Distribution and position of conserved introns within the Oxidored_FMN domain. (A) Schematic comparison of conserved intron distribution of OPR genes. The positions of introns in the coding sequence of the OPR genes are indicated as colored vertical lines and labeled I1 to I12. Introns 3, 8, 10, 11 and 12 (in red) and introns 5 and 7 (in blue) only exist in aquatic plants (green algae). Moreover, introns 5 and 7 most probably arose in aquatic plants. Introns 1, 2, 4, 6 and 9, labeled in black, are present in both aquatic and land plants. (B) Conserved intron positions in OPR proteins with a topology identical to that in (A). The boundary residues between two exons are denoted by a black translucent box.