MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I GENES OF THE COELACANTH LATIMERIA-CHALUMNAE

The coelacanth fish Latimeria chalumnae is the sole surviving species of a phylogenetic lineage that was founded more than 400 million years ago and that has changed morphologically very little since that time. Little is known about the molecular evolution of this ''living fossil ,'' considered by some taxonomists to be the closest living relative o f tetrapods. Here we describe the isolation and characterization of L. chalumnae major histocompatibility complex (MHC) class I genes. The e xon-intron organization of these genes is the same as that of their ma mmalian counterparts. The genes fall into four families, which we desi gnate Lach-UA through Lach-UD. There are multiple loci in all of the f amilies. Genes of the first two families are transcribed. The Lach-UA family bears the characteristics of functional, polymorphic class I ge nes; the other three families may be represented by nonclassical genes . All the Lach loci arose by duplication from an ancestral gene after the foundation of the coelacanth lineage. Intergenic variation is high est at positions corresponding to the mammalian peptide-binding region . The closest relatives of the Lach genes among the MHC genes sequence d thus far are those of the amphibian Xenopus.