Duplication and MHC linkage of the CTX family of genes in Xenopus and in mammals

The effects of whole genome duplications that characterize the evolution of vertebrates have been studied on the gene of the Xenopus thymocyte molecul e CTX and its mammalian relatives. CTX, with an extracellular part consisti ng of one V and one C2 external domain, defines a new subset of the immunog lobulin superfamily and is conserved from amphibians to mammals. The number of CTX loci, their polymorphism, and their genetic linkages have been stud ied in several Xenopus species and in humans. In the genetically simplest s pecies, X. tropicalis (2n = 20), the unique CTX locus is linked to the MHC. In the polyploid species, all CTX genes, unlike many other immune system g enes, have remained in the genome; i.e. there are two CTX loci in the tetra ploid species X: laevis (2n = 6) and six CTX loci in the dode-caploid speci es X. ruwenzoriensis (2n = 108). In X laevis; one CTX gene is linked to the MHC and the other not, presumably because one set of MHC class I and II ha s been deleted from the corresponding linkage group. The various mammalian homologues are less related to each other than are the Xenopus CTX genes am ong each other, and they do not cross-hybridize with each other because the y stem from the ancient polyploidization. Some human CTX homologies are on chromosomes 11 and 21, but others are on chromosomes 1, 6 and 19, which con tain MHC paralogous regions; this suggests that a very ancient linkage grou p has been preserved.