Spatio-temporal variation in the strength and mode of selection acting on major histocompatibility complex diversity in water vole (Arvicola terrestris) metapopulations

Patterns of spatio-temporal genetic variation at a class II major histocompatibility complex (MHC) locus and multiple microsatellite loci were analysed within and between three water vole metapopulations in Scotland, UK. Comparisons of MHC and microsatellite spatial genetic differentiation, based on standardised tests between two demographically asynchronous zones within a metapopulation, suggested that spatial MHC variation was affected by balancing selection, directional selection and random genetic drift, but that the relative effects of these microevolutionary forces vary temporally. At the metapopulation level, between-year differentiation for MHC loci was significantly correlated with that of microsatellites, signifying that neutral factors such as migration and drift were primarily responsible for overall temporal genetic change at the metapopulation scale. Between metapopulations, patterns of genetic differentiation implied that, at large spatial scales, MHC variation was primarily affected by directional selection and drift. Levels of MHC heterozygosity in excess of Hardy–Weinberg expectations were consistent with overdominant balancing selection operating on MHC variation within metapopulations. However, this effect was not constant among all samples, indicating temporal variation in the strength of selection relative to other factors. The results highlight the benefit of contrasting variation at MHC with neutral markers to separate the effects of stochastic and deterministic microevolutionary forces, and add to a growing body of evidence showing that the mode and relative strength of selection acting on MHC diversity varies both spatially and temporally.     

Genetic structure and contrasting selection pattern at two major histocompatibility complex genes in wild house mouse populations

The mammalian major histocompatibility complex (MHC) is a tightly linked cluster of immune genes, and is often thought of as inherited as a unit. This has led to the hope that studying a single MHC gene will reveal patterns of evolution representative of the MHC as a whole. In this study we analyse a 1000-km transect of MHC variation traversing the European house mouse hybrid zone to compare signals of selection and patterns of diversification at two closely linked MHC class II genes, H-2Aa and H-2Eb. We show that although they are 0.01 cM apart (that is, recombination is expected only once in 10 000 meioses), disparate evolutionary patterns were detected. H-2Aa shows higher allelic polymorphism, faster allelic turnover due to higher mutation rates, stronger positive selection at antigen-binding sites and higher population structuring than H-2Eb. H-2Eb alleles are maintained in the gene pool for longer, including over separation of the subspecies, some H-2Eb alleles are positively and others negatively selected and some of the alleles are not expressed. We conclude that studies on MHC genes in wild-living vertebrates can give substantially different results depending on the MHC gene examined and that the level of polymorphism in a related species is a poor criterion for gene choice.     

Sexual selection and the evolutionary dynamics of the major histocompatibility complex

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.     

The diet of Abramis brama (L.) larvae reared in illuminated cages

The paper presents the results of investigations on the feeding of bream larvae reared in illuminated cages in a mesotrophic lake. The same species of zooplankton were found in both the lake and the fish cages. Copepoda predominated in abundance and biomass. During the mixed feeding the bream larvae diet consisted of rotifers (45.8%), small cladocerans (41.6%) and phytoplankton (13.6%). According to Ivlev's selectivity indices (E), the most important food species were: Keratella cochlearis (E = +9.94), Kellicotia longispina (E = +0.93), K. quadrata (E = +0.92), Bosmina longirostris (E = +0.92), Trichocerca sp. (E = +0.89), and Chydorus sphaericus (E = +0.88). The total length (TL) of bream larvae consuming their first nauplii and copepodites of Copepoda was 8.9 mm. As bream larvae grew, cladocerans were more often selected than rotifers. Copepods usually made up an insignificant component of the diet. Bream larvae in illuminated cages generally fed at night; their alimentary tracts were filled to the maximum in the period from sunset to sunrise. Indices thereof reached 755.6 at night and 278.0 ? in the daytime.     

Density-related changes in selection pattern for major histocompatibility complex genes in fluctuating populations of voles

Host-pathogen interactions are of particular interest in studies of the interplay between population dynamics and natural selection. The major histocompatibility complex (MHC) genes of demographically fluctuating species are highly suitable markers for such studies, because they are involved in initiating the immune response against pathogens and display a high level of adaptive genetic variation. We investigated whether two MHC class II genes (DQA1, DRB) were subjected to contemporary selection during increases in the density of fossorial water vole (Arvicola terrestris) populations, by comparing the neutral genetic structure of seven populations with that estimated from MHC genes. Tests for heterozygosity excess indicated that DQA1 was subject to intense balancing selection. No such selection operated on neutral markers. This pattern of selection became more marked with increasing abundance. In the low-abundance phase, when populations were geographically isolated, both overall differentiation and isolation-by-distance were more marked for MHC genes than for neutral markers. Model-based simulations identified DQA1 as an outlier (i.e. under selection) in a single population, suggesting the action of local selection in fragmented populations. The differences between MHC and neutral markers gradually disappeared with increasing effective migration between sites. In the high-abundance year, DQA1 displayed significantly lower levels of overall differentiation than the neutral markers. This gene therefore displayed stronger homogenization than observed under drift and migration alone. The observed signs of selection were much weaker for DRB. Spatial and temporal fluctuations in parasite pressure and locus-specific selection are probably the most plausible mechanisms underlying the observed changes in selection pattern during the demographic cycle.     

Bottom-up effects of bream (Abramis brama L.) in Lake Balaton

Enclosures (17 m³) were used in the mesotrophic area of Lake Balaton to determine the impact of benthivorous bream (Abramis brama L.) on the lower trophic levels during summers of 1984–86. In enclosures with a fish biomass similar to the biomass in the eutrophic area of the lake, the number of phytoplankton species was highest. In enclosures with a low fish biomass the phytoplankton was dominated by the greens. A high biomass of bream in the mesotrophic basin caused bacterial production corresponding to that of the eutrophic part of the lake. Crustaceans were dominated by copepods and were unable to control phytoplankton peaks. Bottom-up effects of bream were more obvious than top-down effects and seem to be more important in the possible control of water quality.     

The red deer (Cervus elaphus) contains two expressed major histocompatibility complex class II DQB genes

The highly variable second exons of the red deer ( Cervus elaphu s) major histocompatibility complex (MHC) DQB genes were cloned and sequenced. Eight different expressed DQB sequences were isolated from four unrelated red deer. Either two or three different DQB sequences were isolated from each individual, demonstrating that more than one DQB gene is expressed in red deer. This is consistent with other ruminants, which also have multiple expressed copies of the DQB gene. The sequences ranged in similarity from 81·3% to 97·6% with an average similarity of 90·2%; this indicates that at least one of the DQB genes is highly polymorphic.     

Is the peacock’s train an honest signal of genetic quality at the major histocompatibility complex?

Peacocks are a classic example of sexual selection, where females preferentially mate with males who have longer, more elaborate trains. One of the central hypotheses of sexual selection theory is that large or elaborate male ‘ornaments’ may signal high genetic quality (good genes). Good genes are thought to be those associated with disease resistance and as diversity at the major histocompatibility complex (MHC) has been shown to equate to superior immune responses, we test whether the peacock’s train reveals genetic diversity at the MHC. We demonstrate via a captive breeding experiment that train length of adult males reflects genetic diversity at the MHC while controlling for genome‐wide diversity and that peahens lay more, and larger, eggs for males with a more diverse MHC, but not for males with longer trains. Our results suggest that females are assessing and responding to male quality in terms of MHC diversity, but this assessment does not appear to be via train length, despite the fact that train length reflects MHC diversity.     

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.     

Allelic and haplotypic diversity at the major histocompatibility class II within domesticated Australian Atlantic salmon (Salmo salar L.)

Variation within the major histocompatibility (MH) class II alpha gene ( Sasa-DAA ) was compared between domesticated Australian Atlantic salmon and their ancestral Canadian population. The level of Sasa-DAA and MH class II beta gene ( Sasa-DAB ) sequence variation was also examined within the Australian population and compared with that published for European Atlantic salmon populations. In contrast to variation previously reported for non-coding microsatellite loci, a high level of MH class II allelic variation has been maintained within the domesticated Australian populations. Furthermore, a high level of Sasa-DAA and Sasa-DAB allele sequence diversity was also observed and exceeded that reported for other cultured Atlantic salmon populations. The number of Sasa-DAB allele sequences (14) surpassed the number of Sasa-DAA allele sequences (9) to produce 14 unique class II haplotypes. We conclude that the Australian Atlantic salmon populations show high MH class II allelic and haplotypic variation compared with both its ancestral Canadian population and other cultured Atlantic salmon populations.     

Considerable haplotypic diversity in the RT1-CE class I gene region of the rat major histocompatibility complex

The major histocompatibility complex (MHC) class I region extending between the Bat1 and Pou5f1 genes shows considerable genomic plasticity in mouse and rhesus macaque but not in human haplotypes. In the rat, this region is known as the RT1-CE region. The recently published rat MHC sequence gave rise to a complete set of class I gene sequences in a single MHC haplotype, namely the RT1ⁿ haplotype of the widely used BN inbred strain. To study the degree of genetic diversity, we compared the RT1-CE region-derived class I genes of the RT1ⁿ haplotype with class I sequences of other rat haplotypes. By using phylogenetic tree analyses, we obtained evidence for extensive presence and absence polymorphisms of single loci and even small subfamilies of class I genes in the rat. Alleles of RT1-CE region class I genes could also be identified, but the rate of allelic nucleotide substitutions appeared rather low, indicating that the diversity in the RT1-CE region is mainly based on genomic plasticity.     

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.     

Expression and characterization of ovine major histocompatibility complex class II (OLA-DR) genes

Previous work made use of nucleic acid probes corresponding to different subtypes of the class II regions of the human and murine major histocompatibility complex (MHC) to isolate seven different alpha and 24 different beta genes of the ovine MHC from two cosmid libraries. In an attempt to identify pairs of alpha and beta genes capable of cell surface expression, all permutations of alpha and beta genes were in turn transfected into mouse L-cells. Two pairs of alpha and beta genes co-expressed and stable ovine MHC class II L-cell lines were developed. The expressed alpha genes had previously been defined as DR-alpha homologues (DRA) by differential Southern hybridization to human subtype specific class II probes. The expressed ovine beta genes were also assigned as ovine DR-beta homologues (DRB) on the basis of their sequence having a higher degree of similarity with human DRB than any other subtype. A total of eight out of 23 anti-sheep class II specific monoclonal antibodies were typed OLA-DR specific by FACScan analysis using the L-cell lines.     

Spawning coloration and sperm quality in a large lake population of Arctic charr (Salmonidae: Salvelinus alpinus L.)

The modern theories of sexual selection predict that male sexual ornaments may have evolved as reliable signals of male fertilization efficiency. However, among the studies of fishes with external fertilization, the results have yielded ambiguous evidence. In the present study, we present data on the phenotypic relationships between red spawning coloration and ejaculate quality (spermatocrit, sperm motility) from Arctic charr, Salvelinus alpinus. We studied two generations (F₁ and F₂) of males from a large lake population, reared in a standardized hatchery environment, to determine whether differential hatchery history, or duration of hatchery selection, affected the variation in ejaculate characteristics or abdominal coloration. After controlling for body length, there was no difference between the hatchery generations in these traits. However, the degree of redness increased with fish size. We found a positive correlation between sperm velocity and sperm longevity, indicating a functional integration between these sperm features across generations. Sperm velocity was also positively correlated with male redness. Therefore, the finding obtained in the present study suggests that the carotenoid‐based ornamentation in Arctic charr may provide information about differences between males in their fertilization potential. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 794–802.     

Analysis of major histocompatibility complex class II Beta genes from rockfishes (genus Sebastes)

Class II major histocompatibility complex (MHC) beta genes were isolated from 12 species of rockfish (genus Sebastes ). Multiple sequences were found in each of the species. The majority of sequences displayed the characteristics of functional MHC genes, with a small group of sequences that were possibly pseudogenes.     

Recombination and mutation shape variations in the major histocompatibility complex

The major histocompatibility complex (MHC) is closely associated with numerous diseases, but its high degree of polymorphism complicates the discovery of disease-associated variants. In principle, recombination and de novo mutations are two critical factors responsible for MHC polymorphisms. However, direct evidence for this hypothesis is lacking. Here, we report the generation of fine-scale MHC recombination and de novo mutation maps of ～5 Mb by deep sequencing (> 100×) of the MHC genome for 17 MHC recombination and 30 non-recombination Han Chinese families (a total of 190 individuals). Recombination hotspots and Han-specific breakpoints are located in close proximity at haplotype block boundaries. The average MHC de novo mutation rate is higher than the genome-wide de novo mutation rate, particularly in MHC recombinant individuals. Notably, mutation and recombination generated polymorphisms are located within and outside linkage disequilibrium regions of the MHC, respectively, and evolution of the MHC locus was mainly controlled by positive selection. These findings provide insights on the evolutionary causes of the MHC diversity and may facilitate the identification of disease-associated genetic variants.     

Molecular characterization of major histocompatibility complex class II alleles in wild tiger salamanders (Ambystoma tigrinum)

Major histocompatibility complex (MHC) class II genes are usually among the most polymorphic in vertebrate genomes because of their critical role (antigen presentation) in immune response. Prior to this study, the MHC was poorly characterized in tiger salamanders (Ambystoma tigrinum), but the congeneric axolotl (Ambystoma mexicanum) is thought to have an unusual MHC. Most notably, axolotl class II genes lack allelic variation and possess a splice variant without a full peptide binding region (PBR). The axolotl is considered immunodeficient, but it is unclear how or to what extent MHC genetics and immunodeficiency are interrelated. To study the evolution of MHC genes in urodele amphibians, we describe for the first time an expressed polymorphic class II gene in wild tiger salamanders. We sequenced the PBR of a class II gene from wild A. tigrinum (n=33) and identified nine distinct alleles. Observed heterozygosity was 73%, and there were a total of 46 polymorphic sites, most of which correspond to amino acid positions that bind peptides. Patterns of nucleotide substitutions exhibit the signature of diversifying selection, but no recombination was detected. Not surprisingly, transspecies evolution of tiger salamander and axolotl class II alleles was apparent. We have no direct data on the immunodeficiency of tiger salamanders, but the levels of polymorphism in our study population should suffice to bind a variety of foreign peptides (unlike axolotls). Our tiger salamander data suggest that the monomorphism and immunodeficiencies associated with axolotl class II genes is a relict of their unique historical demography, not their phylogenetic legacy. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Social pairing and female mating fidelity predicted by restriction fragment length polymorphism similarity at the major histocompatibility complex in a songbird

Female birds often copulate outside the pair-bond to produce broods of mixed paternity, but despite much recent attention the adaptive significance of this behaviour remains elusive. Although several studies support the idea that extra-pair copulations (EPCs) allow females to obtain 'good genes' for their offspring, many others have found no relationship between female mating fidelity and traits likely to reflect male quality. A corollary to the good genes hypothesis proposes that females do use EPCs to increase the quality of young, but it is the interaction between maternal and paternal genomes - and not male quality per se - that is the target of female choice. We tested this 'genetic compatibility' hypothesis in a free-living population of Savannah sparrows (Passerculus sandwichensis) by determining whether females mated nonrandomly with respect to the major histocompatibility complex (Mhc). During both the 1994 and 1995 breeding seasons, female yearlings (but not older birds) avoided pairing with Mhc-similar males (P < 0.005). The Mhc similarity between mates also predicted the occurrence of extra-pair young in first broods (P < 0.007) and covaried with estimates of genome-wide levels of similarity derived from multilocus DNA fingerprinting profiles (P = 0.007). The overall genetic similarity between adults tended to predict female mating fidelity, but with less precision than their Mhc similarity (P = 0.09). In contrast, females appeared insensitive to the size, weight or age of males, none of which explained variation in female mating fidelity. Taken together, these results are consistent with the hypothesis that females sought complementary genes for their offspring and suggest either that the benefits of heterozygosity (at the Mhc) drive female mating patterns or that the avoidance of inbreeding is an ultimate cause of social and genetic mate choice in Savannah sparrows.