Further, there are no complex repeats detected in novel introns of serpins whereas novel introns from medaka and stickleback lineages possess complex repeats. The nucleotides that are flanking the novel introns from these MC receptors form a typical proto-splice site with deviations at the 59end, contrasting the results from novel introns of serpins. However, we found that intron phasing of majority of novel intron Cycloheximide insertions in both MC receptors and serpins is same . Previous large scale analyses of DRY introns indicate that repeated insertions at the same position and with phase c has occurred in different lineages of animals for these introns . After combining these data with ours for novel intron insertions, it appears that intron phasing c is selected during intron insertions, probably due to extra flexibility provided by the wobble at the third base of codons. It is clear that intron insertions have occurred in genomes of ray-finned fishes which underwent genome downsizing and these introns are inserted during period at about 320�C 190 MYA. It is now well established in two independent multigene superfamilies such as GPCR and serpins . In summary, a group of ray-finned fishes exhibits multiple intron insertions in selected GPCRs and serpins at,320�C 190 MYA, which are not present in any other vertebrate. Dabrafenib Compaction in genomic contents of these fishes may have played a crucial role in these intron acquisitions. Fishes exhibit a high diversity after separation from last common ancestor of tetrapods/ fishes lineage and these diversities can be explained by rapid change in DNA contents by processes such as whole genome duplication and genome compaction. Losses/gains in gene contents, introns, and intergenic regions are crucial to these events. Hence, genome compaction may be responsible for novel intron insertions at least in ray-finned fishes. Though one may argue against such possibility given our small sample size of four fishes, novel intron insertions are rare genomic events and they are extremely rare during vertebrate evolution , probably because most such events would lead to deleterious effects. We have compiled a comprehensive catalogue of MC receptors from evolutionary distant vertebrates in this study. With a complete repository of MC receptors at hand, we carried out a study of introns in these genes. We found that introns are inserted in closely linked MC5R and MC2R from a selected set of actinopterygian fishes whose genomic content is reduced. Such insertion events are absent in genomes of zebrafish, lamprey, elephant shark and other tetrapods. Both these receptors share one intron inserted at common site in conserved DRY motif. We demonstrated that P2Y receptors and CHRM3 also had novel intron gains, suggesting that this mechanism is not restricted only to the MC receptors.