Two highly divergent ancient lineages of the TAP gene in Xenopus: further evidence for co-evolution among MHC class I region genes.

In the frog Xenopus, MHC class I antigen presentation and processing genes (the immuno-proteasome LMP2 and LMP7 and the transporter TAP1 and TAP2) seem to be closely linked in a primordial organization. Two distinct lineages of class la and LMP7 loci were previously identified, thus strongly suggesting co-evolution among 'class I region' genes. We now show that the Xenopus MHC 'class I region' lies between class II and class III genes and we have isolated two distinct alleles at both the TAP1 and TAP2 loci. The alleles at each locus are remarkably divergent from each other and phylogenetic tree analysis revealed in both cases that they diverged from each other 60-100 million years ago (MYA). For lineage-frequency and linkage analysis, 25 wild-caught X. laevis and 16 X tropicalis were examined. The two lineages were present in different frequencies for X laevis and X tropicalis. Nevertheless, in all cases, the LMP7, TAP1, and TAP2 lineages were found in a set comprising one of the two lineages. Furthermore, like the LMP7 lineages, the TAP lineages were detected in most Xenopus species that diverged from a common ancestor 80-100 MYA, suggesting that the 'class I region' biallelic lineages are under balancing selection.