EVOLUTION OF MHC CLASS-II-BETA CHAIN-ENCODING GENES IN THE LAKE-TANA BARBEL SPECIES FLOCK (BARBUS-INTERMEDIUS COMPLEX)

Major histocompatibility complex (MHC) class II protein polymorphism i s maintained in allelic lineages which evolve in a trans-specific mann er, passing from one species to descendant species. Selection pressure on peptide binding residues should be greatest during speciation, whe n organisms move into new environments and their MHC molecules encount er new pathogens. The isolation of MHC genes from teleost fishes, the most diverse group of vertebrates, has created possibilities for testi ng this hypothesis. The large barbels of Lake Tana have undergone an a daptive radiation within the last 5 million years, producing 14 morpho types which inhabit different ecological niches within the lake. We st udied the variability II beta chain-encoding genes of four of these mo rphotypes using polymerase chain reaction amplification and DNA sequen cing. The sequences obtained were orthologous to four of the known cla ss II genes from the common carp, from which barbels diverged approxim ately 32 million years ago. When subjected to phylogenetic analysis, t he 48 sequences clustered into groups which represent allelic lineages . A comparison of nonsynonymous and synonymous substitutions between t he peptide binding region codons and non-peptide binding region codons of these sequences revealed that they are under strong selective pres sure.