LINKAGE RELATIONSHIPS AND HAPLOTYPE POLYMORPHISM AMONG CICHLID MHC CLASS-II B-LOCI

The species flocks of cichlid fishes in the Great East African Lakes a re paradigms of adaptive radiation and hence, of great interest to evo lutionary biologists. Phylogenetic studies of these fishes have, howev er, been hampered by the lack of suitable pol!morphic markers. The gen es of the major histocompatibility complex hold the promise to provide , through their extensive polymorphism, a large number of such markers , but their use has been hampered by the complexity of the genetic sys tem and the lack of definition of the individual loci. In this study w e take the first substantial step to alleviate this problem. Using a c ombination of methods, including the typing of single sperm cells, gyn o- or androgenetic individuals, and haploid embryos, as well as sequen cing of class II B restriction fragments isolated from gels for Southe rn blots, Mle identify the previously characterized homolog) groups as distinct loci. At least 17 polymorphic class II B loci, all of which are presumably transcribed, have been found among the different specie s studied. Most of these loci are shared across the various cichlid sp ecies and genera. The number of loci per haplotype varies from individ ual to individual, ranging from 1 to 13. A total of 21 distinct haplot ypes differing in the number of loci they carry has thus far been iden tified. All the polymorphic loci are part of the same cluster ill whic h, however, distances between at least some of the loci las indicated by recombination frequencies) are relatively large. Both the individua l loci and the haplotypes can now be used to study phylogenetic relati onships among the members of the species flocks and the mode in which speciation occurs during adaptive radiation.