Major histocompatibility complex class II A genes in cichlid fishes: identification, expression, linkage relationships, and haplotype variation

Two cichlid species, the haplochromine Aulonocara hansbaenschi and the tila piine Oreochromis niloticus, were used to study the major histocompatibilit y complex (Mhc) class II A variation within this group. Multiple class II A sequences were recovered from A. hansbaenschi and O. niloticus cDNA librar ies and three sequence families, DAA, DBA, and DCA, were identified. Sets o f O. niloticus haploid embryo families were used to determine the linkage r elationships of these genes. Two independently asserting linkage groups wer e detected, DAA and DBA/DCA, neither of which is linked to the previously d escribed Mhc class I gene cluster. Three DCA genes and up to four DBA genes were found to segregate in different haplotypes, whereas DAA occurred as a single locus. Four DBA haplotypes, DBA*H1-H4, were identified and shown to co-segregate with the previously described class II B haplotypes. Four DCA haplotypes, DCA*H1-H4, were found at a distance of 37 cM from the DBA/clas s II B cluster; in one DCA haplotype, DCA*H5, the genes were tightly linked to the DBA/class II B clusters. Transcripts of DAA and DBA genes were foun d in O. niloticus hepatopancreas and spleen; transcripts of DCA genes were detected in the A. hansbaenschi cDNA library, but not in O. niloticus. Thes e findings provide a basis for using class II haplotypes as markers in the study of adaptive radiation in the cichlid species flocks of the East Afric an Great Lakes.