Schematic representation of the evolution trajectory of RLN3 and INSL3. (A) RLN3 and INSL3 of therian mammals evolved via gene duplication, followed by emergence of an LGR7 (RXFP1)-specific daughter gene (RFLCI) in the common ancestor of mammals, and an LGR8 (RXFP2)-specific daughter gene (RFLCII) in the common ancestor of therian mammals. LGR7- and LGR8-specific ligands are indicated by a red and a green hexagonal symbol, respectively. Bifunctional ligands are indicated by hexagonal symbols with a mix of red and green colors. Positions of kidneys and testes in representative vertebrates are indicated by black balls and green balls, respectively. (B) Our results support the hypothesis that natural selection selects signaling pathways with a high signal-to-noise ratio over nonselective ones following the duplication of a ligand gene. A high degree of fitness associated with select mutations can then lead to reproductive success and the selection of these copies. In our example, the two daughter genes, RLN3 and INSL3, were selected through natural selection in a stepwise “divergent resolution” manner (Taylor et al. 2001). Starting with a segmental duplication at RFLC, the LGR7-selective RLN3 and the LGR8-specific INSL3 were fixed in the population after neofunctionalization but occurred at two disparate geological time points. Red bars on a chromatid represent RFLCI and RFLCII daughter genes generated by a segmental duplication event in an ancestral tetrapod. Orange bars on a chromatid represent RLN3 after the acquisition of the LGR7-selective characteristics. Gold bars on a chromatid represent INSL3 following the R → H mutation. Genotypes selected by evolution are indicated by rectangular boxes. Ligand specificities for LGR7 and LGR8 are indicated by arrows.
