THE MAJOR HISTOCOMPATIBILITY COMPLEX IN FISH

The first major histocompatibility complex(MHC) molecule in fish was i dentified in 1990 using a polymerase chain reaction strategy with dege nerate primers (thereby avoiding the problem of low sequence similarit y between higher vertebrates and fish). Since that time, MHC in fish h as attracted much attention, partly due to evolutionary aspects and pa rtly because of the potential practical consequences for future fish b reeding. Knowledge of the MHC molecules in fish has thus been growing extremely rapidly. All current data point to a functional MHC system i n fish. There seem to be many loci and many alleles for both classes o f MHC molecules, but a lack of knowledge about functional loci versus pseudogenes and non-classical loci is making it difficult at present t o establish the exact number found within each species. Fish create a few surprises, such as the lack ota link between class I and class ii regions in at least two teleost species. Data on other molecules which are physically or functionally linked to the MHC genes are currently emerging. To date, no studies have established the functional implicat ions of the MHC molecules in fish but, considering the molecular data available, such an achievement will undoubtedly occur soon. Fish may w ell offer a unique opportunity for identifying correlations of MHC wit h resistance or susceptibility to diseases.