Molecular evolution of the vertebrate immune system

Abstract

Adaptive immunity is unique to the vertebrates, and the molecules involved (including immunoglobulins, T cell receptors and the major histocompatibility complex molecules) seem to have diversified very rapidly early in vertebrate history. Reconstruction of gene phylogenies has yielded insights into the evolutionary origin of a number of molecular systems, including the complement system and the major histocompatibility complex (MHC). These analyses have indicated that the C5 component of complement arose by gene duplication prior to the divergence of C3 and C4, which suggests that the alternative complement pathway was the first to evolve. In the case of the MHC, phylogenetic analysis supports the hypothesis that MHC class II molecules evolved before class I molecules. The fact that the MHC-linked proteasome components that specifically produce peptides for presentation by class I MHC appear to have originated before the separation of jawed and jawless vertebrates suggests that the MHC itself may have been present at this time. Immune system gene families have evolved by gene duplication, interlocus recombination and (in some cases) positive Darwinian selection favoring diversity at the amino acid level.

Two Variable Regions in Carcinoembryonic Antigen-related Cell Adhesion Molecule1 N-terminal Domains Located in or Next to Monoclonal Antibody and Adhesion Epitopes Show Evidence of Recombination in Rat but Not in Human

“…If this locus were evolving under a simple model of purifying selection, one would expect that amino acid mutations would have a negative impact on the functional aspects of the gene product and that synonymous mutations would be more common. An alternative model that is more consistent with the data is one where amino acid changes have some positive effect, a conclusion similar to that of Hughes and Nei (66) and more recent studies of proteins involved in antigen recognition (67). Significant in this regard is the observation that a number of the amino acids involved in the Ig fold (67) were conserved in all of the N-domains.…”

“…An alternative model that is more consistent with the data is one where amino acid changes have some positive effect, a conclusion similar to that of Hughes and Nei (66) and more recent studies of proteins involved in antigen recognition (67). Significant in this regard is the observation that a number of the amino acids involved in the Ig fold (67) were conserved in all of the N-domains. In addition, the ␤-strand locations in the N-domain of CEACAM11 and CEACAM1, which have only a 48% sequence similarity, are virtually identical based on secondary structure predictions using the PSA Sequence Analysis Server from the Biomolecular Engineering Research Center.…”

Evolution in the Fast Lane: Rapidly Evolving Sex-Related Genes in Drosophila

“…The frequent observation of rapid and often adaptive evolution in a select set of genes, such as SRR and immune system genes across different taxonomic groups (Hughes and Yeager 1997;Singh and Kulathinal 2000;, suggests vast heterogeneity in rates of molecular evolution across any genome. In view of the biological species concept (Dobzhansky 1951;Mayr 1963), rapidly evolving SRR genes have received much attention from evolutionary biologists due to their potential role in reproductive isolation.…”

Evolutionary dynamics of the immunoglobulin heavy chain variable region genes in vertebrates

“…The advantage of this organization is thought to be in the facilitation of combinatorial diversification of antibodies (Litman et al 1993). The repertoire of IGHV genes is produced by the combination of gene duplication and the divergence of duplicate genes (Hughes and Yeager 1997;Ota and Nei 1994). Hence, the evolution of the IGHV genes can be explained by two evolutionary processes: the birth-and-death process and diversifying selection (Ota and Nei 1994).…”