Evolutionary genomics of white spot syndrome virus

White spot syndrome virus (WSSV) has been one of the most devastating pathogens affecting the global shrimp industry since its initial outbreaks in Asia in the early 1990s. In this study, we recovered 13 complete metagenome-assembled genomes (MAGs) of Japanese WSSV isolates and 30 draft WSSV MAGs recovered from publicly available sequencing data, to investigate the genomic evolution of WSSV. Phylogenetic analysis revealed two major phylotypes, designated phylotypes I and II. Bayesian divergence time estimates placed the divergence time of the two phylotypes between 1970 and the early 1980s, with an estimated substitution rate of 1.1 x 10(-5 )substitutions per site per year, implying the existence of pre-pandemic genetic diversity of WSSV in Asia. Based on this scenario, phylotype I was responsible for the 1990s pandemic and spread worldwide, whereas phylotype II was localized in Asia and infiltrated Australia. Two cross-phylotype recombinant lineages were identified, which demonstrate the role of genomic recombination in generating the genetic diversity of WSSV. These results provide important insights into the evolution of WSSV and may help uncover the ultimate origins of this devastating pathogen.