The evolutionary path of relaxin family genes. (A) Syntenic mapping indicated that seven human relaxin family genes evolved from three separate ancestral genes (AncRFLA, AncRFLB, and AncRFLC) in the common ancestor of tetrapods and teleosts. These ancestral genes gave rise to five relaxin family genes in pufferfish (RFLA1, RFLA2, RFLB, RFLC1, and RFLC2) as a result of teleost-specific WGD. One of the WGD-derived RFLB daughter genes (RFLB2) has been lost during teleost evolution. In contrast, a segmental duplication at RFLC led to the generation of RFLCI and RFLCII genes in tetrapods. Whereas RFLCI and RFLCII were lost in avian species, these two genes became RLN3 and INSL3, respectively, in therian mammals. Arrows indicate the putative time of emergence of different relaxin family genes during vertebrate evolution. All genes are color-coded to indicate their origins. Lost genes are indicated by embossed letters. Positions of kidneys and testes in representative vertebrates are indicated by black balls and green balls, respectively. (B) Syntenic mapping of RFLC in vertebrates. The orthologs of human RLN3 and INSL3 on RFLC of different vertebrates are indicated by red rectangles whereas neighboring genes are indicated by diamonds. Orthologous genes in syntenic chromosomal regions of different species are identified by color. The chromosomal numbers or the genomic contig numbers are indicated at the top or the left of the schematic representation of each genomic fragment. WGD-derived syntenic chromosomal regions in teleosts are indicated by yellow background. Genomic fragments derived from segmental duplication of the region neighboring RFLC are indicated by rectangles on the right of the human chromosome 19p13 fragment. (**) The co-orthologous relationship of the zebrafish RFLC2 gene on chromosome 24 to the pufferfish counterparts was determined based on sequence similarity analysis.
