No evidence of an MHC-based female mating preference in great reed warblers

Patterns of MHC-dependent sexual selection in a free-living population of sheep

The MHC is one of the most polymorphic gene clusters in vertebrates and play an essential role in adaptive immunity. Apart from pathogen-mediated selection, sexual selection can also contribute to the maintenance of MHC diversity. MHC-dependent sexual selection could occur via several mechanisms but at present there is no consensus as to which of these mechanisms are involved and their importance. Previous studies have often suffered from limited genetic and behavioural data and small sample size, and were rarely able to examine all the mechanisms together, determine whether signatures of MHC-based non-random mating are independent of genomic effects or differentiate whether MHC-dependent sexual selection takes place at the pre- or post-copulatory stage. In this study, we use Monte Carlo simulation to investigate evidence for non-random MHC-dependent mating patterns by all three mechanisms in a free-living population of Soay sheep. Using 1710 sheep diplotyped at the MHC class IIa region and genome-wide SNPs, together with field observations of consorts, we found sexual selection against a particular haplotype in males at the pre-copulatory stage and sexual selection against female MHC heterozygosity during the rut. We also found MHC-dependent disassortative mating at the post-copulatory stage, along with strong evidence of inbreeding avoidance at both stages. However, results from generalized linear mixed models suggest that the pattern of MHC-dependent disassortative mating could be a by-product of inbreeding avoidance. Our results therefore suggest that while multiple apparent mechanisms of non-random mating with respect to the MHC may occur, some of them have alternative explanations.

Molecular Characterization of MHC Class I Genes in Four Species of the Turdidae Family to Assess Genetic Diversity and Selection

In vertebrate animals, the molecules encoded by major histocompatibility complex (MHC) genes play an essential role in the adaptive immunity. MHC class I deals with intracellular pathogens (virus) in birds. MHC class I diversity depends on the consequence of local and global environment selective pressure and gene flow. Here, we evaluated the MHC class I gene in four species of the Turdidae family from a broad geographical area of northeast China. We isolated 77 MHC class I sequences, including 47 putatively functional sequences and 30 pseudosequences from 80 individuals. Using the method based on analysis of cloned amplicons (n = 25) for each species, we found two and seven MHC I sequences per individual indicating more than one MHC I locus identified in all sampled species. Results revealed an overall elevated genetic diversity at MHC class I, evidence of different selection patterns among the domains of PBR and non-PBR. Alleles are found to be divergent with overall polymorphic sites per species ranging between 58 and 70 (out of 291 sites). Moreover, transspecies alleles were evident due to convergent evolution or recent speciation for the genus. Phylogenetic relationships among MHC I show an intermingling of alleles clustering among the Turdidae family rather than between other passerines. Pronounced MHC I gene diversity is essential for the existence of species. Our study signifies a valuable tool for the characterization of evolutionary relevant difference across a population of birds with high conservational concerns.

Mating preferences can drive expansion or contraction of major histocompatibility complex gene family

Major histocompatibility complex (MHC)-based mating rules can evolve as a way to avoid inbreeding or to increase offspring immune competence. While the role of mating preference in shaping the MHC diversity in vertebrates has been acknowledged, its impact on individual MHC diversity has not been considered. Here, we use computer simulations to investigate how simple mating rules favouring MHC-dissimilar partners affect the evolution of the number of MHC variants in individual genomes, accompanying selection for resistance to parasites. We showed that the effect of such preferences could sometimes be dramatic. If preferences are aimed at avoiding identical alleles, the equilibrium number of MHC alleles is much smaller than under random mating. However, if the mating rule minimizes the ratio of shared to different alleles in partners, MHC number is higher than under random mating. Additionally, our simulations revealed that a negative correlation between the numbers of MHC variants in mated individuals can arise from simple rules of MHC-disassortative mating. Our results reveal unexpected potential of MHC-based mating preferences to drive MHC gene family expansions or contractions and highlight the need to study the mechanistic basis of such preferences, which is currently poorly understood.

MHC class I diversity predicts non-random mating in Chinese alligators (Alligator sinensis)

The major histocompatibility complex (MHC) has several important roles in kin recognition, pathogen resistance and mate selection. Research in fish, birds and mammals has suggested that individuals optimise MHC diversity, and therefore offspring fitness, when choosing mates. In reptiles, however, it is unclear whether female mate choice is based on genome-wide genetic characteristics such as microsatellite DNA loci, particular functional-trait loci (e.g., MHC) or both, and MHC's effects on mate choice remain relatively understudied. Herein, we used 13 microsatellite loci and two MHC class I loci to investigate female mate choice of Chinese alligators (Alligator sinensis) in the semi-natural condition. We also determined correlations between the MHC genotype of breeding males and male reproductive success. We found that MHC-heterozygous males harbour a greater reproductive success, which probably is the reason that these males are more preferred by the females than MHC-homozygous males. Furthermore, the MHC class I amino-acid distance and functional distance of true mating pairs were higher compared with those of randomly sampled pairs. Analysis of microsatellites revealed that, despite mate choice, females did not completely avoid inbreeding. These findings are the first evidence of MHC-associated mate choice in Chinese alligators, suggesting that females may adopt different mating strategies after assessing the MHC characteristics of potential mates.

"Balancing" balancing selection? Assortative mating at the major histocompatibility complex despite molecular signatures of balancing selection

In vertebrate animals, genes of the major histocompatibility complex (MHC) determine the set of pathogens to which an individual's adaptive immune system can respond. MHC genes are extraordinarily polymorphic, often showing elevated nonsynonymous relative to synonymous sequence variation and sharing presumably ancient polymorphisms between lineages. These patterns likely reflect pathogen-mediated balancing selection, for example, rare-allele or heterozygote advantage. Such selection is often reinforced by disassortative mating at MHC. We characterized exon 2 of MHC class II, corresponding to the hypervariable peptide-binding region, in song sparrows (Melospiza melodia). We compared nonsynonymous to synonymous sequence variation in order to identify positively selected sites; assessed evidence for trans-species polymorphisms indicating ancient balancing selection; and compared MHC similarity of socially mated pairs to expectations under random mating. Six codons showed elevated ratios of nonsynonymous to synonymous variation, consistent with balancing selection, and we characterized several alleles similar to those occurring in at least four other avian families. Despite this evidence for historical balancing selection, mated pairs were significantly more similar at MHC than were randomly generated pairings. Nonrandom mating at MHC thus appears to partially counteract, not reinforce, pathogen-mediated balancing selection in this system. We suggest that in systems where individual fitness does not increase monotonically with MHC diversity, assortative mating may help to avoid excessive offspring heterozygosity that could otherwise arise from long-standing balancing selection.

Chemical composition of preen wax reflects major histocompatibility complex similarity in songbirds

In jawed vertebrates, genes of the major histocompatibility complex (MHC) play a key role in immunity by encoding cell-surface proteins that recognize and bind non-self antigens. High variability at MHC suggests that these loci may also function in social signalling such as mate choice and kin recognition. This requires that MHC genotype covaries with some perceptible phenotypic trait. In mammals and fish, MHC is signalled chemically through volatile and non-volatile peptide odour cues, facilitating MHC-dependent mate choice and other behaviours. In birds, despite evidence for MHC-dependent mating, candidate mechanisms for MHC signalling remain largely unexplored. However, feather preen wax has recently been implicated as a potential source of odour cues. We examined whether the chemical composition of preen wax correlates with MHC class IIβ genotypes of wild song sparrows (Melospiza melodia). Pairwise chemical distance reflected amino acid distance at MHC for male-female dyads, although not for same-sex dyads. Chemical diversity did not reflect MHC diversity. We used gas chromatography-mass spectrometry (GC-MS) to characterize preen wax compounds, and identified four wax esters that best reflect MHC similarity. Provided songbirds can detect variation in preen wax composition, this cue may allow individuals to assess MHC compatibility of potential mates.

Very high MHC Class IIB diversity without spatial differentiation in the mediterranean population of greater Flamingos

Background

Selective pressure from pathogens is thought to shape the allelic diversity of major histocompatibility complex (MHC) genes in vertebrates. In particular, both local adaptation to pathogens and gene flow are thought to explain a large part of the intraspecific variation observed in MHC allelic diversity. To date, however, evidence that adaptation to locally prevalent pathogens maintains MHC variation is limited to species with limited dispersal and, hence, reduced gene flow. On the one hand high gene flow can disrupt local adaptation in species with high dispersal rates, on the other hand such species are much more likely to experience spatial variation in pathogen pressure, suggesting that there may be intense pathogen mediated selection pressure operating across breeding sites in panmictic species. Such pathogen mediated selection pressure operating across breeding sites should therefore be sufficient to maintain high MHC diversity in high dispersing species in the absence of local adaptation mechanisms. We used the Greater Flamingo, Phoenicopterus roseus, a long-lived colonial bird showing a homogeneous genetic structure of neutral markers at the scale of the Mediterranean region, to test the prediction that higher MHC allelic diversity with no population structure should occur in large panmictic populations of long-distance dispersing birds than in other resident species.

Results

We assessed the level of allelic diversity at the MHC Class IIB exon 2 from 116 individuals born in four different breeding colonies of Greater Flamingo in the Mediterranean region. We found one of the highest allelic diversity (109 alleles, 2 loci) of any non-passerine avian species investigated so far relative to the number of individuals and loci genotyped. There was no evidence of population structure between the four major Mediterranean breeding colonies.

Conclusion

Our results suggest that local adaptation at MHC Class IIB in Greater Flamingos is constrained by high gene flow and high MHC diversity appears to be maintained by population wide pathogen-mediated selection rather than local pathogen-mediated selection. Further understanding of how pathogens vary across space and time will be crucial to further elucidate the mechanisms maintaining MHC diversity in species with large panmictic populations and high dispersal rates.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0905-3) contains supplementary material, which is available to authorized users.

Gene duplication and divergence produce divergent MHC genotypes without disassortative mating

Genes of the major histocompatibility complex (MHC) exhibit heterozygote advantage in immune defence, which in turn can select for MHC-disassortative mate choice. However, many species lack this expected pattern of MHC-disassortative mating. A possible explanation lies in evolutionary processes following gene duplication: if two duplicated MHC genes become functionally diverged from each other, offspring will inherit diverse multilocus genotypes even under random mating. We used locus-specific primers for high-throughput sequencing of two expressed MHC Class II B genes in Leach's storm-petrels, Oceanodroma leucorhoa, and found that exon 2 alleles fall into two gene-specific monophyletic clades. We tested for disassortative vs. random mating at these two functionally diverged Class II B genes, using multiple metrics and different subsets of exon 2 sequence data. With good statistical power, we consistently found random assortment of mates at MHC. Despite random mating, birds had MHC genotypes with functionally diverged alleles, averaging 13 amino acid differences in pairwise comparisons of exon 2 alleles within individuals. To test whether this high MHC diversity in individuals is driven by evolutionary divergence of the two duplicated genes, we built a phylogenetic permutation model. The model showed that genotypic diversity was strongly impacted by sequence divergence between the most common allele of each gene, with a smaller additional impact of monophyly of the two genes. Divergence of allele sequences between genes may have reduced the benefits of actively seeking MHC-dissimilar mates, in which case the evolutionary history of duplicated genes is shaping the adaptive landscape of sexual selection.

Mate choice for neutral and MHC genetic characteristics in Alpine marmots: different targets in different contexts?

Sexual selection through female mate choice for genetic characteristics has been suggested to be an important evolutionary force maintaining genetic variation in animal populations. However, the genetic targets of female mate choice are not clearly identified and whether female mate choice is based on neutral genetic characteristics or on particular functional loci remains an open question. Here, we investigated the genetic targets of female mate choice in Alpine marmots (Marmota marmota), a socially monogamous mammal where extra‐pair paternity (EPP) occurs. We used 16 microsatellites to describe neutral genetic characteristics and two MHC loci belonging to MHC class I and II as functional genetic characteristics. Our results reveal that (1) neutral and MHC genetic characteristics convey different information in this species, (2) social pairs show a higher MHC class II dissimilarity than expected under random mate choice, and (3) the occurrence of EPP increases when social pairs present a high neutral genetic similarity or dissimilarity but also when they present low MHC class II dissimilarity. Thus, female mate choice is based on both neutral and MHC genetic characteristics, and the genetic characteristics targeted seem to be context dependent (i.e., the genes involved in social mate choice and genetic mate choice differ). We emphasize the need for empirical studies of mate choice in the wild using both neutral and MHC genetic characteristics because whether neutral and functional genetic characteristics convey similar information is not universal.

Social pairing of Seychelles warblers under reduced constraints: MHC, neutral heterozygosity, and age

The prevalence and significance of precopulatory mate choice remains keenly debated. The major histocompatibility complex (MHC) plays a key role in vertebrate adaptive immunity, and variation at the MHC influences individual survival. Although MHC-dependent mate choice has been documented in certain species, many other studies find no such pattern. This may be, at least in part, because in natural systems constraints may reduce the choices available to individuals and prevent full expression of underlying preferences. We used translocations to previously unoccupied islands to experimentally reduce constraints on female social mate choice in the Seychelles warbler (Acrocephalus sechellensis), a species in which patterns of MHC-dependent extrapair paternity (EPP), but not social mate choice, have been observed. We find no evidence of MHC-dependent social mate choice in the new populations. Instead, we find that older males and males with more microsatellite heterozygosity are more likely to have successfully paired. Our data cannot resolve whether these patterns in pairing were due to male-male competition or female choice. However, our research does suggest that female Seychelles warblers do not choose social mates using MHC class I to increase fitness. It may also indicate that the MHC-dependent EPP observed in the source population is probably due to mechanisms other than female precopulatory mate choice based on MHC cues.

Females tend to prefer genetically similar mates in an island population of house sparrows

BACKGROUND

It is often proposed that females should select genetically dissimilar mates to maximize offspring genetic diversity and avoid inbreeding. Several recent studies have provided mixed evidence, however, and in some instances females seem to prefer genetically similar males. A preference for genetically similar mates can be adaptive if outbreeding depression is more harmful than inbreeding depression or if females gain inclusive fitness benefits by mating with close kin. Here, we investigated genetic compatibility and mating patterns in an insular population of house sparrow (Passer domesticus), over a three-year period, using 12 microsatellite markers and one major histocompability complex (MHC) class I gene. Given the small population size and the distance from the mainland, we expected a reduced gene flow in this insular population and we predicted that females would show mating preferences for genetically dissimilar mates.

RESULTS

Contrary to our expectation, we found that offspring were less genetically diverse (multi-locus heterozygosity) than expected under a random mating, suggesting that females tended to mate with genetically similar males. We found high levels of extra-pair paternity, and offspring sired by extra-pair males had a better fledging success than those sired by the social male. Again, unexpectedly, females tended to be more closely related to extra-pair mates than to their social mates. Our results did not depend on the type of genetic marker used, since microsatellites and MHC genes provided similar results, and we found only little evidence for MHC-dependent mating patterns.

CONCLUSIONS

These results are in agreement with the idea that mating with genetically similar mates can either avoid the disruption of co-adapted genes or confer a benefit in terms of kin selection.

Extraordinary MHC class II B diversity in a non-passerine, wild bird: the Eurasian Coot Fulica atra (Aves: Rallidae)

The major histocompatibility complex (MHC) hosts the most polymorphic genes ever described in vertebrates. The MHC triggers the adaptive branch of the immune response, and its extraordinary variability is considered an evolutionary consequence of pathogen pressure. The last few years have witnessed the characterization of the MHC multigene family in a large diversity of bird species, unraveling important differences in its polymorphism, complexity, and evolution. Here, we characterize the first MHC class II B sequences isolated from a Rallidae species, the Eurasian Coot Fulica atra. A next-generation sequencing approach revealed up to 265 alleles that translated into 251 different amino acid sequences (β chain, exon 2) in 902 individuals. Bayesian inference identified up to 19 codons within the presumptive peptide-binding region showing pervasive evidence of positive, diversifying selection. Our analyses also detected a significant excess of high-frequency segregating sites (average Tajima's D = 2.36, P < 0.05), indicative of balancing selection. We found one to six different alleles per individual, consistent with the occurrence of at least three MHC class II B gene duplicates. However, the genotypes comprised of three alleles were by far the most abundant in the population investigated (49.4%), followed by those with two (29.6%) and four (17.5%) alleles. We suggest that these proportions are in agreement with the segregation of MHC haplotypes differing in gene copy number. The most widespread segregating haplotypes, according to our findings, would contain one single gene or two genes. The MHC class II of the Eurasian Coot is a valuable system to investigate the evolutionary implications of gene copy variation and extensive variability, the greatest ever found, to the best of our knowledge, in a wild population of a non-passerine bird.

Towards the non-invasive assessment of MHC genotype in wild primates: analysis of wild Assamese macaque MHC-DRB from fecal samples

The major histocompatibility complex (MHC) plays an important role in the immune response and may thus crucially affect an individual's fitness, relevant also for studies on evolutionary ecology and wildlife conservation. Detailed knowledge on the genomic organization, polymorphism and diversity of the MHC has a narrow taxonomic focus though and among macaques is only available for rhesus and long-tailed macaques-the species most commonly kept for biomedical research. The lack of data on wild populations is largely due to the difficulty of obtaining blood or tissue samples necessary for genotyping approaches. Here, we aimed at analyzing MHC-DRB from non-invasively collected fecal samples in wild Assamese macaques (Macaca assamensis), utilizing the MHC-DRB-STR (D6S2878) microsatellite marker. Due to the fecal DNA source incomplete genotypes occurred, which may be improved in the future by method refinement. We detected 28 distinct DRB-STR lengths in 43 individuals with individual genotypes containing 1-9 MHC-DRB-STRs and defined four haplotypes segregating between families in Mendelian fashion. Our results indicate that variability and diversity of MHC-DRB in Assamese macaques is comparable to that of other macaque species and importantly, that fecal samples can be used for non-invasive analysis of MHC genes after refinement of the applied methods, opening a number of opportunities for MHC research on natural populations.

Major histocompatibility complex class I evolution in songbirds: universal primers, rapid evolution and base compositional shifts in exon 3

Genes of the Major Histocompatibility Complex (MHC) have become an important marker for the investigation of adaptive genetic variation in vertebrates because of their critical role in pathogen resistance. However, despite significant advances in the last few years the characterization of MHC variation in non-model species still remains a challenging task due to the redundancy and high variation of this gene complex. Here we report the utility of a single pair of primers for the cross-amplification of the third exon of MHC class I genes, which encodes the more polymorphic half of the peptide-binding region (PBR), in oscine passerines (songbirds; Aves: Passeriformes), a group especially challenging for MHC characterization due to the presence of large and complex MHC multigene families. In our survey, although the primers failed to amplify exon 3 from two suboscine passerine birds, they amplified exon 3 of multiple MHC class I genes in all 16 species of oscine songbirds tested, yielding a total of 120 sequences. The 16 songbird species belong to 14 different families, primarily within the Passerida, but also in the Corvida. Using a conservative approach based on the analysis of cloned amplicons (n = 16) from each species, we found between 3 and 10 MHC sequences per individual. Each allele repertoire was highly divergent, with the overall number of polymorphic sites per species ranging from 33 to 108 (out of 264 sites) and the average number of nucleotide differences between alleles ranging from 14.67 to 43.67. Our survey in songbirds allowed us to compare macroevolutionary dynamics of exon 3 between songbirds and non-passerine birds. We found compelling evidence of positive selection acting specifically upon peptide-binding codons across birds, and we estimate the strength of diversifying selection in songbirds to be about twice that in non-passerines. Analysis using comparative methods suggest weaker evidence for a higher GC content in the 3rd codon position of exon 3 in non-passerine birds, a pattern that contrasts with among-clade GC patterns found in other avian studies and may suggests different mutational mechanisms. Our primers represent a useful tool for the characterization of functional and evolutionarily relevant MHC variation across the hyperdiverse songbirds.

The functional importance of sequence versus expression variability of MHC alleles in parasite resistance

Understanding selection processes driving the pronounced allelic polymorphism of the major histocompatibility complex (MHC) genes and its functional associations to parasite load have been the focus of many recent wildlife studies. Two main selection scenarios are currently debated which explain the susceptibility or resistance to parasite infections either by the effects of (1) specific MHC alleles which are selected frequency-dependent in space and time or (2) a heterozygote or divergent allele advantage. So far, most studies have focused only on structural variance in co-evolutionary processes although this might not be the only trait subject to natural selection. In the present study, we analysed structural variance stretching from exon1 through exon3 of MHC class II DRB genes as well as genotypic expression variance in relation to the gastrointestinal helminth prevalence and infection intensity in wild yellow-necked mice (Apodemus flavicollis). We found support for the functional importance of specific alleles both on the sequence and expression level. By resampling a previously investigated study population we identified specific MHC alleles affected by temporal shifts in parasite pressure and recorded associated changes in allele frequencies. The allele Apfl-DRB*23 was associated with resistance to infections by the oxyurid nematode Syphacia stroma and at the same time with susceptibility to cestode infection intensity. In line with our expectation, MHC mRNA transcript levels tended to be higher in cestode-infected animals carrying the allele Apfl-DRB*23. However, no support for a heterozygote or divergent allele advantage on the sequence or expression level was detected. The individual amino acid distance of genotypes did not explain individual differences in parasite loads and the genetic distance had no effect on MHC genotype expression. For ongoing studies on the functional importance of expression variance in parasite resistance, allele-specific expression data would be preferable.

MHC class I variation in a natural blue tit population (Cyanistes caeruleus)

The major histocompatibility complex (MHC) is central to the vertebrate immune system and its highly polymorphic genes are considered to influence several life-history traits of individuals. To characterize the MHC in a natural population of blue tits (Cyanistes caeruleus) we investigated the class I exon 3 diversity of more than 900 individuals. We designed two pairs of motif-specific primers that reliably amplify independent subsets of MHC alleles. Applying denaturing gradient gel electrophoresis (DGGE) we obtained 48 independently inherited units of unique band patterns (DGGE-haplogroups), which were validated in a segregation analysis within 105 families. In a second approach, we extensively sequenced 6 unrelated individuals to confirm that DGGE-haplogroup composition reflects individual allelic variation. The highest number of different DGGE-haplogroups in a single individual corresponded in 19 MHC exon 3 sequences, suggesting a minimum of 10 amplified MHC class I loci in the blue tit. In total, we identified 50 unique functional and 3 non-functional sequences. Functional sequences showed high levels of recombination and strong positive selection in the antigen binding region, whereas nucleotide diversity was comparatively low in the range of all passerine species. Finally, in a phylogenetic comparison of passerine MHC class I exon 3 sequences we discuss conflicting evolutionary signals possibly due to recent gene duplication, recombination events and concerted evolution. Our results indicate that the described method is suitable to effectively explore the MHC diversity and its ecological impacts in blue tits in future studies.

Social and extra-pair mating in relation to major histocompatibility complex variation in common yellowthroats

Females are thought to gain better-quality genes for their offspring by mating with particular males. Genes of the major histocompatibility complex (MHC) play a critical role in adaptive immunity, and several studies have examined female mate choice in relation to MHC variation. In common yellowthroats, females prefer males that have larger black facial masks, an ornament associated with MHC variation, immune function and condition. Here we also tested whether mating patterns are directly correlated with MHC diversity or similarity. Using pyrosequencing, we found that the presence of extra-pair young in the brood was not related to male MHC diversity or similarity between the female and her within-pair mate. Furthermore, extra-pair sires did not differ in overall diversity from males they cuckolded, or in their similarity to the female. MHC diversity is extremely high in this species, and it may limit the ability of females to assess MHC variation in males. Thus, mating may be based on ornaments, such as mask size, which are better indicators of overall male health and genetic quality.

Major histocompatibility complex class II compatibility, but not class I, predicts mate choice in a bird with highly developed olfaction

Mate choice for major histocompatibility complex (MHC) compatibility has been found in several taxa, although rarely in birds. MHC is a crucial component in adaptive immunity and by choosing an MHC-dissimilar partner, heterozygosity and potentially broad pathogen resistance is maximized in the offspring. The MHC genotype influences odour cues and preferences in mammals and fish and hence olfactory-based mate choice can occur. We tested whether blue petrels, Halobaena caerulea, choose partners based on MHC compatibility. This bird is long-lived, monogamous and can discriminate between individual odours using olfaction, which makes it exceptionally well suited for this analysis. We screened MHC class I and II B alleles in blue petrels using 454-pyrosequencing and quantified the phylogenetic, functional and allele-sharing similarity between individuals. Partners were functionally more dissimilar at the MHC class II B loci than expected from random mating (p = 0.033), whereas there was no such difference at the MHC class I loci. Phylogenetic and non-sequence-based MHC allele-sharing measures detected no MHC dissimilarity between partners for either MHC class I or II B. Our study provides evidence of mate choice for MHC compatibility in a bird with a high dependency on odour cues, suggesting that MHC odour-mediated mate choice occurs in birds.

MHC signaling during social communication

The major histocompatibility complex (MHC) has been known to play a critical role in immune recognition since the 1950s. It was a surprise, then, in the 1970s when the first report appeared indicating MHC might also function in social signaling. Since this seminal discovery, MHC signaling has been found throughout vertebrates and its known functions have expanded beyond mate choice to include a suite of behaviors from kin-biased cooperation, parent-progeny recognition to pregnancy block. The widespread occurrence of MHC in social signaling has revealed conserved behavioral-genetic mechanisms that span vertebrates and includes humans. The identity of the signal's chemical constituents and the receptors responsible for the perception of the signal have remained elusive, but recent advances have enabled the identification of the key components of the behavioral circuit. In this chapter we organize recent findings from the literature and discuss them in relation to four nonmutually exclusive models wherein MHC functions as a signal of (i) individuality, (ii) relatedness, (iii) genetic compatibility and (iv) quality. We also synthesize current mechanistic studies, showing how knowledge about the molecular basis of MHC signaling can lead to elegant and informative experimental manipulations. Finally, we discuss current evidence relating to the primordial functions of the MHC, including the possibility that its role in social signaling may be ancestral to its central role in adaptive immunity.

Characterization of MHC class I and II genes in a subantarctic seabird, the blue petrel, Halobaena caerulea (Procellariiformes)

The great polymorphism observed in the major histocompatibility complex (MHC) genes is thought to be maintained by pathogen-mediated selection possibly combined with MHC-disassortative mating, guided by MHC-determined olfactory cues. Here, we partly characterize the MHC class I and II B of the blue petrel, Halobaena caerulea (Procellariiformes), a bird with significant olfactory abilities that lives under presumably low pathogen burdens in Subantarctica. Blue petrels are long-lived, monogamous birds which suggest the necessity of an accurate mate choice process. The species is ancestral to songbirds (Passeriformes; many MHC loci), although not to gamefowls (Galliformes; few MHC loci). Considering the phylogenetic relationships and the low subantarctic pathogen burden, we expected few rather than many MHC loci in the blue petrel. However, when we analysed partial MHC class I and class II B cDNA and gDNA sequences we found evidence for as many as at least eight MHC class I loci and at least two class II B loci. These class I and II B sequences showed classical MHC characteristics, e.g. high nucleotide diversity, especially in putative peptide-binding regions where signatures of positive selection was detected. Trans-species polymorphism was found between MHC class II B sequences of the blue petrel and those of thin-billed prion, Pachyptila belcheri, two species that diverged ∼25 MYA. The observed MHC allele richness in the blue petrel may well serve as a basis for mate choice, especially since olfactory discrimination of MHC types may be possible in this species.

Female house sparrows "count on" male genes: experimental evidence for MHC-dependent mate preference in birds

Background

Females can potentially assess the quality of potential mates using their secondary sexual traits, and obtain "good genes" that increase offspring fitness. Another potential indirect benefit from mating preferences is genetic compatibility, which does not require extravagant or viability indicator traits. Several studies with mammals and fish indicate that the genes of the major histocompatibility complex (MHC) influence olfactory cues and mating preferences, and such preferences confer genetic benefits to offspring. We investigated whether individual MHC diversity (class I) influences mating preferences in house sparrows (Passer domesticus).

Results

Overall, we found no evidence that females preferred males with high individual MHC diversity. Yet, when we considered individual MHC allelic diversity of the females, we found that females with a low number of alleles were most attracted to males carrying a high number of MHC alleles, which might reflect a mating-up preference by allele counting.

Conclusions

This is the first experimental evidence for MHC-dependent mating preferences in an avian species to our knowledge. Our findings raise questions about the underlying mechanisms through which birds discriminate individual MHC diversity among conspecifics, and they suggest a novel mechanism through which mating preferences might promote the evolution of MHC polymorphisms and generate positive selection for duplicated MHC loci.

Putative causes and consequences of MHC variation within and between locally adapted stickleback demes

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.

MHC, mate choice and heterozygote advantage in a wild social primate

Preferences for mates carrying dissimilar genes at the major histocompatibility complex (MHC) may help animals increase offspring pathogen resistance or avoid inbreeding. Such preferences have been reported across a range of vertebrates, but have rarely been investigated in social species other than humans. We investigated mate choice and MHC dynamics in wild baboons (Papio ursinus). MHC Class II DRB genes and 16 microsatellite loci were genotyped across six groups (199 individuals). Based on the survey of a key segment of the gene-rich MHC, we found no evidence of mate choice for MHC dissimilarity, diversity or rare MHC genotypes. First, MHC dissimilarity did not differ from random expectation either between parents of the same offspring or between immigrant males and females from the same troop. Second, female reproductive success was not influenced by MHC diversity or genotype frequency. Third, population genetic structure analysis revealed equally high genotypic differentiation among troops, and comparable excess heterozygosity within troops for juveniles, at both Mhc-DRB and neutral loci. Nevertheless, the age structure of Mhc-DRB heterozygosity suggested higher longevity for heterozygotes, which should favour preferences for MHC dissimilarity. We propose that high levels of within-group outbreeding, resulting from group-living and sex-biased dispersal, might weaken selection for MHC-disassortative mate choice.

Opposites attract: MHC-associated mate choice in a polygynous primate

We investigated reproduction in a semi-free-ranging population of a polygynous primate, the mandrill, in relation to genetic relatedness and male genetic characteristics, using neutral microsatellite and major histocompatibility complex (MHC) genotyping. We compared genetic dissimilarity to the mother and genetic characteristics of the sire with all other potential sires present at the conception of each offspring (193 offspring for microsatellite genetics, 180 for MHC). The probability that a given male sired increased as pedigree relatedness with the mother decreased, and overall genetic dissimilarity and MHC dissimilarity with the mother increased. Reproductive success also increased with male microsatellite heterozygosity and MHC diversity. These effects were apparent despite the strong influence of dominance rank on male reproductive success. The closed nature of our study population is comparable to human populations for which MHC-associated mate choice has been reported, suggesting that such mate choice may be especially important in relatively isolated populations with little migration to introduce genetic variation.

Individual odor recognition in procellariiform chicks: potential role for the major histocompatibility complex

Since the groundbreaking work of Wenzel, Bang, and Grubb in the 1960s, enormous progress has been made toward elucidating the sense of smell in procellariiform seabirds. Although it is now well established that adult procellariiforms use olfaction in many behaviors, such as for foraging, nest relocation, and mate recognition, the olfactory abilities of petrel chicks are less well understood. Recent studies have shown that petrel chicks can recognize prey-related odors and odors associated with their nest before leaving their burrow for the first time. The recognition of burrow odors by petrel chicks is unlikely to be used for homing, and we have suggested that chicks may be learning personal odors associated with the nest's occupants for use later in life in the context of kin recognition or mate choice. The source of personal odors in petrels is unknown. However, in other vertebrates, the major histocompatibility complex influences body odors, which in turn influence mating preferences. It is not currently known whether this highly polymorphic gene region influences body odors and individual recognition in the procellariiforms, but this could be a fruitful area of future research.

Evidence that pairing with genetically similar mates is maladaptive in a monogamous bird

Background

Evidence of multiple genetic criteria of mate choice is accumulating in numerous taxa. In many species, females have been shown to pair with genetically dissimilar mates or with extra-pair partners that are more genetically compatible than their social mates, thereby increasing their offsprings' heterozygosity which often correlates with offspring fitness. While most studies have focused on genetically promiscuous species, few studies have addressed genetically monogamous species, in which mate choice tends to be mutual.

Results

Here, we used microsatellite markers to assess individual global heterozygosity and genetic similarity of pairs in a socially and genetically monogamous seabird, the black-legged kittiwake Rissa tridactyla. We found that pairs were more genetically dissimilar than expected by chance. We also identified fitness costs of breeding with genetically similar partners: (i) genetic similarity of pairs was negatively correlated with the number of chicks hatched, and (ii) offspring heterozygosity was positively correlated with growth rate and survival.

Conclusion

These findings provide evidence that breeders in a genetically monogamous species may avoid the fitness costs of reproducing with a genetically similar mate. In such species that lack the opportunity to obtain extra-pair fertilizations, mate choice may therefore be under high selective pressure.

Influence of major histocompatibility complex genotype on mating success in a free-ranging reptile population

Major histocompatibility complex (MHC) genes are highly polymorphic components of the vertebrate immune system, which play a key role in pathogen resistance. MHC genes may also function as odour-related cues for mate choice, thus ensuring optimal MHC diversity in offspring. MHC-associated mate choice has been demonstrated in some fish, bird and mammal species but it is not known whether this is a general vertebrate phenomenon. We investigated whether MHC-associated mate choice occurs in a wild population of tuatara (Sphenodon punctatus), a territorial and sexually dimorphic reptile. We found weak evidence for MHC-disassortative mating, based on amino acid genotypic distance between pairs, when mated pairs were directly compared with potential pairs in close spatial proximity. No significant association was found between male mating success, number of MHC sequences, microsatellite heterozygosity or MHC lineage. The major determinant of mating success in tuatara was male body size, which was not related to MHC lineage or microsatellite heterozygosity. Our results suggest that male competitive ability is the primary driver of mating success in tuatara. However, MHC-associated preferences also appear to play a role, possibly as a kin avoidance mechanism during territory formation.

Selection, diversity and evolutionary patterns of the MHC class II DAB in free-ranging Neotropical marsupials

BACKGROUND

Research on the genetic architecture and diversity of the MHC has focused mainly on eutherian mammals, birds and fish. So far, studies on model marsupials used in laboratory investigations indicated very little or even no variation in MHC class II genes. However, natural levels of diversity and selection are unknown in marsupials as studies on wild populations are virtually absent. We used two endemic South American mouse opossums, Gracilinanus microtarsus and Marmosops incanus, to investigate characteristic features of MHC selection. This study is the first investigation of MHC selection in free-ranging Neotropical marsupials. In addition, the evolutionary history of MHC lineages within the group of marsupials was examined.

RESULTS

G. microtarsus showed extensive levels of MHC diversity within and among individuals as 47 MHC-DAB alleles and high levels of sequence divergence were detected at a minimum of four loci. Positively selected codon sites were identified, of which most were congruent with human antigen binding sites. The diversity in M. incanus was rather low with only eight observed alleles at presumably two loci. However, these alleles also revealed high sequence divergence. Again, positive selection was identified on specific codon sites, all congruent with human ABS and with positively selected sites observed in G. microtarsus. In a phylogenetic comparison alleles of M. incanus interspersed widely within alleles of G. microtarsus with four alleles being present in both species.

CONCLUSION

Our investigations revealed extensive MHC class II polymorphism in a natural marsupial population, contrary to previous assumptions. Furthermore, our study confirms for the first time in marsupials the presence of three characteristic features common at MHC loci of eutherian mammals, birds and fish: large allelic sequence divergence, positive selection on specific sites and trans-specific polymorphism.

Influence of genetic dissimilarity in the reproductive success and mate choice of brown trout - females fishing for optimal MHC dissimilarity

We examined the reproductive success of 48 adult brown trout (Salmo trutta L.) which were allowed to reproduce in a stream that was controlled for the absence of other trout. Parentage analyses based on 11 microsatellites permitted us to infer reproductive success and mate choice preferences in situ. We found that pairs with intermediate major histocompatibility complex (MHC) dissimilarity mated more often than expected by chance. It appears that female choice was the driving force behind this observation because, compared with other individuals, males with intermediate MHC dissimilarity produced a larger proportion of offspring, whereas female reproductive output did not show this pattern. Hence, rather than seeking mates with maximal MHC dissimilarity, as found in several species, brown trout seemed to prefer mates of intermediate MHC difference, thus supporting an optimality-based model for MHC-dependent mate choice.

Genetic basis for MHC-dependent mate choice

Genes in the major histocompatibility complex (MHC), best known for their role in immune recognition and transplantation success, are also involved in modulating mate choice in mice. Early studies with inbred, congenic mouse lines showed that mate choice tended to favor nonself MHC types. A similar phenomenon was demonstrated with semi-wild mice as well. Subsequent studies showed that, rather than nonself choices, it was more accurate to say that mice chose nonparental MHC types for mates since preferences for nonself could be reversed if mice were fostered from birth on parents with nonself MHC types. Other studies have demonstrated that parent-offspring recognition is also regulated by MHC-determined signals suggesting that this system is one of general importance for mouse behavior. Many studies have now demonstrated that volatile mouse body odors are regulated by MHC genes and it is presumably these odor differences that underlie mate choice and familial recognition. Recent studies have shown that many odorants are controlled by the MHC but the mechanism by which MHC genes exert their influence has not been identified. Surprisingly, not only are volatile body odors influenced by MHC genes but so too are nonvolatile signals. Peptides bound to the MHC protein may also function in individual recognition. The extent to which this system is involved in mate choice of other species is unclear although there are some suggestive studies. Indeed, there is tentative evidence that MHC differences, presumably acting via odor changes, may influence human partner selection. Further studies should clarify both the mechanism underlying MHC influence on body odors as well as the generality of their importance in mate selection.

Complex Mhc-based mate choice in a wild passerine

The extreme polymorphism of the vertebrate major histocompatibility complex (Mhc) is famous for protecting hosts against constantly evolving pathogens. Mate choice is often evoked as a means of maintaining Mhc variability through avoidance of partners with similar Mhc alleles or preference for heterozygotes. Evidence for these two hypotheses mostly comes from studies on humans and laboratory mice. Here, we tested these hypotheses in a wild outbred population of house sparrows (Passer domesticus). Females were not more or less closely related to the males they paired with when considering neutral genetic variation. However, males failed to form breeding pairs when they had too few Mhc alleles and when they were too dissimilar from females at Mhc loci (i.e. had no common alleles). Furthermore, pairs did not form at random as Mhc diversity positively correlated in mating pairs. These results suggest that mate choice evolves in response to (i) benefits in terms of parasite resistance acquired from allelic diversity, and (ii) costs associated with the disruption of co-adapted genes.

The evolutionary ecology of the major histocompatibility complex

The major histocompatibility complex (MHC) has become a paradigm for how selection can act to maintain adaptively important genetic diversity in natural populations. Here, we review the contribution of studies on the MHC in non-model species to our understanding of how selection affects MHC diversity, emphasising how ecological and ethological processes influence the tempo and mode of evolution at the MHC, and conversely, how variability at the MHC affects individual fitness, population dynamics and viability. We focus on three main areas: the types of information that have been used to detect the action of selection on MHC genes; the relative contributions of parasite-mediated and sexual selection on the maintenance of MHC diversity; and possible future lines of research that may help resolve some of the unanswered issues associated with MHC evolution.