The nature of selection on the major histocompatibility complex

Inability of bm14 mice to respond to Theiler's murine encephalomyelitis virus is caused by defective antigen presentation, not repertoire selection

Natural selection drives diversification of MHC class I proteins, but the mechanism by which selection for polymorphism occurs is not known. New variant class I alleles differ from parental alleles both in the nature of the CD8 T cell repertoire formed and the ability to present pathogen-derived peptides. In the current study, we examined whether T cell repertoire differences, Ag presentation differences, or both account for differential viral resistance between mice bearing variant and parental alleles. We demonstrate that nonresponsive mice have inadequate presentation of viral Ag, but have T cell repertoires capable of mounting Ag-specific responses. Although previous work suggests a correlation between the ability to present an Ag and the ability to generate a repertoire responsive to that Ag, we show that the two functions of MHC class I are independent.

Genetic quality and sexual selection: an integrated framework for good genes and compatible genes

Why are females so choosy when it comes to mating? This question has puzzled and marveled evolutionary and behavioral ecologists for decades. In mating systems in which males provide direct benefits to the female or her offspring, such as food or shelter, the answer seems straightforward--females should prefer to mate with males that are able to provide more resources. The answer is less clear in other mating systems in which males provide no resources (other than sperm) to females. Theoretical models that account for the evolution of mate choice in such nonresource-based mating systems require that females obtain a genetic benefit through increased offspring fitness from their choice. Empirical studies of nonresource-based mating systems that are characterized by strong female choice for males with elaborate sexual traits (like the large tail of peacocks) suggest that additive genetic benefits can explain only a small percentage of the variation in fitness. Other research on genetic benefits has examined nonadditive effects as another source of genetic variation in fitness and a potential benefit to female mate choice. In this paper, we review the sexual selection literature on genetic quality to address five objectives. First, we attempt to provide an integrated framework for discussing genetic quality. We propose that the term 'good gene' be used exclusively to refer to additive genetic variation in fitness, 'compatible gene' be used to refer to nonadditive genetic variation in fitness, and 'genetic quality' be defined as the sum of the two effects. Second, we review empirical approaches used to calculate the effect size of genetic quality and discuss these approaches in the context of measuring benefits from good genes, compatible genes and both types of genes. Third, we discuss biological mechanisms for acquiring and promoting offspring genetic quality and categorize these into three stages during breeding: (i) precopulatory (mate choice); (ii) postcopulatory, prefertilization (sperm utilization); and (iii) postcopulatory, postfertilization (differential investment). Fourth, we present a verbal model of the effect of good genes sexual selection and compatible genes sexual selection on population genetic variation in fitness, and discuss the potential trade-offs that might exist between mate choice for good genes and mate choice for compatible genes. Fifth, we discuss some future directions for research on genetic quality and sexual selection.

Recombination and the origin of sequence diversity in the DRB MHC class II locus in chamois (Rupicapra spp.)

We examined the evolutionary processes contributing to genetic diversity at the major histocompatibility complex (MHC) class II DRB locus in chamois (Rupicapra spp., subfamily Caprinae). We characterised the pattern of intragenic recombination (or homologous gene conversion) and quantified the amount of recombination in the genealogical history of the two chamois species, Pyrenean chamois (Rupicapra pyrenaica) and Alpine chamois (Rupicapra rupicapra). We found evidence for intragenic recombination, and the estimated amount of population recombination suggests that recombination has been a significant process in generating DRB allelic diversity in the genealogical history of the genus Rupicapra. Moreover, positive selection appears to act on the same peptide-binding residues in both analysed chamois species, but not in identical intensity. Recombination coupled with positive selection drives the rapid evolution at the peptide-binding sites in the MHC class II DRB gene. Many chamois MHC class II DRB alleles are thus much younger than previously assumed.

Mate choice decisions of stickleback females predictably modified by MHC peptide ligands

Sexual selection has been proposed as one mechanism to explain the maintenance of high allelic diversity in MHC genes that control the extent of resistance against pathogens and parasites in natural populations. MHC-based sexual selection is known to involve olfactory mechanisms in fish, mice, and humans. During mate choice, females of the three-spined stickleback (Gasterosteus aculeatus) use an odor-based selection strategy to achieve an optimal level of MHC diversity in their offspring, equipping them with optimal resistance toward pathogens and parasites. The molecular mechanism of odor-based mate-selection strategies is unknown. Because peptide ligands for MHC class I molecules function as individuality signals in mice, we hypothesized that female sticklebacks might assess the degree of MHC diversity of potential partners by means of the structural diversity of the corresponding peptide ligands in perceived odor signals. We show that structurally diverse MHC ligands interact with natural odors of male sticklebacks to predictably modify MHC-related mate choice. For a mating pair with suboptimal numbers of MHC alleles, peptides increase the attractiveness of male water, whereas for a mating pair with superoptimal numbers, attractiveness is decreased. Our results suggest that female sticklebacks use evolutionarily conserved structural features of MHC peptide ligands to evaluate MHC diversity of their prospective mating partners.

Diverse evolutionary mechanisms shape the type III effector virulence factor repertoire in the plant pathogen Pseudomonas syringae

Many gram-negative pathogenic bacteria directly translocate effector proteins into eukaryotic host cells via type III delivery systems. Type III effector proteins are determinants of virulence on susceptible plant hosts; they are also the proteins that trigger specific disease resistance in resistant plant hosts. Evolution of type III effectors is dominated by competing forces: the likely requirement for conservation of virulence function, the avoidance of host defenses, and possible adaptation to new hosts. To understand the evolutionary history of type III effectors in Pseudomonas syringae, we searched for homologs to 44 known or candidate P. syringae type III effectors and two effector chaperones. We examined 24 gene families for distribution among bacterial species, amino acid sequence diversity, and features indicative of horizontal transfer. We assessed the role of diversifying and purifying selection in the evolution of these gene families. While some P. syringae type III effectors were acquired recently, others have evolved predominantly by descent. The majority of codons in most of these genes were subjected to purifying selection, suggesting selective pressure to maintain presumed virulence function. However, members of 7 families had domains subject to diversifying selection.

Patterns of variability at the major histocompatibility class II alpha locus in Atlantic salmon contrast with those at the class I locus

In order to investigate the mechanisms creating and maintaining variability at the major histocompatibility (MH) class II alpha (DAA) locus we examined patterns of polymorphism in two isolated Atlantic salmon populations which share a common post-glacial origin. As expected from their common origin, but contrary to the observation at the MH class I locus, these populations shared the majority of DAA alleles: out of 17 sequences observed, 11 were common to both populations. Recombination seems to play a more important role in the origin of new alleles at the class II alpha locus than at the class I locus. A greater than expected proportion of sites inferred to be positively selected (potentially peptide binding residues, PBRs) were found to be involved in recombination events, suggesting a mechanism for increasing MH variability through an interaction between recombination and natural selection. Thus it appears that although selection and recombination are important mechanisms for the evolution of both class II alpha and class I loci in the Atlantic salmon, the pattern of variability differs markedly between these classes of MH loci.

Genetic drift outweighs balancing selection in shaping post-bottleneck major histocompatibility complex variation in New Zealand robins (Petroicidae)

The Chatham Island black robin, Petroica traversi, is a highly inbred, endangered passerine with extremely low levels of variation at hypervariable neutral DNA markers. In this study we investigated variation in major histocompatibility complex (MHC) class II genes in both the black robin and its nonendangered relative, the South Island robin Petroica australis australis. Previous studies have shown that Petroica have at least four expressed class II B MHC genes. In this study, the sequences of introns flanking exon 2 of these loci were characterized to design primers for peptide-binding region (PBR) sequence analysis. Intron sequences were comprised of varying numbers of repeated units, with highly conserved regions immediately flanking exon 2. Polymerase chain reaction primers designed to this region amplified three or four sequences per black robin individual, and eight to 14 sequences per South Island robin individual. MHC genes are fitness-related genes thought to be under balancing selection, so they may be more likely to retain variation in bottlenecked populations. To test this, we compared MHC variation in the black robin with artificially bottlenecked populations of South Island robin, and with their respective source populations, using restriction fragment length polymorphism analyses and DNA sequencing of the PBR. Our results indicate that the black robin is monomorphic at class II B MHC loci, while both source and bottlenecked populations of South Island robin have retained moderate levels of variation. Comparison of MHC variation with minisatellite DNA variation indicates that genetic drift outweighs balancing selection in determining MHC diversity in the bottlenecked populations. However, balancing selection appears to influence MHC diversity over evolutionary timescales, and the effects of gene conversion are evident.

Frequency-dependent selection with dominance: a window onto the behavior of the mean fitness

Selection in which fitnesses vary with the changing genetic composition of the population may facilitate the maintenance of genetic diversity in a wide range of organisms. Here, a detailed theoretical investigation is made of a frequency-dependent selection model, in which fitnesses are based on pairwise interactions between the two phenotypes at a diploid, diallelic, autosomal locus with complete dominance. The allele frequency dynamics are fully delimited analytically, along with all possible shapes of the mean fitness function in terms of where it increases or decreases as a function of the current allele frequency in the population. These results in turn allow possibly the first complete characterization of the dynamical behavior by the mean fitness through time under frequency-dependent selection. Here the mean fitness (i) monotonically increases, (ii) monotonically decreases, (iii) initially increases and then decreases, or (iv) initially decreases and then increases as equilibrium is approached. We analytically derive the exact initial and fitness conditions that produce each dynamic and how often each arises. Computer simulations with random initial conditions and fitnesses reveal that the potential decline in mean fitness is not negligible; on average a net decrease occurs 20% of the time and reduces the mean fitness by >17%.

Genetic aspects of immune-mediated adverse drug effects

Adverse drug effects (ADEs) are of great importance in medicine and account for up to 5% of all hospital admissions. ADEs can arise from several mechanisms and a wide range of drugs can cause immune-mediated ADEs (IMADEs). For a drug to elicit an IMADE, it must be both immunogenic (that is, able to sensitize the immune system) and antigenic (that is, able to evoke a response from a sensitized immune system). Unlike protein therapeutics, small-molecule drugs (or xenobiotics) are usually neither immunogenic nor antigenic. IMADEs are therefore the result of complex interactions between drug-metabolizing enzymes, immune sensitization and immune effectors. The genetic aspects of this interplay are discussed in this review.

The Evolution of Alloimmunity and the Genesis of Adaptive Immunity

Infectious agents select for host immune responses that destroy infectious nonself yet maintain tolerance to self. Here we propose that retroviruses and other host-antigen associated pathogens (HAAPs) select for the genetic, biochemical, and cell biological properties of alloimmunity, also known as the histocompatibility or tissue rejection response. This hypothesis predicts the major observations regarding histocompatibility responses, including: (i) their existence in animals as diverse as sponges and humans; (ii) extreme polymorphism and balanced allele frequencies at histocompatibility loci, including the human MHC and blood group loci; (iii) the frequency dependent selection of histocompatibility alleles; (iv) the ancient age of many alloantigenic polymorphisms; (v) the high ratio of nonsynonymous mutations to synonymous mutations at histocompatibility loci; (vi) disassortative mating based on MHC alleles; (vii) the inability to explain the existence and continuing selection of histocompatibility alleles by other more conventional biochemical and genetic paradigms; and (viii) the susceptibility of HAAPs, particularly retroviruses such as HIV (human immunodeficiency virus), to histocompatibility reactions. In addition, the hypothesis that HAAPs select the forms and molecules of alloimmunity offers simple explanations for the evolution of histocompatibility systems over time, the initial selection of hypervariable immune mechanisms, and the genesis of adaptive immunity.

Choosing mates: good genes versus genes that are a good fit

Female choice for male ornamental traits is widely accepted as a mechanism by which females maximize their reproductive success and/or offspring quality. However, there is an increasing empirical literature that shows a fitness benefit of genetic diversity and a tendency for females to use genetic dissimilarity as a criterion for mate choice. This genetic compatibility hypothesis for female mate choice presents a paradox. How can females use both an absolute criterion, such as male ornamentation, and a relative criterion, such as genetic dissimilarity, to choose their mates? Here, we present potential solutions for this dilemma and the empirical evidence supporting them. The interplay between these two contrasting forms of female mate choice presents an exciting empirical and theoretical challenge for evolutionary ecologists.

Parasites and individual major histocompatibility complex diversity—an optimal choice?

Overdominant selection or heterozygote advantage can partly explain the extraordinary polymorphism found at classical major histocompatibility complex (MHC) loci. However, several studies employing only single infectious agents often failed to detect it. Here, we review recent studies suggesting that due to the dominant nature of MHC-mediated resistance, a heterozygote advantage is most likely to be detected in multiple pathogen challenges.

Major histocompatibility complex controls the trajectory but not host-specific adaptation during virulence evolution of the pathogenic fungus Cryptococcus neoformans

Genes of the major histocompatibility complex (MHC) play a critical role in immune recognition and are the most genetically diverse loci known. One hypothesis to explain this diversity postulates that pathogens adapt to common MHC haplotypes and thus favour selection of new or rare alleles. To determine whether the pathogenic yeast Cryptococcus neoformans adapts to MHC-dependent immune responses, it was serially passaged in two independent replicate lines of five B10 MHC-congenic strains and Balb/c mice. All passaged lines increased in virulence as measured by reduced host survival. MHC influenced the rate (trajectory) of virulence increase during passages as measured by significant differences in mortality rate (p < 0.001). However, when the post-passage strains were tested, no MHC differences in mortality rate remained and only minor differences in titres were observed. Also contrary to expectations, increased virulence in three lines passaged in B10 mice had a larger effect in Balb/c mice, and the evolution of virulence in lines passaged in alternating hosts was not retarded. To our knowledge, these data represent the first experimental test of MHC-specific adaptation in a non-viral pathogen. The failure to observe MHC effects despite dramatically increased virulence and host-genotype-specific adaptation to non-MHC genes suggests that escape of MHC-dependent immune recognition may be difficult for pathogens with unlimited epitopes or that other virulence factors can swamp MHC effects.

Evolution of MHC-DRB class II polymorphism in the genus Apodemus and a comparison of DRB sequences within the family Muridae (Mammalia: Rodentia)

Allelic diversity at major histocompatibility complex (MHC) genes is thought to be maintained by balancing selection over long periods of time, even across multiple speciation events. Trans-species sharing of MHC alleles among genera has been supported by many studies on mammals and fish, but in rodents, the results are ambiguous. We investigated natural levels of MHC- DRB variability and evolutionary processes in the wood mouse (Apodemus sylvaticus) and the yellow-necked mouse (Apodemus flavicollis), which are common, sympatric murid rodents in European forests. Using single-strand conformation polymorphism analysis and DNA sequencing, 38 DRB exon 2 alleles were detected among 162 A. sylvaticus from nine different locations in Germany and Switzerland, and 15 DRB exon 2 alleles were detected among 60 A. flavicollis from three different locations in northern Germany. There was evidence for balancing selection in both species. Phylogenetic analysis, including additional murid taxa, showed that the DRB exon 2 sequences did not separate according to species, consistent with trans-species evolution of the MHC in these taxa.

Primer effects by conspecific odors in house mice: a new perspective in the study of primer effects on reproductive activities

Half a century has passed since the first report of the influence of odors on mice. Odors are known to influence behavior (signaling effect) and affect the physiology (primer effect) of mice. This review focuses on summarizing the primer effects found so far in female and male mice. Odors from conspecifics of the opposite sex had the tendency to enhance reproductive activities, whereas odors from conspecifics of the same sex diminish them. Only 2,5-dimethylpyrazine, one of the odor components identified in group-housed females, has been reported to have a suppressive influence on both males and females by lowering reproductive activities. Studies showed progress from the discovery of phenomena to the identification of odor components that cause such changes in reproductive organs and related neuroendocrinological changes. Compared to studies on the mechanisms of primer effects in females, the mechanisms in males are not yet clarified, and detailed studies on effects on the reproductive organs are still in primitive stages especially for males. Hypotheses on the influence of changes in the concentration of testosterone, estrogen, and prolactin on spermatogenesis and sperm maturation after exposure to odors are discussed.

Costs of immunity: immune responsiveness reduces survival in a vertebrate

Immune defences are undoubtedly of great benefit to the host, reducing the impact of infectious organisms. However, mounting immune responses also entails costs, which may be measured by inducing immune responses against artificial infections. We injected common eider (Somateria mollissima) females with three different non-pathogenic antigens, sheep red blood cells (SRBC), diphtheria toxoid and tetanus toxoid, early in their incubation period. In the group of females that mounted a humoral immune response against SRBC, the return rate was only 27%, whereas the group of females that did not mount a response against SRBC had a return rate of 72%. Moreover, responding against diphtheria toxoid when also responding against SRBC led to a further reduction in return rate. These results are repeatable, as the same effect occurred independently in two study years. The severely reduced return rate of females producing antibodies against SRBC and diphtheria toxoid implies that these birds experienced considerably impaired long-term survival. This study thus documents severe costs of mounting humoral immune responses in a vertebrate. Such costs may explain why many organisms suppress immunity when under stress or when malnourished, and why infections may sometimes be tolerated without eliciting immune responses.

Major histocompatibility complex diversity influences parasite resistance and innate immunity in sticklebacks

Proteins of the major histocompatibility complex (MHC) play a central role in the presentation of antigens to the adaptive immune system. The MHC also influences the odour-based choice of mates in humans and several animal taxa. It has recently been shown that female three-spined sticklebacks (Gasterosteus aculeatus) aim at a moderately high MHC diversity in their offspring when choosing a mate. Do they optimize the immune systems of their offspring? Using three-spined sticklebacks that varied in their individual numbers of MHC class IIB molecules, we tested, experimentally, whether allelic diversity at the MHC influences parasite resistance and immune parameters. We found that sticklebacks with low MHC diversity suffered more from parasite infection after experimental exposure to Schistocephalus solidus tapeworms and Glugea anomala microsporidians. They also showed the highest proportion of granulocytes and the strongest respiratory burst reaction, which are correlates of innate immunity. This indicates a strong activity of the innate immune system after challenge by parasites when MHC diversity is suboptimal. Individuals with very high allelic diversity at the MHC seemed inferior to those with moderately high diversity. Such a pattern is consistent with theoretical expectations of an optimal balance between the number of recognizable antigens and self-tolerance.

MHC-linked susceptibility to a bacterial infection, but no MHC-linked cryptic female choice in whitefish

Non-random gamete fusion is one of several potential cryptic female choice mechanisms that have been postulated and that may enhance the survival probability of the offspring. Previous studies have found that gamete fusion in mice is influenced by genes of the major histocompatibility complex (MHC) region. Here we test (i) whether there is MHC-dependent gamete fusion in whitefish (Coregonus sp.) and (ii) whether there is a link between the MHC and embryo susceptibility to an infection by the bacterium Pseudomonas fuorescens. We experimentally bred whitefish and reared sibships in several batches that either experienced or did not experience strong selection by P. fluorescens. We then determined the MHC class II B1 genotype of 1016 surviving larvae of several full sibships. We found no evidence for MHC-linked gamete fusion. However, in one of seven sibships we found a strong connection between the MHC class II genotype and embryo susceptibility to P. fluorescens. This connection was still significant after correcting for multiple testing. Hence, the MHC class II genotype can considerably influence embryo survival in whitefish, but gamete fusion seems to be random with respect to the MHC.

Diversity of Mhc class I and IIB genes in house sparrows (Passer domesticus)

In order to understand the expression and evolution of host resistance to pathogens, we need to examine the links between genetic variability at the major histocompatibility complex ( Mhc), phenotypic expression of the immune response and parasite resistance in natural populations. To do so, we characterized the Mhc class I and IIB genes of house sparrows with the goal of designing a PCR-based genotyping method for the Mhc genes using denaturing gradient gel electrophoresis (DGGE). The incredible success of house sparrows in colonizing habitats worldwide allows us to assess the importance of the variability of Mhc genes in the face of various pathogenic pressures. Isolation and sequencing of Mhc class I and IIB alleles revealed that house sparrows have fewer loci and fewer alleles than great reed warblers. In addition, the Mhc class I genes divided in two distinct lineages with different levels of polymorphism, possibly indicating different functional roles for each gene family. This organization is reminiscent of the chicken B complex and Rfp-Y system. The house sparrow Mhc hence appears to be intermediate between the great reed warbler and the chicken Mhc, both in terms of numbers of alleles and existence of within-class lineages. We specifically amplified one Mhc class I gene family and ran the PCR products on DGGE gels. The individuals screened displayed between one and ten DGGE bands, indicating that this method can be used in future studies to explore the ecological impacts of Mhc diversity.

MHC polymorphism under host-pathogen coevolution

The genes encoding major histocompatibility (MHC) molecules are among the most polymorphic genes known for vertebrates. Since MHC molecules play an important role in the induction of immune responses, the evolution of MHC polymorphism is often explained in terms of increased protection of hosts against pathogens. Two selective pressures that are thought to be involved are (1) selection favoring MHC heterozygous hosts, and (2) selection for rare MHC alleles by host-pathogen coevolution. We have developed a computer simulation of coevolving hosts and pathogens to study the relative impact of these two mechanisms on the evolution of MHC polymorphism. We found that heterozygote advantage per se is insufficient to explain the high degree of polymorphism at the MHC, even in very large host populations. Host-pathogen coevolution, on the other hand, can easily account for realistic polymorphisms of more than 50 alleles per MHC locus. Since evolving pathogens mainly evade presentation by the most common MHC alleles in the host population, they provide a selective pressure for a large variety of rare MHC alleles. Provided that the host population is sufficiently large, a large set of MHC alleles can persist over many host generations under host-pathogen coevolution, despite the fact that allele frequencies continuously change.

Heterozygote advantage fails to explain the high degree of polymorphism of the MHC

Major histocompatibility (MHC) molecules are encoded by extremely polymorphic genes and play a crucial role in vertebrate immunity. Natural selection favors MHC heterozygous hosts because individuals heterozygous at the MHC can present a larger diversity of peptides from infectious pathogens than homozygous individuals. Whether or not heterozygote advantage is sufficient to account for a high degree of polymorphism is controversial, however. Using mathematical models we studied the degree of MHC polymorphism arising when heterozygote advantage is the only selection pressure. We argue that existing models are misleading in that the fitness of heterozygotes is not related to the MHC alleles they harbor. To correct for this, we have developed novel models in which the genotypic fitness of a host directly reflects the fitness contributions of its MHC alleles. The mathematical analysis suggests that a high degree of polymorphism can only be accounted for if the different MHC alleles confer unrealistically similar fitnesses. This conclusion was confirmed by stochastic simulations, including mutation, genetic drift, and a finite population size. Heterozygote advantage on its own is insufficient to explain the high population diversity of the MHC.

HLA class II diversity in seven Amerindian populations. Clues about the origins of the Ache

The study of the HLA variability of Native American populations revealed several alleles specific to one or more of the Latin American indigenous populations. The analysis of Amerindian groups distributed all over the continent might inform about the area of origin and the dispersal of these alleles and shed light on the evolution of this remarkable polymorphism. Moreover, HLA alleles and haplotypes are excellent markers to understand the genetic relationships between populations. For these reasons, we characterized the HLA class II polymorphism in seven South American Amerindian populations and compared the results with those previously reported for other Amerindian groups. The Guarani-Kaiowá (n = 160) and Guarani-Nandeva (n = 87) were from the Brazilian state of Mato Grosso do Sul, the Guarani-M'byá (n = 93) and Kaingang (n = 235) from Paraná state, the Aché (n = 89) from eastern Paraguay, the Quechua (n = 44) from Andean Peru. From Amazonia, a heterogeneous group was analyzed (n = 45). The most frequent alleles and haplotypes are common also in other Amerindian populations. Each HLA-DRB1 allele was typically found in combination with just one DQA1-DQB1 haplotype, most likely as a result of some form of random genetic drift and reduced gene flow from non-Amerindians. The frequency distribution differed significantly among all populations, although differences were less pronounced between the Guarani subgroups. Marker alleles allowed an estimate of European and sub-Saharan African gene flow into these populations: Quechua 23%, Guarani-Nandeva 14%, Kaingang 7%, Guarani-M'byá 4%, Guarani-Kaiowá, Amazonia, and Aché 0%. Interestingly, the DRB1*1413 allele, previously found only among the Guarani-M'byá (frequency 15%), appeared in the Aché (8%). The relationship of the Aché to other Amerindian populations is unclear, and this finding reveals a link with the Guarani. On the basis of genetic distance and the HLA allele/haplotype set, we propose that the Aché are differentiated Tupi-Guarani group, most closely related to the Guarani-M'byá.

Parasitic exploitation as an engine of diversity

Parasitic exploitation occurs within and between a wide variety of taxa in a plethora of diverse contexts. Theoretical and empirical analyses indicate that parasitic exploitation can generate substantial genetic and phenotypic polymorphism within species. Under some circumstances, parasitic exploitation may also be an important factor causing reproductive isolation and promoting speciation. Here we review research relevant to the relationship between parasitic exploitation, within species-polymorphism, and speciation in some of the major arenas in which such exploitation has been studied. This includes research on the vertebrate major histocompatibility loci, plant-pathogen interactions, the evolution of sexual reproduction, intragenomic conflict, sexual conflict, kin mimicry and social parasitism, tropical forest diversity and the evolution of language. We conclude by discussing some of the issues raised by comparing the effect of parasitic exploitation on polymorphism and speciation in different contexts.

Geographic patterns of functional categories of HLA-DRB1 alleles: a new approach to analyse associations between HLA-DRB1 and disease

Because specific amino acids found within the peptide-binding cleft of human leukocyte antigen (HLA) molecules have been implicated in HLA/disease associations, an approach which consists in grouping the alleles according to their functional properties at the protein level may enable us to better understand HLA associations than the conventional allelic classification. In this study, we applied this methodology to investigate the associations between HLA-DRB1 and rheumatoid arthritis. The alleles were first classified into seven functional categories [restrictive supertype patterns (RSPs)], among which three were known to be significantly associated with susceptibility (one category) or resistance (two categories) to rheumatoid arthritis. The frequencies of these categories were then estimated in 104 population samples previously tested for HLA-DRB1, and their variability was analysed spatially on a worldwide scale by applying an original methodology for detecting discontinuities in geographically patterned data. RSP frequencies were also compared to known values of rheumatoid arthritis prevalence in some populations. The results indicated that the three RSP frequency distributions were geographically structured, and that these patterns could generally be explained by the history of human migrations. However, the peculiar pattern observed for RSP 'A' (conferring susceptibility to rheumatoid arthritis) indicated a possible association with some latitude-dependent disease. Furthermore, the very high correlation coefficient found between RSP 'A' frequencies and rheumatoid arthritis prevalence confirmed the significant disease association of this functional category. In contrast, the putative protective effect of the other RSPs ('De' and 'Q') was not detectable at the worldwide level, but may be significant in specific geographic areas. This study shows that population genetic diversity analyses based on a functional grouping of HLA alleles provide an efficient way to explore the mutual influence of HLA genetic variation and disease.

MHC-congenic mice (C57BL/6J and B6-H-2K) show differences in speed but not accuracy in learning the Hebb-Williams Maze

We compared spatial learning and memory in male and female mice of two MHC-congenic strains (C57BL/6J and B6-H-2K) in two versions of the Hebb-Williams Maze. In the food-reward paradigm, males required fewer sessions to learn than females, but there were no strain differences in acquisition. There were no strain or sex differences in the number of errors during the test phase, but the B6-H-2K mice reached the goal box faster than the C57BL/6J mice. In the water-escape paradigm, the C57BL/6J mice required more sessions than the B6-H-2K mice during acquisition. There were no strain or sex differences in the number of errors or in the latency to swim to the goal box in the test phase of the water-escape task. There were no significant correlations between the number of sessions to learn the two mazes; the number of errors made or the latencies to reach the goal box in each maze. These results indicate that these two strains show differences in performance in the Hebb-Williams Maze, but do not differ in cognitive ability.

From Eden to a hell of uniformity? Directed evolution in humans

For the first time during evolution of life on this planet, a species has acquired the ability to direct its own genetic destiny. Following 200,000 years of evolution, modern man now has the technologies not only to eradicate genetic disease but also to prolong life and enhance desired physical and mental traits. These technologies include preimplantation diagnosis, cloning, and gene therapy in the germline on native chromosomes or by adding artificial ones. At first glance, we should all be in favor of eliminating genetic diseases and enhancing genetic traits. Evolutionary considerations, however, uncover hidden dangers and suggest caution against the total embracement of such actions. The first major concern is that the genome will never be a completely reliable crystal ball for predicting human phenotypes. This is especially true for predictions concerning the performance of alleles in future generations whose populations might be subjected to different environmental and social challenges. The second, and perhaps more important, concern is that the end result of germline intervention and genetic enhancement will likely lead to the impoverishment of gene variants in the human population and deprive us of one of our most valued assets for survival in the future, our genetic diversity.

MHC studies in nonmodel vertebrates: what have we learned about natural selection in 15 years?

Elucidating how natural selection promotes local adaptation in interaction with migration, genetic drift and mutation is a central aim of evolutionary biology. While several conceptual and practical limitations are still restraining our ability to study these processes at the DNA level, genes of the major histocompatibility complex (MHC) offer several assets that make them unique candidates for this purpose. Yet, it is unclear what general conclusions can be drawn after 15 years of empirical research that documented MHC diversity in the wild. The general objective of this review is to complement earlier literature syntheses on this topic by focusing on MHC studies other than humans and mice. This review first revealed a strong taxonomic bias, whereby many more studies of MHC diversity in natural populations have dealt with mammals than all other vertebrate classes combined. Secondly, it confirmed that positive selection has a determinant role in shaping patterns of nucleotide diversity in MHC genes in all vertebrates studied. Yet, future tests of positive selection would greatly benefit from making better use of the increasing number of models potentially offering more statistical rigour and higher resolution in detecting the effect and form of selection. Thirdly, studies that compared patterns of MHC diversity within and among natural populations with neutral expectations have reported higher population differentiation at MHC than expected either under neutrality or simple models of balancing selection. Fourthly, several studies showed that MHC-dependent mate preference and kin recognition may provide selective factors maintaining polymorphism in wild outbred populations. However, they also showed that such reproductive mechanisms are complex and context-based. Fifthly, several studies provided evidence that MHC may significantly influence fitness, either by affecting reproductive success or progeny survival to pathogens infections. Overall, the evidence is compelling that the MHC currently represents the best system available in vertebrates to investigate how natural selection can promote local adaptation at the gene level despite the counteracting actions of migration and genetic drift. We conclude this review by proposing several directions where future research is needed.

Major histocompatibility complex heterozygote superiority during coinfection

Genes of the major histocompatibility complex (MHC) play a critical role in immune recognition, and many alleles confer susceptibility to infectious and autoimmune diseases. How these deleterious alleles persist in populations is controversial. One hypothesis postulates that MHC heterozygote superiority emerges over multiple infections because MHC-mediated resistance is generally dominant and many allele-specific susceptibilities to pathogens will be masked by the resistant allele in heterozygotes. We tested this hypothesis by using experimental coinfections with Salmonella enterica (serovar Typhimurium C5TS) and Theiler's murine encephalomyelitis virus (TMEV) in MHC-congenic mouse strains where one haplotype was resistant to Salmonella and the other was resistant to TMEV. MHC heterozygotes were superior to both homozygotes in 7 out of 8 comparisons (P = 0.0024), and the mean standardized pathogen load of heterozygotes was reduced by 41% over that of homozygotes (P = 0.01). In contrast, no heterozygote superiority was observed when the MHC haplotype combinations had similar susceptibility profiles to the two pathogens. This is the first experimental evidence for MHC heterozygote superiority against multiple pathogens, a mechanism that would contribute to the evolution of MHC diversity and explain the persistence of alleles conferring susceptibility to disease.

Enzyme kinetics, substitutable resources and competition: from biochemistry to frequency-dependent selection in lac

Trade-offs in catalytic efficiency at the lac permease of Escherichia coli produce alleles with different substrate specializations that are selectively favored on different galactosides. We show that differential resource utilization during competition for mixtures of galactosides produces frequency-dependent selection at lac. However, the polymorphism is protected only in a narrow range of galactoside ratios despite intense selection on the pure galactosides. Hence, stabilizing frequency-dependent selection protecting natural allozyme polymorphisms through differential resource utilization will be sporadic and ephemeral in randomly changing environments. A comparison of predictions, based on first principles, with experimental outcomes reveals an additional, unanticipated source of weak selection.

Multiple parasites are driving major histocompatibility complex polymorphism in the wild

Parasite mediated selection may result in arms races between host defence and parasite virulence. In particular, simultaneous infections from multiple parasite species should cause diversification (i.e. balancing selection) in resistance genes both at the population and the individual level. Here, we tested these ideas in highly polymorphic major histocompatibility complex (MHC) genes from three-spined sticklebacks (Gasterosteus aculeatus L.). In eight natural populations, parasite diversity (15 different species), and MHC class IIB diversity varied strongly between habitat types (lakes vs. rivers vs. estuaries) with lowest values in rivers. Partial correlation analysis revealed an influence of parasite diversity on MHC class IIB variation whereas general genetic diversity assessed at seven microsatellite loci was not significantly correlated with parasite diversity. Within individual fish, intermediate, rather than maximal allele numbers were associated with minimal parasite load, supporting theoretical models of self-reactive T-cell elimination. The optimal individual diversity matched those values female fish try to achieve in their offspring by mate choice. We thus present correlative evidence supporting the 'allele counting' strategy for optimizing the immunocompetence in stickleback offspring.

The immunogenomics of minor histocompatibility antigens

Minor histocompatibility (H) antigens are a diverse assemblage of major histocompatibility complex (MHC)-bound peptides with the unifying property of acting as alloantigens that induce allogeneic tissue rejection. They are a consequence of any form of accumulated genetic variation that translates to differential MHC-presented peptide epitopes, the most common form of which is simple sequence polymorphisms. The universe of potential minor H antigens is large when transplantation is performed between genetically unrelated, MHC-matched individuals, especially considering the remarkable discriminative sensitivity of T cells. However, the phenomenon of immunodominance greatly simplifies immune responses that ensue. One mouse minor H antigen, H60, stands out in that the preponderance of the CD8 T cell response elicited in a complex alloantigenic setting is directed against this single minor H antigen epitope. Its immunodominance is because mice lacking H60 develop an unusually robust T cell repertoire dedicated to this single minor H antigen. The now well-characterized mouse minor H antigen system should provide a vehicle to assess the degree to which immunodominant alloantigens contribute to transplant rejection.

Biological and biomedical implications of the co-evolution of pathogens and their hosts

Co-evolution between host and pathogen is, in principle, a powerful determinant of the biology and genetics of infection and disease. Yet co-evolution has proven difficult to demonstrate rigorously in practice, and co-evolutionary thinking is only just beginning to inform medical or veterinary research in any meaningful way, even though it can have a major influence on how genetic variation in biomedically important traits is interpreted. Improving our understanding of the biomedical significance of co-evolution will require changing the way in which we look for it, complementing the phenomenological approach traditionally favored by evolutionary biologists with the exploitation of the extensive data becoming available on the molecular biology and molecular genetics of host-pathogen interactions.

Experimental evidence for major histocompatibility complex-allele-specific resistance to a bacterial infection

The extreme polymorphism found at some major histocompatibility complex (MHC) loci is believed to be maintained by balancing selection caused by infectious pathogens. Experimental support for this is inconclusive. We have studied the interaction between certain MHC alleles and the bacterium Aeromonas salmonicida, which causes the severe disease furunculosis, in Atlantic salmon (Salmo salar L.). We designed full-sibling broods consisting of combinations of homozygote and heterozygote genotypes with respect to resistance or susceptibility alleles. The juveniles were experimentally infected with A. salmonicida and their individual survival was monitored. By comparing full siblings carrying different MHC genotypes the effects on survival due to other segregating genes were minimized. We show that a pathogen has the potential to cause very intense selection pressure on particular MHC alleles; the relative fitness difference between individuals carrying different MHC alleles was as high as 0.5. A co-dominant pattern of disease resistance/susceptibility was found, indicative of qualitative difference in the immune response between individuals carrying the high- and low-resistance alleles. Rather unexpectedly, survival was not higher among heterozygous individuals as compared with homozygous ones.

Pathogen resistance and genetic variation at MHC loci

Balancing selection in the form of heterozygote advantage, frequency-dependent selection, or selection that varies in time and/or space, has been proposed to explain the high variation at major histocompatibility complex (MHC) genes. Here the effect of variation of the presence and absence of pathogens over time on genetic variation at multiallelic loci is examined. In the basic model, resistance to each pathogen is conferred by a given allele, and this allele is assumed to be dominant. Given that s is the selective disadvantage for homozygotes (and heterozygotes) without the resistance allele and the proportion of generations, which a pathogen is present, is e, fitnesses for homozygotes become (1 - s)((n - 1)e) and the fitnesses for heterozygotes become (1 - s)((n - 2)e), where n is the number of alleles. In this situation, the conditions for a stable, multiallelic polymorphism are met even though there is no intrinsic heterozygote advantage. The distribution of allele frequencies and consequently heterozygosity are a function of the autocorrelation of the presence of the pathogen in subsequent generations. When there is a positive autocorrelation over generations, the observed heterozygosity is reduced. In addition, the effects of lower levels of selection and dominance and the influence of genetic drift were examined. These effects were compared to the observed heterozygosity for two MHC genes in several South American Indian samples. Overall, resistance conferred by specific alleles to temporally variable pathogens may contribute to the observed polymorphism at MHC genes and other similar host defense loci.

Molecular immunogenetics in susceptibility to bovine dermatophilosis: a candidate gene approach and a concrete field application

To identify molecular genetic markers of resistance or susceptibility to dermatophilosis in cattle, we used a functional candidate gene approach to analyze the DNA polymorphisms of targeted genes encoding molecules implicated in known mechanisms of both nonspecific and specific immune responses existing in the pathogen/host interface mechanisms. The most significant results were obtained within the Major Histocompatibility Complex (MHC) where the BoLA-DRB3 and DQB genes encode molecules involved in the antigen presentation to T cell receptors. A unique BoLA class II haplotype, made up of one DRB3 exon 2 allele and one DQB allele, highly correlates with the susceptibility character (P < 0.001). This haplotype marker of susceptibility was also found and validated in other bovine populations. A eugenic marker-assisted selection was developed in the field by eliminating only the animals having this haplotype. The disease prevalence was thereby reduced from 0.76 to 0.02 over 5 years. A crossbreeding plan is in progress to study the genetic transmission of the genotypic and phenotypic characters of susceptibility to dermatophilosis. In conclusion, we discuss several hypotheses at the molecular and cellular levels to better define the exact role of the MHC molecules in disease control and to answer the question: How is MHC diversity selectively maintained by natural selection imposed by pathogens?

Do pheromones reveal male immunocompetence?

Pheromones function not only as mate attractors, but they may also relay important information to prospective mates. It has been shown that vertebrates can distinguish, via olfactory mechanisms, major histocompatibility complex types in their prospective mates. However, whether pheromones can transmit information about immunocompetence is unknown. Here, we show that female mealworm beetles (Tenebrio molitor) prefer pheromones from males with better immunocompetence, indicated by a faster encapsulation rate against a novel antigen, and higher levels of phenoloxidase in haemolymph. Thus, the present study indicates that pheromones could transmit information about males' parasite resistance ability and may work as a reliable sexual ornament for female choice.

MHC heterozygosity confers a selective advantage against multiple-strain infections

Genetic heterozygosity is thought to enhance resistance of hosts to infectious diseases, but few tests of this idea exist. In particular, heterozygosity at the MHC, the highly polymorphic loci that control immunological recognition of pathogens, is suspected to confer a selective advantage by enhancing resistance to infectious diseases (the "heterozygote advantage" hypothesis). To test this hypothesis, we released mice into large population enclosures and challenged them with multiple strains of Salmonella and one of Listeria. We found that during Salmonella infections with three avirulent strains, MHC heterozygotes had greater survival and weight than homozygotes (unlike sham controls), and they were more likely to clear chronic Salmonella infection than homozygotes. In laboratory experiments, we found that MHC heterozygosity enhanced the clearance of multiple-strain Salmonella infections. Yet, contrary to what is widely assumed, the benefits of heterozygosity were due to resistance being dominant rather than overdominant, i.e., heterozygotes were more resistant than the average of parental homozygotes, but they were not more resistant than both. The fact that MHC heterozygotes were more resistant to infection and had higher fitness than homozygotes provides a functional explanation for MHC-disassortative mating preferences.

Levels of soluble human leukocyte antigen class 1 are increased in Graves' disease and toxic multinodular goiter and correlate with the levels of triiodothyronine

Soluble human leukocyte antigens class 1 (sHLA-1) are postulated to play a role in immunomodulation. Thyroid hormones and thyroid stimulating hormone influence expression of major histocompatibility complex (MHC) class 1 genes. We hypothesized similar changes in sHLA-1 with changes in the thyroid hormone. We measured serum levels of sHLA-1 in 140 normal controls, 11 patients with toxic multinodular goiter and 26 patients with active Graves' disease. On follow-up 1 year after treatment for Graves' disease, we repeated the measurement of the levels of sHLA-1 in 21 patients. Levels of sHLA-1 were markedly elevated in the group of patients with active Graves' disease compared to the normal controls (p = 0.00016). The levels of sHLA-1 were also elevated significantly in patients with toxic multinodular goiter compared to normal controls (p = 0.034). There is no significant difference between genders and the two races tested among patients with Graves' disease in contrast to controls. Free triiodothyronine (T(3)) levels positively correlated with the levels of sHLA-1 (r = 0.61). On follow-up after treatment for Graves' disease, levels of sHLA-1 decreased by 62% with p = 0.0006 (active Graves vs. follow-up). We conclude that there is a definite association between thyroid hormones and sHLA-1 levels.

Increased heterozygosity for MHC class II lineages in newborn males

In plants, fungi and marine invertebrates, there are genetic compatibility systems to ensure diversity in the offspring. The importance of genetic compatibility in gametic union and selective abortion in vertebrate animals has also been appreciated recently. There have been suggestions that the major histocompatibility complex (HLA in humans) may be a compatibility system in vertebrates. HLA class II haplotypes often contain a second expressed DRB locus which can be either DRB3, DRB4 or DRB5. These encode the supertypical specificities and mark the ancestral lineages. The members of each lineage have related DNA sequences at the main class II locus HLA-DRB1. We analysed 415 newborns at all expressed DRB loci by PCR analysis to seek evidence for sex-specific prenatal selection events. While there was no significant change in heterozygosity rates between males and females at DRB1, the proportion of males carrying two DRB1 specificities from different ancestral lineages was significantly increased (53.7% in males vs 39.3% in females, P = 0.003). The genotypes consisting of phylogenetically most distinct ones, namely the DRB3 and DRB4 haplotypes, showed the most striking difference between sexes (P = 0.007). These results suggested a more favourable outcome for male concepti heterozygous for supertypical haplotypes. Heterozygosity for most divergent haplotypical families ensures the highest degree of functional heterozygosity at the main HLA class II locus DRB1 while increasing the likelihood of heterozygosity also at other MHC loci. Our observations agree with the previously reported heterozygote excess in male newborn rats and mice. Correlations between MHC class II heterozygosity and advertised male quality in deer and pheasant as well as increased reproductive success in MHC class II heterozygous male macaques are examples of postnatal benefits of heterozygosity in males that may be behind the development of prenatal selection mechanisms. The MHC-mediated prenatal selection of males may also be one of the selective events suggested by the very high primary (male-to-female) sex ratio at fertilization reaching close to unity at birth in humans. These results provide an appealing working hypothesis for further studies in humans and other vertebrates.

Geographic heterogeneity in natural selection on an MHC locus in sockeye salmon

Balancing selection maintains high levels of polymorphism and heterozygosity in genes of the MHC (major histocompatibility complex) of vertebrate organisms, and promotes long evolutionary persistence of individual alleles and strongly differentiated allelic lineages. In this study, genetic variation at the MHC class II DAB-beta1 locus was examined in 31 populations of sockeye salmon (Oncorhynchus nerka) inhabiting the Fraser River drainage of British Columbia, Canada. Twenty-five percent of variation at the locus was partitioned among sockeye populations, as compared with 5% at neutral genetic markers. Geographic heterogeneity of balancing selection was detected among four regions in the Fraser River drainage and among lake systems within regions. High levels of beta1 allelic diversity and heterozygosity, as well as distributions of alleles and allelic lineages that were more even than expected for a neutral locus, indicated the presence of balancing selection in populations throughout much of the interior Fraser drainage. However, proximate populations in the upper Fraser region, and four of six populations from the lower Fraser drainage, exhibited much lower levels of genetic diversity and had beta1 allele frequency distributions in conformance with those expected for a neutral locus, or a locus under directional selection. Pair-wise FST values for beta1 averaged 0.19 and tended to exceed the corresponding values estimated for neutral loci at all levels of population structure, although they were lower among populations experiencing balancing selection than among other populations. The apparent heterogeneity in selection resulted in strong genetic differentiation between geographically proximate populations with and without detectable levels of balancing selection, in stark contrast to observations at neutral loci. The strong partitioning and complex structure of beta1 diversity within and among sockeye populations on a small geographic scale illustrates the value of incorporating adaptive variation into conservation planning for the species.

Discrimination of MHC-derived odors by untrained mice is consistent with divergence in peptide-binding region residues

Genes of the major histocompatibility complex (MHC) play a central role in immune recognition, yet they also influence the odor of individuals. Mice can be trained to distinguish odors mediated by classical MHC loci; however, training can introduce confounding behavioral artifacts. This study demonstrates that mice can distinguish some, but not all, naturally occurring allelic variants at classical MHC loci without prior training. This result suggests that MHC-disassortative mating preferences might operate by means of small MHC-based odor differences, and could therefore contribute to diversifying selection acting on MHC loci. Here we show that odors of two MHC mutant mouse strains (bm1 and bm3) can be distinguished, even after genetic background is controlled by intercrossing strains. These two strains differ by five amino acids, three of which are predicted to chemically contact peptides bound to the peptide-binding region (PBR), the site of antigen presentation for T cell recognition. However, the odors of neither bm1 nor bm3 were distinguished from their parental B6 haplotype after randomizing genomic background, despite discrimination of pure-bred B6 and bm1 strain odors. These combined results suggest that (i) there may be an MHC odor discrimination threshold based on divergence in PBR residues, providing a more logical pattern of MHC-based odor discrimination than found in previous training studies, where discrimination ability was not correlated with PBR divergence; and (ii) additional (non-MHC) mutations that influence odor have accumulated in these strains during the 100 generations of divergence between pure B6 and bm1 strains.