Bracamonte, Seraina E.

English

Natural populations are exposed to a diverse community of parasites. As a consequence, hosts have evolved defence strategies. In jawed vertebrates, the major histocompatibility complex (MHC) is an essential part of the immune system. These genes show an exceptional polymorphism which is assumed to be maintained mainly by parasite-mediated selection but also by natural and demographic events. At the sequence level, this usually results in gene duplications and positive selection at sites involved in binding of parasite derived antigens. At the population level, balancing selection is assumed to maintain of a large and diverse allele pool. However, genetic drift can have a major impact on the evolution and population structure at these genes and selection might become undetectable in strongly differentiated populations. In the European eel on the other hand, there is extensive gene flow due to its near-panmictic life history, wherefore it might be a particularly suitable system to study the effects of natural selection on MHC evolution in the absence of confounding neutral processes. In this species, up to four expressed MHC IIA and MHC IIB loci were isolated, suggesting the presence of at least two potentially functional loci. Both genes show  signs of positive selection and recombination, indicating that natural selection was an important factor for their evolution. However, in contrast to the current view, MHC IIA seemed to experience stronger selection than MHC IIB. Nevertheless, an extraordinary variability of MHC IIB alleles was found in a large population survey. The different populations did not show signs of divergent selection on the MHC pool. However, the differences did not correlate with neutral distances, suggesting some form of selection on MHC but its drivers remain elusive

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