Gene expression and molecular evolution of the core genome and flexible genome of Prochlorococcus MED4. (a) Box plot of the correlation between gene expression levels and the nonsynonymous substitution rates (Ka). The line was drawn through the median. A circle represents an outlier, and an asterisk represents an extreme data point. (b) Nonsynonymous substitution rate comparison between CEG and VEG (Mann–Whitney U Test, two-tailed). A circle represents an outlier, and an asterisk represents an extreme data point. (c) Comparison of five expression subclasses between the core genome and flexible genome (Fisher’s exact test, one-tailed). P-value ≤ 0.05 was indicated in figure. HEG, highly expressed genes; MEG, moderately expressed genes; LEG, lowly expressed genes; NEG, non expressed genes; CEG, constantly expressed genes (including four expression subclasses mentioned above); VEG, variably expressed genes.