The haplochromine cichlid species flocks of the East African Great Lakes are one of the best
examples of adaptive radiation. Analysis of genetic variation among these species provides
valuable information on species relationships and timing of speciation events. Although the
haplochromine cichlids generally display little genetic variation, the major histocompatibility
complex (Mhc) genes have been found to be highly variable. A study of the linkage relationships
of the Mhc class I A genes in the cichlid fish Oreochromis niloticus was therefore
undertaken. Class I loci were identified, and their segregation in seven mothers and their
haploid embryos was determined. In total, 56 class I A sequences were found among the seven
families. A strong concordance of segregation was observed in five haplotypes among the embryos,
indicating a close linkage of all loci. The number of loci per haplotype varied from 11 to 17,
while the total number of distinct loci found among all families was 22. These findings show
that all class I A loci are linked in a single genetic cluster in O. niloticus.