Genome dynamics and evolution of codon usage patterns in shrimp viruses.

We analysed the genomes and codon usage patterns of seven small (DNA and RNA) shrimp viruses. Effective number of codon (ENC) values indicated moderate (35 < ENC < 50) codon usage bias in shrimp viruses. Correlation analysis between GC compositions at non-synonymous codon and synonymous codon positions (GC1, 2 and GC3) as well as GC3 versus ENC curves indicated varying influences of mutational pressure on codon usage. The presence of deoptimized codons and host-antagonistic codon usage trends in shrimp viruses suggested the adaptation of a slow replication strategy by these viruses to avoid host defences. Low CpG frequencies indicated that shrimp viruses have evolved with underrepresentation of CpGs to avoid the host's immune response.