The study has changed fundamental ideas about the evolution of the immune system in vertebrates. The international team, led by researchers from the University of Oslo, found that an important component of the Atlantic cod immune system is absent. In other vertebrates, including humans, this component helps fight disease due to bacterial infections and parasites.
The overall aim of this initiative was to obtain the entire genome sequence of the Atlantic cod by utilising new technology, says Professor Kjetill S. Jakobsen at the Centre for Ecological and Evolutionary Synthesis (CEES) University of Oslo, who led the team of researchers.
We had not expected to find that Atlantic cod has lost such a crucial component of its immune system. Prof. Jakobsen explains: All vertebrates, including humans, have a complex immune system that fights disease. This immune system, like the genetic code, originated only once in evolutionary history. Therefore, all vertebrates share similar components of this system and we expected that Atlantic cod possessed a minimum set of genes responsible for these components. Nevertheless, despite some very hard searching, we concluded that some genes were missing.
In fact, Atlantic cod has lost the genes that are essential for the function of the major histocompatibility complex (MHC) II pathway. In humans, the malfunction of this pathway leads to severe disease or even death. Atlantic cod, however, is not exceptionally susceptible to disease under natural conditions. Interestingly, other genes in the Atlantic cod genome are far more numerous than expected.
For example, large increase in the numbers of MHC I genes and Toll-like receptor (TLR) genes were found, indicating that Atlantic cod relies relatively more on these genes for its immune response and has developed unique mechanisms to deal with bacterial infections.
Knowledge of such alternative mechanisms will greatly help understanding of the evolution of immune function in vertebrates and even humans, and will allow for more targeted vaccine development aiding disease management and the process of domestication of Atlantic cod.