Major histocompatibility complex genes in the common carp, Cyprinus carpio L.

This thesis describes a study of the major histocompatibility complex (Mhc) genes of the common carp (Cyprinus carpio L.). The molecules encoded by Mhc genes play an essential role in the specific immune response, by presenting antigens to T lymphocytes. Knowledge of the Mhc of carp, therefore, contributes to our understanding of the immune response mechanisms in this species. In addition, it may give important insights in the phylogenetic development of these genes. The common carp was found to contain several distinct lineages of Mhc class I genes, denoted as Cyca-U, Cyca-Z, Cyca- TC16 and Cyca- C4. The Cyca-U sequences probably represent classical Mhc class I genes, of which most likely only a single locus is expressed in each individual. Cyca-Z genes are present in multiple polymorphic copies in the genome, but it is not clear whether these genes are expressed. The sequence of Cyca- TC16 is most similar to the class I genes of the coelacanth, a fish which is thought to be a representative of the evolutionary lineage leading to the tetrapods. It is, however, not clear whether Cyca -TC16 is expressed. In addition, the sequence encoding carp β ₂ -microglobulin was isolated. Although two B2m genes were detected in each individual, apparently only one of these is expressed. In contrast, at least four class II B genes may be expressed in a single animal. These genes are linked in two pairs, which, however, segregate independently. In addition, two expressed class II A sequences were identified, most likely derived from two separate loci. Both the class II A and B genes are likely to encode bona fide class II chains, the components of the cell-surface class II α-βheterodimer. Although carp thus possess a complete set of class I and class II genes, it is not yet clear whether these genes reside in a single genetic Mhc region.