Evidence that the separation of Mhc class II from class I loci in the zebrafish, Danio rerio, occurred by translocation.

In the zebrafish, Danio rerio, and other teleosts, the class I and class II loci of the major histocompatibility complex ( Mhc) reside on different chromosomes. To shed light on the events that might have generated this difference from tetrapods, in which these two types of loci are clustered in a single chromosomal region, the organization of the class II loci in linkage group 8 of the zebrafish was determined by the characterization of contigs of PAC clones. Three contigs were defined: DAB, DCB, and DBB. The 350-kb-long DAB contig contained only four genes: DDB, DAB, SLC7A4, and DAA. The 150-kb-long DCB contig contained the DCB, DCA, and fz10 genes at an undetermined distance from the DAB contig. And the 120-kb-long DBB contig comprised the DBB gene presumably in another linkage group. The low gene density of the linkage group 8 contigs, contrasting with the high gene density of the zebrafish class I region, and the close association with genes [ SLC7A4 coding for an amino acid transporter, and fz10 (frizzled 10) coding for a receptor of the WNT glycoprotein] that are not linked with the tetrapod Mhc, is interpreted to mean that the separation of the class II from class I loci in teleosts occurred by translocation rather than by genomic or chromosomal duplication.