Comparative genomic analysis reveals new evidence of genus boundary for family Iridoviridae and explores qualified hallmark genes

Members of the family Iridoviridae (iridovirids) are globally distributed and trigger adverse economic and ecological impacts on aquaculture and wildlife. Iridovirids taxonomy has previously been studied based on a limited number of genomes, but this is not suitable for the current and future virological studies as more iridovirids are emerging. In our study, 57 representative iridovirids genomes were selected from a total of 179 whole genomes available on NCBI. Then 18 core genes were screened out for members of the family Iridoviridae. Average amino acid sequence identity (AAI) analysis indicated that a cut-off value of 70% is more suitable for the current iridovirids genome database than ICTV-defined 50% threshold to better clarify viral genus boundaries. In addition, more subgroups were divided at genus level with the AAI threshold of 70%. This observation was further confirmed by genomic synteny analysis, codon usage preference analysis, genome GC content and length analysis, and phylogenic analysis. According to the pairwise comparison analysis of core genes, 9 hallmark genes were screened out to conduct preliminary identification and investigation at the genus level of iridovirids in a more convenient and economical manner.