Conservation of sequence motifs suggests that the nonclassical MHC class I lineages CD1/PROCR and UT were established before the emergence of tetrapod species.

Humans have a number of nonclassical major histocompatibility complex (MHC) class I molecules that are quite divergent from the classical ones, and that may have separated from the classical lineage in pre-mammalian times. To estimate when in evolution the respective nonclassical lineages separated from the classical lineage, we first identified "phylogenetic marker motifs" within the evolution of classical MHC class I; the selected motifs are rather specific for and rather stably inherited within clades of species. Distribution of these motifs in nonclassical MHC class I molecules indicates that the lineage including the nonclassical MHC class I molecules CD1 and PROCR separated from the classical lineage before the emergence of tetrapod species, and that the human nonclassical MHC class I molecules FCGRT, MIC/ULBP/RAET, HFE, MR1, and ZAG show similarity with classical MHC class I at the avian/reptilian level. An MR1-like α1 exon sequence was identified in turtle. Our system furthermore indicates that the lineage UT, hitherto only found in non-eutherian mammals, predates tetrapod existence, and we identified UT genes in reptiles. If only accepting wide distribution of a lineage among extant species as true evidence for ancientness, the oldest identified nonclassical MHC class I lineage remains the fish-specific lineage Z, which was corroborated in the present study by finding both Z and classical-type MHC class I sequences in a primitive fish, the bichir. In short, we gained important new insights into the evolution of classical MHC class I motifs and the probable time of origin of nonclassical MHC class I lineages.