Interferons (IFNs) are important antiviral agents in the immune system of vertebrates. Based on the type of receptor through which they signal, IFNs can be classified into three major types: type I IFN, type II IFN, and type III IFN. In reptiles, birds and mammals, type I and type III IFN are structurally related cytokines with similar antiviral functions. However, they have different genomic organizations and bind to distinct receptor complexes. Unlike type I IFN genes which do not contain any intron, type III IFN genes contain four introns.  

Recent studies have shown that IFNs discovered in the lower vertebrates (e.g. fish) have the same genomic structure to the type III IFNs, but contain motifs which are conserved in the type I IFN proteins. Studies on the receptors are also inconclusive, as to whether the receptors utilized by fish IFNs are homologues to type I or type III IFN receptors. Whether the recently identified intron containing IFN genes in fish and amphibians belong to the type I or III IFN family and when the type I and III genes diverged in vertebrates are fascinating topics debated by comparative immunologists. 

By analyzing the genome sequences of the vertebrates, Dr. Zhitao Qi, jointly supervised by Prof Pin Nie, Institute of Hydrobiology, Chinese Academy of Sciences (IHB), and Dr Jun Zou, Scottish Fish Immunology Research Centre, Institute of Biological and Environmental Sciences, University of Aberdeen, has found that intron containing type I and type III IFN genes co-exist in the Xenopus tropicalis genome and hence demonstrated for the first time that intron containing type I and III genes diverged relatively early in vertebrate evolution, and at least by the appearance of early tetrapods, a transition period when vertebrates migrated from an aquatic environment to land. Their data also suggest that the intronless type I IFN genes seen in reptiles, birds, and human civilizationmammals have originated from a type I IFN transcript via a retrotransposition event that occurred prior to the appearance of reptiles.  

The new findings, recently published in Journal of Immunology (184: 5038-5046), provide important clues for further understanding evolution of the vertebrate IFN system.