Unique Composition of Intronless and Intron-Containing Type I IFNs in the Tibetan Frog Nanorana parkeri Provides New Evidence To Support Independent Retroposition Hypothesis for Type I IFN Genes in Amphibians.

Abstract

In vertebrates, intron-containing and intronless type I IFN genes have recently been reported in amphibian model species Xenopus tropicalis and X. laevis. However, whether intronless type I IFNs in amphibians are the ancestral genes of type I IFNs in amniotes or just represent the independent divergence in amphibians is unknown or even uninvestigated. In this study, both intron-containing and intronless type I IFN genes, as well as their receptor genes, were identified in the Tibetan frog Nanorana parkeri The evidence obtained from homology, synteny, phylogeny, and divergence time showed that intronless type I IFN genes in N. parkeri and in Xenopus might have arisen from two independent retroposition events occurred in these two lineages, and the retrotransposition causing the generation of intronless type I IFN genes in amniotes is another independent event beyond the two in amphibians. It can then be proposed that intronless type I IFNs in N. parkeri and Xenopus may not be the ancestral genes of intronless type I IFNs in amniotes but may just represent two independent bifurcations in the amphibian lineage. Furthermore, both intronless and intron-containing type I IFNs in N. parkeri showed strong ability in inducing the expression of IFN-stimulated genes and the strong antiviral activity against frog virus 3. The present study thus provides the evolutionary evidence to support the independent retroposition hypothesis for the occurrence of intronless type I IFN genes in amphibians and contributes to a functional understanding of type I IFNs in this group of vertebrates.