Expansion and contraction of major histocompatibility complex genes: a teleostean example

The MHC is a multigene family that has arisen through recurrent expansion and contraction of genes, and a continuum of the evolutionary process is observed in the teleost fishes. The number of duplicated genes observed in different phylogenetic groups of teleost fish varies from one to 42, with only a few genes observed in the primitive euteleost species, and greater numbers of genes observed in the more advanced neoteleost species. In this study, an attempt is made to isolate all of the Mhc class I genes of an early neoteleost species, Atlantic cod (Gadus morhua L.), in the superorder Paracanthopterygii. Eighty-three sequences were isolated from the cDNA of an individual G. morhua. The level of gene duplication observed within each of the lineages and sublineages was similar, and most contained an estimated two to four duplicated genes. Mhc class I gene duplication in G. morhua was independent of, and possibly more recent than, extensive duplication in the Acanthopterygian superorder. Only limited contraction of Mhc genes is observed in G. morhua. A low level of haplotype diversity is observed, with most individuals containing at least one copy of each of the lineages tested. Divergence of the conserved N- and C-terminal residues of the antigen recognition site is observed, indicative of the initial stage of degeneration from classical to non-classical genes. However, most or all of the lineages are still polymorphic, and degeneration is present both within and among lineages. Thus, the outcome (i.e., which genes will remain classical) is as yet undetermined.