Gene necessity analysis and COG functional enrichment of HEG. All coding-sequence genes were searched on the Database of Essential Genes (DEG8.0 ) using BLASTx (E-value = 1 × 10-4). (a) Comparison of the DEG-hit genes in the core and flexible genomes. (b) Comparisons of gene expression subclasses between DEG-hit and DEG-miss genes. (c) COG functional enrichment of HEG in the core genome. Statistic significance was performed by Fisher’s exact test (one-tailed). P-value ≤ 0.05 was indicated in figure. COG, clusters of orthologous groups; Core, the core genome; DEG-hit, genes with homologs identified in the database; DEG-miss, genes without any known homologs; Flexible, the flexible genome; Unk, unknown function.