Explanation of the nomenclature used for the hypothetical ancestral genes that gave rise to the three gene families discussed in this study (receptors <i>RXFP3/4, RXFP1/2</i> and their ligands <i>RLN/INSL</i>) via three rounds of WGD (<i>1R, 2R</i> and teleost-specific <i>3R</i>).

*We show that the gene known as “RXFP4” is one of the three ohnologs of RXFP3-1, hence based on its origin it should be termed “RXFP3-4”; **The origins of RXFP2-like (present in zebrafish, amphibians, birds, reptiles and marsupials) remain controversial, it is possible that RXFP2-like is a post-2R descendant of AncRxfp2, in which case it should be called “RXFP2-2”, while the ortholog of human RXFP2 should be called “RXFP2-1”; ‡rxfp3-3a was locally duplicated in the Post-3R ancestor of zebrafish, medaka, stickleback and pufferfishes (hence rxfp3-3a and rxfp3-3b); in zebrafish there are three paralogous rxfp3-3a genes: 3-3a1, 3-3a2 and 3-3a3; †Gene loss