Evolution of antibody diversity in bony vertebrates

This paper is a rapid description of,the different systems which, in I,oste ichthyan vertebrates, led to. antibody diversity. Two in major groups can t herefore be distinguished. In the first one, diversity is., mainly obtained by the combinatorial recombination of a large number of the genomic DNA se gments that encode the V, (D) and J regions of variable domains of immunogl obulin (Ig) chains. This grout, p includes man, mouse, amphibians and actin opterygians. In the second group, combinatorial diversity cannot operates e fficiently, due to the loss of the majority of the genomic V segments. Dive rsity is thus. obtained by alter native strategies, like gene conversion or somatic hypermutation that, to a lesser extend, also operate in the first group. This second group includes chicken, rabbit, sheep, ox and pig. These two groups obviously have nothing to do with phylogeny. The phylogenetic d isparity of the second group suggests that their immunity systems evolved i ndependently I in each domesticated species. As a working hypothesis, we pr opose that the situation of the second group could be a side effect of arti ficial selection. All these species are domesticated farm animals which hav e been selected to improve some phenotypic characters, A possible effect of this selection would be a drastic loss of V genes. This loss of combinator ial diversity was then compensated by alternative strategies of antibody di versification. This hypothesis may help, to elucidate a complex situation a nd could easily be tested, by analyzing the Ig repertoires in the wild and comparing them to the ones found in,species of the, second group.