kinetics and significantly lower viral titers compared with GP64-containing viruses. Structural analysis using cryo-electron microscopy revealed reduced incorporation of ATHOV-1 EFP into viral envelopes, explaining the lower infection efficiency. Overall, the findings provide experimental evidence supporting the hypothesis that baculovirus GP64 originated from thogotovirus glycoproteins through horizontal gene transfer and highlight functional constraints that shaped the evolution of viral envelope fusion proteins.
(TMR)