Maximum parsimony tree with bootstrap confidence levels based on nucleotide coding sequence of the various vertebrate opsin classes rooted with human melanopsin (see Materials and Methods). Cone opsin class nomenclature after Hunt et al. . To the right of the tree is a schematic representation of the intron insertion sites in the various opsin genes based on Figure 3. Conserved intron positions between opsin classes are indicated by a dashed vertical line. The relative position of the second rod and cone opsin intron is indicated by an arrow head in the pinopsin and VA-opsin classes to show their shifted second intron positions (2a and 2b).