Characterization of MHC class II genes from an ancient reptile lineage, Sphenodon (tuatara)

The organization and evolution of major histocompatibility complex (MHC) genes vary considerably among vertebrate lineages. MHC genes have been well characterized in mammals, birds, amphibians and fish, but little is known about their organization in reptiles, despite the fact that reptiles occupy an important phylogenetic position for understanding the evolutionary history of both mammalian and avian MHC genes. Here we describe the characterization of the first MHC class II B cDNA sequences from a non-avian reptile, the tuatara (Sphenodon spp.). Three class II B sequences were isolated from a tuatara cDNA library, and four additional partial sequences were isolated by reverse transcriptase-polymerase chain reaction. Six of these sequences appear to belong to the same gene family, which we have named SppuDAB. The remaining sequence (named SppuDBB) shares only 43.9% amino acid similarity with SppuDAB and thus appears to represent a separate gene family. SppuDBB may be a non-classical locus as it does not contain all the conserved residues expected of a classical MHC class II gene. Southern blot analysis indicates that only a single copy of SppuDBB exists in tuatara, but that multiple loci related to SppuDAB are present. The SppuDAB sequences have the highest amino acid similarity (57.2-62.4%) with class II B sequences from the spectacled caiman, but only 26.4-48.7% similarity with sequences from other vertebrates. The tuatara sequences do not strongly group with other reptile sequences on a phylogenetic tree, reflecting the antiquity of the Sphenodon lineage and the lack of closely related sequences for comparison.