Fig. 2.

Phylogenetic analysis of bilaterian rhodopsin and secretin receptors. Maximum likelihood tree of bilaterian rhodopsin β (A), γ-type (B), and secretin (C) receptors, according to the GRAFS classification established in ref. 3. The tree is structured in well-supported subtrees containing both clusters of protostome (blue) and deuterostome (pink) groups of sequences. At the root of blue-pink subtrees (shown as black or green solid circles), a prototypic receptor of each subtype was already present in the urbilaterian. Black solid circles indicate well-supported bilaterian GPCR families, and green solid circles show hypothetical evolutionary relationships among bilaterian families. The bilaterian (b-), protostomian (p-), deuterostomian (d-), chordate (c-), lophotrochozoan (-l), or arthropod (a-) origin is indicated by an initial letter before each peptide GPCR acronym. Ancestral bilaterian clusters containing receptors characterized only in either protostomes or deuterostomes (e.g., b-TRHR and b-ETHR) were colored with alternating blue and pink bands, and bilaterian clusters containing no characterized receptors were shaded in gray. Photoreceptors and aminergic receptors were used as an outgroup for rhodopsin β receptors (A), and human adhesion GPCRs were used as an outgroup for the secretin receptors (C).