Positive selection assessment of the OPT gene family in Arabidopsis, Populus, Vitis and rice. A. Selection pressure (K
) of the full-length OPT protein sequences for the different phylogenetic groups. Two different evolution models (M8/M7) were used. The M8 model was the only one that predicted the presence of positively selected sites (shown in red). B. Likelihood values and parameter estimates for the OPT genes predicted to undergo positive selection pressure as described in A. C. Detailed distribution of the positive selection sites of Group 4 predicted by the M8 model. Ten potential positive-selection sites are marked with arrows and shown in red in the tertiary structure of the PtYSL2 protein.