Maintenance of MHC polymorphism

Genetic Variation of the MHCDQBLocus in the Finless Porpoise (Neophocaena phocaenoides)

The Major Histocompatibility Complex (MHC) is a large multigene coding for glycoproteins that play a key role in the initiation of immune responses in vertebrates. The exon 2 region of the MHC DQB locus was analyzed using 160 finless porpoises from 5 populations in Japanese waters. The 5 populations were based on a previous mitochondrial DNA control region analysis, which showed distinct geographical separation. Eight DQB alleles were detected, and the geographical distribution of the alleles indicated that most of them are shared among the populations. Heterozygosity of the DQB alleles in each population ranged from 0.55 to 0.78, and for all 5 populations was 0.78. Low MHC variability is not a common feature in marine mammals, but the finless porpoise populations inhabiting coastal waters had a relatively high MHC heterozygosity. Balancing selection in the MHC DQB alleles of the finless porpoise was indicated by the higher rate of nonsynonymous than synonymous substitutions for PBR; however, an excess of hetrozygotes compared to expectation was not observed. This suggests that the MHC DQB locus in the finless porpoise may have been under balancing selection for a long evolutionary time period, and is influenced by genetic drift beyond the effect of balancing selection for short time periods in small local populations.

The evolution of immune-related genes from disease carrying mosquitoes: diversity in a peptidoglycan- and a thioester-recognizing protein

Adaptive polymorphism may be common in immune system genes as co-evolutionary interactions foster diversity; either through ongoing positive selection (arms races), or balancing selection. DNA sequence diversity in two putative immune system genes was examined in species of the genus Anopheles and from Aedes aegypti. For one gene, encoding the peptidoglycan recognizing protein PGRPLB, there was evidence of purifying selection, suggesting that selection acts to eliminate sequence variation. For another gene, encoding the thioester-containing protein TEP3, higher levels of amino acid replacement were found than would be expected under neutral models of evolution - an indication that this gene has been subject to repeated bouts of positive selection.

Molecular evolution of daphnia immunity genes: polymorphism in a gram-negative binding protein gene and an alpha-2-macroglobulin gene

Studies of DNA polymorphism have shown that some immune system genes of mammals and plants are exceptionally diverse, indicating that coevolution between these taxa and their parasites mediates positive selective sweeps and/or balancing selection. The genes of the arthropod immune system remain comparatively unstudied. We isolated two putative immune system genes from the cladoceran crustacean Daphnia and examined DNA sequence diversity. For one gene, encoding a putative gram-negative binding protein, we found evidence of only purifying selection, indicating that this gene is under strong functional constraint and that selection acts to eliminate amino acid variation. For another gene, encoding a putative alpha-2-macroglobulin, we found evidence of positive selection, indicating the possible involvement of this gene in a host-parasite arms race. We discuss the assumed function of these genes and offer speculation regarding which components of the arthropod immune system might experience diversifying adaptive evolution.

Population biology of antigen presentation by MHC class I molecules

In principle, the function of major histocompatibility complex (MHC) molecules is simple: to bind a peptide and engage a T cell. In practice, placing this function within the context of the immune response begs questions of population biology; How does the immune response emerge from the interactions among populations of peptides, T cells and MHC molecules? Within a population of vertebrates, how does MHC polymorphism stamp individuality on the response? Does polymorphism confer differential advantages in responding to parasites? How are the pressures on the MHC reflected in turnover of alleles? The role of mutation, recombination, selection, and drift in the generation and maintenance of MHC class 1 polymorphism are considered.

The myth of Eve: molecular biology and human origins

It has been proposed that modern humans descended from a single woman, the "mitochondrial Eve" who lived in Africa 100,000 to 200,000 years ago. The human immune system DRB1 genes are extremely polymorphic, with gene lineages that coalesce into an ancestor who lived around 60 million years ago, a time before the divergence of the apes from the Old World monkeys. The theory of gene coalescence suggests that, throughout the last 60 million years, human ancestral populations had an effective size of 100,000 individuals or greater. Molecular evolution data favor the African origin of modern humans, but the weight of the evidence is against a population bottleneck before their emergence. The mitochondrial Eve hypothesis emanates from a confusion between gene genealogies and individual genealogies.

Human leukocyte antigens and natural selection by malaria

The extraordinary polymorphism of human leukocyte antigens (HLA) poses a question as to how this remarkable diversity arose and is maintained. The explanation that infectious pathogens are largely responsible is theoretically attractive but clear and consistent associations between HLA alleles and major infectious diseases have rarely been identified. Large case-control studies of HLA types in African children with severe malaria indicate that HLA associations with this parasitic infection do exist and it is becoming possible to investigate the underlying mechanisms by identification of peptide epitopes in parasite antigens. Such analysis reveals how the magnitude and detectability of HLA associations may be influenced by numerous genetic and environmental factors. These complex interactions will give rise to variation over time and space in the selective pressures exerted by infectious diseases and this fluctuation may, in itself, contribute to the maintenance of HLA polymorphism.