With no clear Z-VAD-FMK supply orthologs from invertebrate species, it is evident that these MC NVP-BEZ235 receptors originated at the beginning of vertebrate evolution at around 450/500 MY ago. The presence of MC receptors at different loci across the vertebrate spectrum, from teleost fishes to humans, suggests that they evolved by a process of duplication that happened very early during vertebrate evolution. Other than D. rerio, none of the teleost fishes contain fish-specific loci of MC receptors. D. rerio possesses two copies of MC5 receptors. Previously, it was reported that these two D. rerio MC5R sequences are not a very recent duplicates and the duplication event that created these two receptors took place in the teleost lineage after the divergence of tetrapods, which is usually dated to 300 MY ago as estimated by molecular clock calculations . These two MC5Rs from D. rerio have similar pharmacological and expression profile in different tissues supporting the developed degeneration complementation model for the fates of duplicated genes . Further, we found that MC2Rs and MC5Rs of selected group of fishes have unusual gene structures when compared to the orthologous receptors from tetrapods . Normally, MC receptors do not possess introns, but MC5R and MC2R from T. rubripes, T. nigroviridis, O. latipes, and G. aculeatus contain one and three introns, respectively, at identical positions. However, we did not detect introns in MC2R and MC5R receptors from D. rerio, where all MC receptors were intron-less as found in tetrapod MC receptor gene structures. In different studies, unusual exon-intron boundaries of MC5R and MC2R have been reported previously in pufferfishes and in O. latipes and G. aculeatus for different purposes. In this study, we have systematically studied the properties of these introns for possible explanations for novel intron insertions. Intron insertions are considered to be rare genomic changes across a wide range of metazoan lineages such as mammals and puffer fishes . Our data suggest that the MC5R genes of the four ray-finned fishes represent a clear proof of novel intron insertions, which is absent in tetrapods, D. rerio, and in elephant shark; all of which possess the typical single intron-less gene structure. There are three introns in orthologs of MC5R from a group of selected ray-finned fishes that are found after the split of the D. rerio lineage from the superorder Acanthopterygii . Similarly, unusual gene structures of MC2R from the same group of ray-finned fishes were also found after diversification of D. rerio from these fishes. Some introns are ultrasmall in size as found in case of novel insertion at position 230c and 236a in MC2R from T. rubripes, and G. aculeatus, respectively. Such small introns are rare; however, it cannot be completely excluded as the T. rubripes genome has some smaller introns .