Figure 3.

Correlation heatmap of codon usage indices in three Medicago species. codon No: codon number; GC1: GC content of codon base 1; GC2: GC content of codon base 2; GC3: GC content of codon base 3; GC_all: total codon GC content; GC3s: GC content of mitochondrial genome synonymous codon base 3; CAI: codon adaptation index; CBI: codon preference index; Fop: frequency of optimal codon usage; ENC: effective codon count; **: significant correlation at the 0.01 level; *: significant correlation at the 0.05 level. In the upper-right section, numerical values quantify the correlation coefficients between the indices. The lower-left section uses colored blocks to visually depict these correlations, where red signifies a positive correlation and blue indicates a negative one. The intensity of the color reflects the strength of the correlation, with darker shades representing stronger associations.