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1.
Major histocompatibility complex (MHC) genes play an important role in the immune response of vertebrates. Allelic polymorphism and evolutionary mechanism of MHC genes have been investigated in many mammals, but much less is known in teleosts. We examined the polymorphism, gene duplication and balancing selection of the MHC class II DAB gene of the half-smooth tongue sole (Cynoglossus semilaevis); 23 alleles were found in this species. Gene duplication manifested as three to six distinct sequences at each domain in the same individuals. Non-synonymous substitutions occurred at a significantly higher frequency than synonymous substitutions in the PBR domain, suggesting balancing selection for maintaining polymorphisms at the MHC II DAB locus. Many positive selection sites were found to act very intensely on antigen-binding sites of MHC class II DAB gene.  相似文献   

2.
Xu TJ  Sun YN  Wang RX 《Marine Genomics》2010,3(2):117-123
Allelic polymorphism and evolution mechanism of major histocompatibility complex (MHC) genes has been investigated in many mammals, however, much less is known in teleost. In order to investigate the mechanisms creating and maintaining variability at the MHC class II DAA locus, we examined the polymorphism, gene duplication and balancing selection of MHC class II DAA gene of the half-smooth tongue sole (Cynoglossus semilaevis). We described 33 alleles in the C. semilaevis, recombination and gene duplication seems to play more important roles in the origin of new alleles. The rate of non-synonymous substitutions (d(N)) occurred at a significantly higher frequency than that of synonymous substitutions (d(S)) in peptide-binding region (PBR) and non-PBR, suggesting balancing selection for maintaining polymorphisms at the MHC II DAA locus. Many positive selection sites were found to act very intensively on antigen-binding sites. Our founding suggests a snapshot in an evolutionary process of MHC-DAA gene evolution of the C. semilaevis.  相似文献   

3.
Mate choice is thought to contribute to the maintenance of the spectacularly high polymorphism of the Major Histocompatibility Complex (MHC) genes, along with balancing selection from parasites, but the relative contribution of the former mechanism is debated. Here, we investigated the association between male MHC genotype and mating success in the brown bear. We analysed fragments of sequences coding for the peptide-binding region of the highly polymorphic MHC class I and class II DRB genes, while controlling for genome-wide effects using a panel of 18 microsatellite markers. Male mating success did not depend on the number of alleles shared with the female or amino-acid distance between potential mates at either locus. Furthermore, we found no indication of female mating preferences for MHC similarity being contingent on the number of alleles the females carried. Finally, we found no significant association between the number of MHC alleles a male carried and his mating success. Thus, our results provided no support for the role of mate choice in shaping MHC polymorphism in the brown bear.  相似文献   

4.
Cutrera AP  Lacey EA 《Immunogenetics》2007,59(12):937-948
Balancing selection acting over the evolutionary history of a lineage can result in the retention of alleles among species for longer than expected under neutral evolution. The associated pattern of trans-species polymorphism, in which similar or even identical alleles are shared among species, is often used to infer that balancing selection has occurred. The genes of the major histocompatibility complex (MHC) are thought to be subject to balancing selection that maintains alleles associated with response to specific pathogens. To explore the role of balancing selection in shaping MHC diversity in ctenomyid rodents, we examined allelic variability at the class II DRB and DQA loci in 18 species in the genus Ctenomys. Previous studies of four of these species had revealed significant within-population evidence of positive selection on MHC loci. The current study expands upon these analyses to (1) evaluate among-species evidence of positive selection and (2) explore the potential for balancing selection on MHC genes. Interspecific nucleotide sequence variation revealed significant evidence of positive selection on the DRB and DQA loci. At the same time, comparisons of phylogenetic trees for these MHC loci with a putative species tree based on mitochondrial sequence data revealed multiple examples of trans-specific polymorphism, including sharing of identical DRB and DQA alleles among distantly related species of Ctenomys. These findings suggest that MHC genes in these animals have historically been subject to balancing selection and yield new insights into the complex suite of forces shaping MHC diversity in free-living vertebrates.  相似文献   

5.
The genes of the major histocompatibility complex (MHC) are a key component of the adaptive immune system and among the most variable loci in the vertebrate genome. Pathogen-mediated natural selection and MHC-based disassortative mating are both thought to structure MHC polymorphism, but their effects have proven difficult to discriminate in natural systems. Using the first model of MHC dynamics incorporating both survival and reproduction, we demonstrate that natural and sexual selection produce distinctive signatures of MHC allelic diversity with critical implications for understanding host–pathogen dynamics. While natural selection produces the Red Queen dynamics characteristic of host–parasite interactions, disassortative mating stabilizes allele frequencies, damping major fluctuations in dominant alleles and protecting functional variants against drift. This subtle difference generates a complex interaction between MHC allelic diversity and population size. In small populations, the stabilizing effects of sexual selection moderate the effects of drift, whereas pathogen-mediated selection accelerates the loss of functionally important genetic diversity. Natural selection enhances MHC allelic variation in larger populations, with the highest levels of diversity generated by the combined action of pathogen-mediated selection and disassortative mating. MHC-based sexual selection may help to explain how functionally important genetic variation can be maintained in populations of conservation concern.  相似文献   

6.
Selection pressure from parasites is thought to be a major force shaping the extreme polymorphism of the major histocompatibility complex (MHC) genes, but the modes and consequences of selection remain unclear. Here, we analyse MHC class II and microsatellite diversity in 16 guppy (Poecilia reticulata) populations from two islands (Trinidad and Tobago) that have been separated for at least 10 ky. Within-population MHC diversity was high, but allele sharing was limited within islands and even lower between islands, suggesting relatively fast turnover of alleles. Allelic lineages strongly supported in phylogenetic analyses tended to be island-specific, suggesting rapid lineage sorting, and an expansion of an allelic lineage private to Tobago was observed. New alleles appear to be generated locally at a detectably high frequency. We did not detect a consistent signature of local adaptation, but FST outlier analysis suggested that balancing selection may be the more general process behind spatial variation in MHC allele frequencies in this system, particularly within Trinidad. We found no evidence for divergent allele advantage within populations, or for decreased genetic structuring of MHC supertypes compared to MHC alleles. The dynamic and complex nature of MHC evolution we observed in guppies, coupled with some evidence for balancing selection shaping MHC allele frequencies, are consistent with Red Queen processes of host-parasite coevolution.Subject terms: Population genetics, Evolutionary genetics  相似文献   

7.
The effects of sexual selection on population mean fitness are unclear and a subject of debate. Recent models propose that, because reproductive success may be condition dependent, much of the genome may be a target of sexual selection. Under this scenario, mutations that reduce health, and thus nonsexual fitness, may also be deleterious with respect to reproductive success, meaning that sexual selection may contribute to the purging of deleterious alleles. We tested this hypothesis directly by subjecting replicate Drosophila melanogaster populations to two treatments that altered the opportunity for sexual selection and then tracked changes in the frequency of six separate deleterious alleles with recessive and visible phenotypic effects. While natural selection acted to decrease the frequency of all six mutations, the addition of sexual selection did not aid in the purging of any of them, and for three of them appears to have hampered it. Courtship and mating have harmful effects in this species and mate choice assays showed that males directed more courtship and mating behavior toward wild-type over mutant females, providing a likely explanation for sexual selection's cost. Whether this cost extends to other mutations (e.g., those lacking visible phenotypic effects) is an important topic for future research.  相似文献   

8.
The genes of the major histocompatibility complex (MHC) code for proteins involved in antigen recognition and triggering of the adaptive immune response, and are therefore likely to be under selection from parasites. These selection regimes may vary in space and time. Here we report a strong geographical structure in MHC class II B genes of a migrating bird, the great snipe (Gallinago media). Genetic differentiation in the MHC between two ecologically distinct distributional regions (Scandinavian mountain populations vs. East European lowland populations) was still present after statistically controlling for the effect of selectively neutral variation (microsatellites) using partial Mantel tests. This suggests a role for selection in generating this spatial structure and that it represents local adaptation to different environments. Differentiation between populations within the two regions was negligible. Overall, we found a high number of MHC alleles (50, from 175 individuals). This, together with a tendency for a higher rate of nonsynonymous than synonymous substitutions in the peptide binding sites, and high Tajima's D in certain regions of the gene, suggests a history of balancing selection. MHC variation is often thought to be maintained by some form of balancing selection, but the nature of this selection remains unclear. Our results support the hypothesis that spatial variation in selection regimes contributes to the high polymorphism.  相似文献   

9.
Major histocompatibility complex (MHC) genes encode proteins that play a central role in vertebrates' adaptive immunity to parasites. MHC loci are among the most polymorphic in vertebrates' genomes, inspiring many studies to identify evolutionary processes driving MHC polymorphism within populations and divergence between populations. Leading hypotheses include balancing selection favouring rare alleles within populations, and spatially divergent selection. These hypotheses do not always produce diagnosably distinct predictions, causing many studies of MHC to yield inconsistent or ambiguous results. We suggest a novel strategy to distinguish balancing vs. divergent selection on MHC, taking advantage of natural admixture between parapatric populations. With divergent selection, individuals with immigrant alleles will be more infected and less fit because they are susceptible to novel parasites in their new habitat. With balancing selection, individuals with locally rare immigrant alleles will be more fit (less infected). We tested these contrasting predictions using three‐spine stickleback from three replicate pairs of parapatric lake and stream habitats. We found numerous positive and negative associations between particular MHC IIβ alleles and particular parasite taxa. A few allele–parasite comparisons supported balancing selection, and others supported divergent selection between habitats. But, there was no overall tendency for fish with immigrant MHC alleles to be more or less heavily infected. Instead, locally rare MHC alleles (not necessarily immigrants) were associated with heavier infections. Our results illustrate the complex relationship between MHC IIβ allelic variation and spatially varying multispecies parasite communities: different hypotheses may be concurrently true for different allele–parasite combinations.  相似文献   

10.
The extraordinary polymorphism of major histocompatibility complex (MHC) genes is considered a paradigm of pathogen‐mediated balancing selection, although empirical evidence is still scarce. Furthermore, the relative contribution of balancing selection to shape MHC population structure and diversity, compared to that of neutral forces, as well as its interaction with other evolutionary processes such as hybridization, remains largely unclear. To investigate these issues, we analyzed adaptive (MHC‐DAB gene) and neutral (11 microsatellite loci) variation in 156 brown trout (Salmo trutta complex) from six wild populations in central Italy exposed to introgression from domestic hatchery lineages (assessed with the LDH gene). MHC diversity and structuring correlated with those at microsatellites, indicating the substantial role of neutral forces. However, individuals carrying locally rare MHC alleles/supertypes were in better body condition (a proxy of individual fitness/parasite load) regardless of the zygosity status and degree of sequence dissimilarity of MHC, hence supporting balancing selection under rare allele advantage, but not heterozygote advantage or divergent allele advantage. The association between specific MHC supertypes and body condition confirmed in part this finding. Across populations, MHC allelic richness increased with increasing admixture between native and domestic lineages, indicating introgression as a source of MHC variation. Furthermore, introgression across populations appeared more pronounced for MHC than microsatellites, possibly because initially rare MHC variants are expected to introgress more readily under rare allele advantage. Providing evidence for the complex interplay among neutral evolutionary forces, balancing selection, and human‐mediated introgression in shaping the pattern of MHC (functional) variation, our findings contribute to a deeper understanding of the evolution of MHC genes in wild populations exposed to anthropogenic disturbance.  相似文献   

11.
Xu TJ  Sun YN  Chen SL 《Genetica》2010,138(11-12):1251-1259
Major histocompatibility complex (MHC) genes play an important role in the immune response of vertebrates. Allelic polymorphism and pattern of evolution in MHC genes has been investigated in many mammals, however, much less is known in teleost. In the present study, we have investigated complete MHC Iα gene consists of 7 exons and 6 introns in Olive flounder (Paralichthys olivaceus). Genetic variation in the MHC class Iα gene was also tested in flounder. In 32 individuals, a total of 62 alleles were detected from exon 2 of MHC class Iα gene. The rate of non-synonymous substitutions (d ( N )) occurred at a significantly higher frequency than that of synonymous substitutions (d ( S )) in PBR and non-PBR, suggesting that balancing selection for maintaining polymorphisms at the MHC Iα locus. Many positive selection sites were found to act very intensively on antigen binding sites. Our founding suggests a snapshot in an evolutionary process of MHC Iα gene evolution of the P. olivaceus.  相似文献   

12.
Genes of the major histocompatibility complex, which are the most polymorphic of all vertebrate genes, are a pre‐eminent system for the study of selective pressures that arise from host–pathogen interactions. Balancing selection capable of maintaining high polymorphism should lead to the homogenization of MHC allele frequencies among populations, but there is some evidence to suggest that diversifying selection also operates on the MHC. However, the pattern of population structure observed at MHC loci is likely to depend on the spatial and/or temporal scale examined. Here, we investigated selection acting on MHC genes at different geographic scales using Venezuelan guppy populations inhabiting four regions. We found a significant correlation between MHC and microsatellite allelic richness across populations, which suggests the role of genetic drift in shaping MHC diversity. However, compared to microsatellites, more MHC variation was explained by differences between populations within larger geographic regions and less by the differences between the regions. Furthermore, among proximate populations, variation in MHC allele frequencies was significantly higher compared to microsatellites, indicating that selection acting on MHC may increase population structure at small spatial scales. However, in populations that have significantly diverged at neutral markers, the population‐genetic signature of diversifying selection may be eradicated in the long term by that of balancing selection, which acts to preserve rare alleles and thus maintain a common pool of MHC alleles.  相似文献   

13.
The major histocompatibility complex (MHC) genes code for proteins that play a critical role in the immune system response. The MHC genes are among the most polymorphic genes in vertebrates, presumably due to balancing selection. The two MHC classes appear to differ in the rate of evolution, but the reasons for this variation are not well understood. Here, we investigate the level of polymorphism and the evolution of sequences that code for the peptide-binding regions of MHC class I and class II DRB genes in the Alpine marmot (Marmota marmota). We found evidence for four expressed MHC class I loci and two expressed MHC class II loci. MHC genes in marmots were characterized by low polymorphism, as one to eight alleles per putative locus were detected in 38 individuals from three French Alps populations. The generally limited degree of polymorphism, which was more pronounced in class I genes, is likely due to bottleneck the populations undergone. Additionally, gene duplication within each class might have compensated for the loss of polymorphism at particular loci. The two gene classes showed different patterns of evolution. The most polymorphic of the putative loci, Mama-DRB1, showed clear evidence of historical positive selection for amino acid replacements. However, no signal of positive selection was evident in the MHC class I genes. These contrasting patterns of sequence evolution may reflect differences in selection pressures acting on class I and class II genes.  相似文献   

14.
The unprecedented polymorphism in the major histocompatibility complex (MHC) genes is thought to be maintained by balancing selection from parasites. However, do parasites also drive divergence at MHC loci between host populations, or do the effects of balancing selection maintain similarities among populations? We examined MHC variation in populations of the livebearing fish Poecilia mexicana and characterized their parasite communities. Poecilia mexicana populations in the Cueva del Azufre system are locally adapted to darkness and the presence of toxic hydrogen sulphide, representing highly divergent ecotypes or incipient species. Parasite communities differed significantly across populations, and populations with higher parasite loads had higher levels of diversity at class II MHC genes. However, despite different parasite communities, marked divergence in adaptive traits and in neutral genetic markers, we found MHC alleles to be remarkably similar among host populations. Our findings indicate that balancing selection from parasites maintains immunogenetic diversity of hosts, but this process does not promote MHC divergence in this system. On the contrary, we suggest that balancing selection on immunogenetic loci may outweigh divergent selection causing divergence, thereby hindering host divergence and speciation. Our findings support the hypothesis that balancing selection maintains MHC similarities among lineages during and after speciation (trans‐species evolution).  相似文献   

15.
Selection forces that favour different phenotypes in different environments can change frequencies of genes between populations along environmental clines. Clines are also compatible with balancing forces, such as negative frequency‐dependent selection (NFDS), which maintains phenotypic polymorphisms within populations. For example, NFDS is hypothesized to maintain partial migration, a dimorphic behavioural trait prominent in species where only a fraction of the population seasonally migrates. Overall, NFDS is believed to be a common phenomenon in nature, yet a scarcity of studies were published linking naturally occurring allelic variation with bimodal or multimodal phenotypes and balancing selection. We applied a Pool‐seq approach and detected selection on alleles associated with environmental variables along a North–South gradient in western North American caribou, a species displaying partially migratory behaviour. On 51 loci, we found a signature of balancing selection, which could be related to NFDS and ultimately the maintenance of the phenotypic polymorphisms known within these populations. Yet, remarkably, we detected directional selection on a locus when our sample was divided into two behaviourally distinctive groups regardless of geographic provenance (a subset of GPS‐collared migratory or sedentary individuals), indicating that, within populations, phenotypically homogeneous groups were genetically distinctive. Loci under selection were linked to functional genes involved in oxidative stress response, body development and taste perception. Overall, results indicated genetic differentiation along an environmental gradient of caribou populations, which we found characterized by genes potentially undergoing balancing selection. We suggest that the underlining balancing force, NFDS, plays a strong role within populations harbouring multiple haplotypes and phenotypes, as it is the norm in animals, plants and humans too.  相似文献   

16.
The depletion of variation at MHC loci, which play a crucial role in pathogen recognition, has been postulated to be one of important extinction risk factors for endangered populations. Thus, it is important to understand how selection affects the level of polymorphism in these genes when populations undergo a reduction in size. We followed MHC diversity in computer simulations of population bottlenecks. The fates of MHC alleles in the simulations were determined either by drift, or by balancing selection resulting from host–parasite coevolution. We found that the impact of selection on MHC polymorphism in bottlenecked populations was dependent upon the timescales involved. Initially, selection maintained lower number of alleles than drift, but after ~40 generations of hosts selection maintained higher MHC diversity, as compared to drift. The adverse effects of decreased MHC polymorphism on population viability may be, to some extent, compensated for if selection helps to retain MHC alleles which show high functional diversity, which should allow protection against a broader range of pathogens. Our simulation shows, however, that the mean divergence of alleles retained under selection in bottlenecked populations is not, on average, significantly higher than the divergence due to drift.  相似文献   

17.
Genes of the major histocompatibility complex (MHC) have been a source of considerable research interest, owing in large part to the growing body of evidence that they may be subject to both natural and sexual selection. However, much remains to be learned about the dynamics of MHC genes in subdivided populations, particularly those characterized by divergent ecological pressures. In this study, we attempt to disentangle the relative roles of both parasite-mediated selection and MHC-mediated mate choice in an open estuarine system inhabited by two parapatric, adaptively divergent threespine stickleback (Gasterosteus aculeatus) demes. We sequenced the putative peptide-binding region (PBR) of an estimated four Class IIβ loci from 127 individuals, identifying 329 sequence variants (276 translated amino acid sequences). Demes differed significantly both in the frequency of MHC alleles and in the communities of helminth parasites infecting resident sticklebacks. Strong signatures of natural selection were inferred from analyses of codon substitutions, particularly in the derived (freshwater) rather than the ancestral (marine) deme. Relationships between parasite load and MHC diversity were indicative of balancing selection, but only within the freshwater deme. Signals of MHC-mediated mate choice were weak and differed significantly between demes. Moreover, MHC-mediated mate choice was significantly influenced by environmental salinity and appeared of secondary importance to tendencies towards assortative mating. We discuss the implications of these findings in respect to ecological adaptation and the potential demographic consequences of possible outcomes of MHC-mediated mate choice.  相似文献   

18.

Background  

The extreme polymorphism that is observed in major histocompatibility complex (MHC) genes, which code for proteins involved in recognition of non-self oligopeptides, is thought to result from a pressure exerted by parasites because parasite antigens are more likely to be recognized by MHC heterozygotes (heterozygote advantage) and/or by rare MHC alleles (negative frequency-dependent selection). The Ewens-Watterson test (EW) is often used to detect selection acting on MHC genes over the recent history of a population. EW is based on the expectation that allele frequencies under balancing selection should be more even than under neutrality. We used computer simulations to investigate whether this expectation holds for selection exerted by parasites on host MHC genes under conditions of heterozygote advantage and negative frequency-dependent selection acting either simultaneously or separately.  相似文献   

19.
Balancing selection, an evolutionary force that retains genetic diversity, has been detected in multiple genes and organisms, such as the sexual mating loci in fungi. However, to quantify the strength of balancing selection and define the mating-related genes require a large number of strains. In tetrapolar basidiomycete fungi, sexual type is determined by two unlinked loci, MATA and MATB. Genes in both loci define mating type identity, control successful mating and completion of the life cycle. These loci are usually highly diverse. Previous studies have speculated, based on culture crosses, that species of the non-model genus Trichaptum (Hymenochaetales, Basidiomycota) possess a tetrapolar mating system, with multiple alleles. Here, we sequenced a hundred and eighty strains of three Trichaptum species. We characterized the chromosomal location of MATA and MATB, the molecular structure of MAT regions and their allelic richness. The sequencing effort was sufficient to molecularly characterize multiple MAT alleles segregating before the speciation event of Trichaptum species. Analyses suggested that long-term balancing selection has generated trans-species polymorphisms. Mating sequences were classified in different allelic classes based on an amino acid identity (AAI) threshold supported by phylogenetics. 17,550 mating types were predicted based on the allelic classes. In vitro crosses allowed us to support the degree of allelic divergence needed for successful mating. Even with the high amount of divergence, key amino acids in functional domains are conserved. We conclude that the genetic diversity of mating loci in Trichaptum is due to long-term balancing selection, with limited recombination and duplication activity. The large number of sequenced strains highlighted the importance of sequencing multiple individuals from different species to detect the mating-related genes, the mechanisms generating diversity and the evolutionary forces maintaining them.  相似文献   

20.
The extreme polymorphism of antigen‐presenting genes of the major histocompatibility complex (MHC) has spurred intense research unparalleled for any other gene family. This applies also to teleosts where sequence information is available for 3559 MHC class I and class II allelic variants from 137 species. This review summarizes current knowledge on the origin and maintenance of diversity at classical MHC loci. Most studies identified positive selection (i.e. elevated rates of non‐synonymous over synonymous substitutions, dN/dS) as a sign of balancing selection. A meta‐analysis on nine species with sufficient numbers of class I and class II sequences revealed that recombination rate and intensity of positive selection were positively correlated, suggesting that recombination and gene conversion played a significant role in shaping the allelic repertoire. Processes that create diversity over long timescales need to be complemented by contemporary balancing selection, either through overdominance or frequency‐dependent selection, in order to explain the high allelic diversity observed today. While some evidence for overdominance exists for a few taxa (mainly salmonids) by correlating parasite infection data or survival to MHC genotypes, field or experimental data on negative frequency‐dependent selection are lacking altogether, even though some fish species are particularly suitable as model systems. Theoretical predictions suggest that negative frequency‐dependent selection is necessary to maintain the existing polymorphism. Hence, future empirical studies should focus on detecting signals that differentiate between mechanisms of contemporary selection rather than repeatedly showing historical selection events.  相似文献   

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