Nonlinkage of major histocompatibility complex class I and class II loci in bony fishes

In tetrapods, the functional (classical) class I and class II B loci of the major histocompatibility complex (Mhc) are tightly linked in a single chro mosomal region. In an earlier study, we demonstrated that in the zebrafish, Danio rerio, order Cypriniformes, the two classes are present on different chromosomes. Here, we show that the situation is similar in the sticklebac k, Gasterosteus aculeatus, order Gasterosteiformes, the common guppy, Poeci lia reticulata, order Cyprinodontiformes, and the cichlid fish Oreochromis niloticus, order Perciformes. These data, together with unpublished results from other laboratories suggest that in all Euteleostei, the classical cla ss I and class II B loci are in separate linkage groups, and that in at lea st some of these taxa, the class II loci are in two different groups. Since Euteleostei are at least as numerous as tetrapods, in approximately one-ha lf of jawed vertebrates, the class I and class II regions are not linked.