Nonsynonymous/synonymous substitution rate of the liver fluke transcripts. Ka/Ks ratio distribution (%) of a total of 12,394 Dugesiidae or C. elegans orthologous F. hepatica transcripts is shown. The majority of the transcripts [89.63% (87.90% at P < 0.05)] were found to be associated with negative selection (Ka/Ks < 1). A group of transcripts [6.75% (1.88% at P < 0.05)] could be under directional selection (0.5 < Ka/Ks < 1). Another group of transcripts (0.24%) were related to neutral selection. Only a small portion of the transcripts (3.37%, n = 418) showed the signs of positive selection (Ka/Ks > 1) and, of this, a minority (0.64%, n = 79) reached statistical significance (P < 0.05).