Evolution of the immune repertoire with and without somatic DNA recombination.

Repertoire of an immune system is a set of antigen receptors each having a unique specificity to bind an antigen. In many vertebrate species, antigen receptors are produced via combinatorial arrangements of DNA segments in specialized immune cells. Due to this molecular mechanism, repertoire of vertebrate species is potentially very large. The diversity of repertoire is thought to guarantee recognition of most ill-causing micro-organisms. In vertebrate species however, similar editing of DNA segments has not been demonstrated to take place. Immune system of invertebrate species therefore seems to operate in a distinct manner from that of vertebrate species. Using an evolutionary model in which organisms struggle to fight infections, we attempt to understand why some species use a more diverse set of antigen receptors than others. Individuals in our model either use somatic DNA recombination to produce antigen receptors (as in vertebrates) or do not use such a mechanism (as in vertebrates). We found that individuals having an invertebrate-like immune system came to employ only a few antigen receptors to recognize a set of pathogens whereas those with a vertebrate-like immune system use a larger set of more specific antigen receptors to recognize the same set of pathogens. Our interpretation of this finding is that because the genetics of the immune system imposed different constraints on the evolutionary process, two distinct recognition strategies have been adapted by these species.