Codon usage patterns and adaptive evolution of marine unicellular cyanobacteria Synechococcus and Prochlorococcus

► Synechococcus (Syn) and Prochlorococcus (Pro) have a close phylogenetic relationship. ► Their codon usages are highly divergent and so is the potential driving. ► Translational selection contributes to codon usage bias in Syn. ► Mutation and genome compositional constraints contribute to codon usage bias in Pro. ► Average dN/dS value of core genes in Syn is significantly higher than that in Pro.