Fig. 7

Diagram showing the phylogenetic distribution of RXFP/LGR3-type proteins in the Bilateria, with representative species from phyla/sub-phyla illustrated. Taxa in which genes encoding RXFP/LGR3-type proteins have not been found are shown with an empty square. Taxa in which genes encoding RXFP/LGR3-type proteins have been found but without experimental identification of a cognate ligand(s) are shown with a filled square without an asterisk. Taxa in which genes encoding RXFP/LGR3-type proteins have been found and with experimental identification of a cognate ligand(s) are shown with a filled square and an asterisk together with the name of the ligand(s). The identification of an RXFP/LGR3-type protein as the receptor for RGP1 and RGP2 in starfish (A. rubens, A. cf solaris), as reported in this study and highlighted here in red, is the first experimental characterization of a relaxin-type signaling system in a deuterostome invertebrate. The findings of this study indicate that the evolutionary origin of relaxin-RXFP1,2/LGR3-type signaling can be traced back to the common ancestor of the Bilateria, but with subsequent loss in some taxa. Abbreviations: D, Deuterostomia; P, Protostomia; C, Chordata; A, Ambulacraria; S, Spiralia; E, Ecdysozoa. Images of representative animals from each phylum/sub-phylum were obtained from http://phylopic.org or were created by the authors and their collaborators