Directional and balancing selection in human beta-defensins

A single amino acid polymorphism in natural Metchnikowin alleles of Drosophila results in systemic immunity and life history tradeoffs

Antimicrobial peptides (AMPs) are at the interface of interactions between hosts and microbes and are therefore expected to be rapidly evolving in a coevolutionary arms race with pathogens. In contrast, previous work demonstrated that insect AMPs tend to evolve more slowly than the genome average. Metchikowin (Mtk) is a Drosophila AMP that has a single amino acid residue that segregates as either proline (P) or arginine (R) in populations of four different species, some of which diverged more than 10 million years ago. These results suggest that there is a distinct functional importance to each allele. The most likely hypotheses are driven by two main questions: does each allele have a different efficacy against different specific pathogens (specificity hypothesis)? Or, is one allele a more potent antimicrobial, but with a host fitness cost (autoimmune hypothesis)? To assess their functional differences, we created D. melanogaster lines with the P allele, R allele, or Mtk null mutation using CRISPR/Cas9 genome editing and performed a series of life history and infection assays to assess them. In males, testing of systemic immune responses to a repertoire of bacteria and fungi demonstrated that the R allele performs as well or better than the P and null alleles with most infections. Females show some results that contrast with males, with Mtk alleles either not contributing to survival or with the P allele outperforming the R allele. In addition, measurements of life history traits demonstrate that the R allele is more costly in the absence of infection for both sexes. These results are consistent with both the specificity hypothesis (either allele can perform better against certain pathogens depending on context), and the autoimmune hypothesis (the R allele is generally the more potent antimicrobial in males, and carries a fitness cost). These results provide strong in vivo evidence that differential fitness with or without infection and sex-based functional differences in alleles may be adaptive mechanisms of maintaining immune gene polymorphisms in contrast with expectations of rapid evolution. Therefore, a complex interplay of forces including pathogen species and host sex may lead to balancing selection for immune genotypes. Strikingly, this selection may act on even a single amino acid polymorphism in an AMP.

Balancing selection on the complement system of a wild rodent

BACKGROUND

Selection pressure exerted by pathogens can influence patterns of genetic diversity in the host. In the immune system especially, numerous genes encode proteins involved in antagonistic interactions with pathogens, paving the way for coevolution that results in increased genetic diversity as a consequence of balancing selection. The complement system is a key component of innate immunity. Many complement proteins interact directly with pathogens, either by recognising pathogen molecules for complement activation, or by serving as targets of pathogen immune evasion mechanisms. Complement genes can therefore be expected to be important targets of pathogen-mediated balancing selection, but analyses of such selection on this part of the immune system have been limited.

RESULTS

Using a population sample of whole-genome resequencing data from wild bank voles (n = 31), we estimated the extent of genetic diversity and tested for signatures of balancing selection in multiple complement genes (n = 44). Complement genes showed higher values of standardised β (a statistic expected to be high under balancing selection) than the genome-wide average of protein coding genes. One complement gene, FCNA, a pattern recognition molecule that interacts directly with pathogens, was found to have a signature of balancing selection, as indicated by the Hudson-Kreitman-Aguadé test (HKA) test. Scans for localised signatures of balancing selection in this gene indicated that the target of balancing selection was found in exonic regions involved in ligand binding.

CONCLUSION

The present study adds to the growing evidence that balancing selection may be an important evolutionary force on components of the innate immune system. The identified target in the complement system typifies the expectation that balancing selection acts on genes encoding proteins involved in direct interactions with pathogens.

Drosophila immunity: the Drosocin gene encodes two host defence peptides with pathogen-specific roles

Antimicrobial peptides (AMPs) are key to defence against infection in plants and animals. Use of AMP mutations in Drosophila has now revealed that AMPs can additively or synergistically contribute to defence in vivo. However, these studies also revealed high specificity, wherein just one AMP contributes an outsized role in combatting a specific pathogen. Here, we show the Drosocin locus (CG10816) is more complex than previously described. In addition to its namesake peptide 'Drosocin', it encodes a second mature peptide from a precursor via furin cleavage. This peptide corresponds to the previously uncharacterized 'Immune-induced Molecule 7'. A polymorphism (Thr52Ala) in the Drosocin precursor protein previously masked the identification of this peptide, which we name 'Buletin'. Using mutations differently affecting Drosocin and Buletin, we show that only Drosocin contributes to Drosocin gene-mediated defence against Enterobacter cloacae. Strikingly, we observed that Buletin, but not Drosocin, contributes to the Drosocin gene-mediated defence against Providencia burhodogranariea, including an importance of the Thr52Ala polymorphism for survival. Our study reveals that the Drosocin gene encodes two prominent host defence peptides with different specificity against distinct pathogens. This finding emphasizes the complexity of the Drosophila humoral response and demonstrates how natural polymorphisms can affect host susceptibility.

Evolving and assembling to pierce through: Evolutionary and structural aspects of antimicrobial peptides

The burgeoning menace of antimicrobial resistance across the globe has necessitated investigations into other chemotherapeutic strategies to combat infections. Antimicrobial peptides, or host defense peptides, are a set of promising therapeutic candidates in this regard. Most of them cause membrane permeabilization and are a key component of the innate immune response to pathogenic invasion. It has also been reported that peptide self-assembly is a driving factor governing the microbicidal activity of these peptide candidates. While efforts have been made to develop novel synthetic peptides against various microbes, many clinical trials of such peptides have failed due to toxicity and hemolytic activity to the host. A function-guided rational peptide engineering, based on evolutionary principles, physicochemical properties and activity determinants of AMP activity, is expected to help in targeting specific microbes. Furthermore, it is important to develop a unified understanding of the evolution of AMPs in order to fully appreciate their importance in host defense. This review seeks to explore the evolution of AMPs and the physicochemical determinants of AMP activity. The specific interactions driving AMP self-assembly have also been reviewed, emphasizing implications of this self-assembly on microbicidal and immunomodulatory activity.

Evolutionary history of type II transmembrane serine proteases involved in viral priming

Type II transmembrane serine proteases (TTSPs) are a family of trypsin-like membrane-anchored serine proteases that play key roles in the regulation of some crucial processes in physiological conditions, including cardiac function, digestion, cellular iron homeostasis, epidermal differentiation, and immune responses. However, some of them, in particular TTSPs expressed in the human airways, were identified as host factors that promote the proteolytic activation and spread of respiratory viruses such as influenza virus, human metapneumovirus, and coronaviruses, including SARS-CoV-2. Given their involvement in viral priming, we hypothesized that members of the TTSP family may represent targets of positive selection, possibly as the result of virus-driven pressure. Thus, we investigated the evolutionary history of sixteen TTSP genes in mammals. Evolutionary analyses indicate that most of the TTSP genes that have a verified role in viral proteolytic activation present signals of pervasive positive selection, suggesting that viral infections represent a selective pressure driving the evolution of these proteases. We also evaluated genetic diversity in human populations and we identified targets of balancing selection in TMPRSS2 and TMPRSS4. This scenario may be the result of an ancestral and still ongoing host–pathogen arms race. Overall, our results provide evolutionary information about candidate functional sites and polymorphic positions in TTSP genes.

Sex-specific phenotypic effects and evolutionary history of an ancient polymorphic deletion of the human growth hormone receptor

The common deletion of the third exon of the growth hormone receptor gene (GHRd3) in humans is associated with birth weight, growth after birth, and time of puberty. However, its evolutionary history and the molecular mechanisms through which it affects phenotypes remain unresolved. We present evidence that this deletion was nearly fixed in the ancestral population of anatomically modern humans and Neanderthals but underwent a recent adaptive reduction in frequency in East Asia. We documented that GHRd3 is associated with protection from severe malnutrition. Using a novel mouse model, we found that, under calorie restriction, Ghrd3 leads to the female-like gene expression in male livers and the disappearance of sexual dimorphism in weight. The sex- and diet-dependent effects of GHRd3 in our mouse model are consistent with a model in which the allele frequency of GHRd3 varies throughout human evolution as a response to fluctuations in resource availability.

Unusual interplay of contrasting selective pressures on β-defensin genes implicated in male fertility of the Buffalo (Bubalus bubalis)

BACKGROUND

The buffalo, despite its superior milk-producing ability, suffers from reproductive limitations that constrain its lifetime productivity. Male sub-fertility, manifested as low conception rates (CRs), is a major concern in buffaloes. The epididymal sperm surface-binding proteins which participate in the sperm surface remodelling (SSR) events affect the survival and performance of the spermatozoa in the female reproductive tract (FRT). A mutation in an epididymal secreted protein, beta-defensin 126 (DEFB-126/BD-126), a class-A beta-defensin (CA-BD), resulted in decreased CRs in human cohorts across the globe. To better understand the role of CA-BDs in buffalo reproduction, this study aimed to identify the BD genes for characterization of the selection pressure(s) acting on them, and to identify the most abundant CA-BD transcript in the buffalo male reproductive tract (MRT) for predicting its reproductive functional significance.

RESULTS

Despite the low protein sequence homology with their orthologs, the CA-BDs have maintained the molecular framework and the structural core vital to their biological functions. Their coding-sequences in ruminants revealed evidence of pervasive purifying and episodic diversifying selection pressures. The buffalo CA-BD genes were expressed in the major reproductive and non-reproductive tissues exhibiting spatial variations. The Buffalo BD-129 (BuBD-129) was the most abundant and the longest CA-BD in the distal-MRT segments and was predicted to be heavily O-glycosylated.

CONCLUSIONS

The maintenance of the structural core, despite the sequence divergence, indicated the conservation of the molecular functions of the CA-BDs. The expression of the buffalo CA-BDs in both the distal-MRT segments and non-reproductive tissues indicate the retention the primordial microbicidal activity, which was also predicted by in silico sequence analyses. However, the observed spatial variations in their expression across the MRT hint at their region-specific roles. Their comparison across mammalian species revealed a pattern in which the various CA-BDs appeared to follow dissimilar evolutionary paths. This pattern appears to maintain only the highly efficacious CA-BD alleles and diversify their functional repertoire in the ruminants. Our preliminary results and analyses indicated that BuBD-129 could be the functional ortholog of the primate DEFB-126. Further studies are warranted to assess its molecular functions to elucidate its role in immunity, reproduction and fertility.

An Evolutionary Perspective on the Impact of Genomic Copy Number Variation on Human Health

Copy number variants (CNVs), deletions and duplications of segments of DNA, account for at least five times more variable base pairs in humans than single-nucleotide variants. Several common CNVs were shown to change coding and regulatory sequences and thus dramatically affect adaptive phenotypes involving immunity, perception, metabolism, skin structure, among others. Some of these CNVs were also associated with susceptibility to cancer, infection, and metabolic disorders. These observations raise the possibility that CNVs are a primary contributor to human phenotypic variation and consequently evolve under selective pressures. Indeed, locus-specific haplotype-level analyses revealed signatures of natural selection on several CNVs. However, more traditional tests of selection which are often applied to single-nucleotide variation often have diminished statistical power when applied to CNVs because they often do not show strong linkage disequilibrium with nearby variants. Recombination-based formation mechanisms of CNVs lead to frequent recurrence and gene conversion events, breaking the linkage disequilibrium involving CNVs. Similar methodological challenges also prevent routine genome-wide association studies to adequately investigate the impact of CNVs on heritable human disease. Thus, we argue that the full relevance of CNVs to human health and evolution is yet to be elucidated. We further argue that a holistic investigation of formation mechanisms within an evolutionary framework would provide a powerful framework to understand the functional and biomedical impact of CNVs. In this paper, we review several cases where studies reveal diverse evolutionary histories and unexpected functional consequences of CNVs. We hope that this review will encourage further work on CNVs by both evolutionary and medical geneticists.

Balancing Selection Drives the Maintenance of Genetic Variation in Drosophila Antimicrobial Peptides

Genes involved in immune defense against pathogens provide some of the most well-known examples of both directional and balancing selection. Antimicrobial peptides (AMPs) are innate immune effector genes, playing a key role in pathogen clearance in many species, including Drosophila. Conflicting lines of evidence have suggested that AMPs may be under directional, balancing, or purifying selection. Here, we use both a linear model and control-gene-based approach to show that balancing selection is an important force shaping AMP diversity in Drosophila. In Drosophila melanogaster, this is most clearly observed in ancestral African populations. Furthermore, the signature of balancing selection is even more striking once background selection has been accounted for. Balancing selection also acts on AMPs in Drosophila mauritiana, an isolated island endemic separated from D. melanogaster by about 4 Myr of evolution. This suggests that balancing selection may be broadly acting to maintain adaptive diversity in Drosophila AMPs, as has been found in other taxa.

The role of DEFB126 variation in male infertility and medically assisted reproduction technique outcome

RESEARCH QUESTION

Human DEFB126 is an important component of the glycocalyx of human spermatozoa. Beta-defensins play a primary role in male infertility due to their involvement in maturation and capacitation of spermatozoa. A 2-nt deletion of DEFB126 affects sperm function and so this study investigated the possible association between DEFB126 variants and its protein expression on medically assisted reproduction (MAR) technique outcome in Iranian infertile males.

DESIGN

The presence of a 2-nt deletion of DEFB126, and its protein expression in spermatozoa, were investigated by standard polymerase chain reaction (PCR) sequencing and immunocytochemistry, respectively. MAR technique outcome according to clinical pregnancy rates was assessed in 277 Iranian males with unexplained infertility, including 139 patients who underwent intrauterine insemination (IUI) and 103 patients who underwent IVF/intracytoplasmic sperm injection (ICSI), as well as 35 infertile males who declined to use any MAR treatment. As the control group, 100 fertile males with a normal spermiogram were enrolled.

RESULTS

The 2-nt deletion of DEFB126 was significantly higher in infertile patients than controls (P ≤ 0.05). The presence of this deletion resulted in significantly lower clinical pregnancy rates following IUI (P ≤ 0.05); however, there were no differences in IVF/ICSI outcomes according to genotype. The protein expression in del/del males was also remarkably lower than that of the other genotypes.

CONCLUSIONS

This sequence variation of DEFB126 may impair male reproductive function and can be related to male infertility. Interestingly, males with the del/del genotype have a normal spermiogram; however, their spermatozoa are evidently functionally impaired, which can affect IUI treatment outcome, but not treatment by IVF/ICSI.

Fine-Scale Characterization of Genomic Structural Variation in the Human Genome Reveals Adaptive and Biomedically Relevant Hotspots

Genomic structural variants (SVs) are distributed nonrandomly across the human genome. The "hotspots" of SVs have been implicated in evolutionary innovations, as well as medical conditions. However, the evolutionary and biomedical features of these hotspots remain incompletely understood. Here, we analyzed data from 2,504 genomes to construct a refined map of 1,148 SV hotspots in human genomes. We confirmed that segmental duplication-related nonallelic homologous recombination is an important mechanistic driver of SV hotspot formation. However, to our surprise, we also found that a majority of SVs in hotspots do not form through such recombination-based mechanisms, suggesting diverse mechanistic and selective forces shaping hotspots. Indeed, our evolutionary analyses showed that the majority of SV hotspots are within gene-poor regions and evolve under relaxed negative selection or neutrality. However, we still found a small subset of SV hotspots harboring genes that are enriched for anthropologically crucial functions and evolve under geography-specific and balancing adaptive forces. These include two independent hotspots on different chromosomes affecting alpha and beta hemoglobin gene clusters. Biomedically, we found that the SV hotspots coincide with breakpoints of clinically relevant, large de novo SVs, significantly more often than genome-wide expectations. For example, we showed that the breakpoints of multiple large SVs, which lead to idiopathic short stature, coincide with SV hotspots. Therefore, the mutational instability in SV hotpots likely enables chromosomal breaks that lead to pathogenic structural variation formations. Overall, our study contributes to a better understanding of the mutational and adaptive landscape of the genome.

Perspectives on the evolutionary ecology of arthropod antimicrobial peptides

Antimicrobial peptides (AMPs) are important elements of the innate immune defence in multicellular organisms that target and kill microbes. Here, we reflect on the various points that are raised by the authors of the 11 contributions to a special issue of Philosophical Transactions on the 'evolutionary ecology of arthropod antimicrobial peptides'. We see five interesting topics emerging. (i) AMP genes in insects, and perhaps in arthropods more generally, evolve much slower than most other immune genes. One explanation refers to the constraints set by AMPs being part of a finely tuned defence system. A new view argues that AMPs are under strong stabilizing selection. Regardless, this striking observation still invites many more questions than have been answered so far. (ii) AMPs almost always are expressed in combinations and sometimes show expression patterns that are dependent on the infectious agent. While it is often assumed that this can be explained by synergistic interactions, such interactions have rarely been demonstrated and need to be studied further. Moreover, how to define synergy in the first place remains difficult and needs to be addressed. (iii) AMPs play a very important role in mediating the interaction between a host and its mutualistic or commensal microbes. This has only been studied in a very small number of (insect) species. It has become clear that the very same AMPs play different roles in different situations and hence are under concurrent selection. (iv) Different environments shape the physiology of organisms; especially the host-associated microbial communities should impact on the evolution host AMPs. Studies in social insects and some organisms from extreme environments seem to support this notion, but, overall, the evidence for adaptation of AMPs to a given environment is scant. (v) AMPs are considered or already developed as new drugs in medicine. However, bacteria can evolve resistance to AMPs. Therefore, in the light of our limited understanding of AMP evolution in the natural context, and also the very limited understanding of the evolution of resistance against AMPs in bacteria in particular, caution is recommended. What is clear though is that study of the ecology and evolution of AMPs in natural systems could inform many of these outstanding questions, including those related to medical applications and pathogen control.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

Prediction of the impact of coding missense and nonsense single nucleotide polymorphisms on HD5 and HBD1 antibacterial activity against Escherichia coli

Defensins confer host defense against microorganisms and are important for human health. Single nucleotide polymorphisms (SNPs) in defensin gene-coding regions could lead to less active variants. Using SNP data available at the dbSNP database and frequency information from the 1000 Genomes Project, two DEFA5 (L26I and R13H) and eight DEFB1 (C35S, K31T, K33R, R29G, V06I, C12Y, Y28* and C05*) missense and nonsense SNPs that are located within mature regions of the coded defensins were retrieved. Such SNPs are rare and population restricted. In order to assess their antibacterial activity against Escherichia coli, two linear regression models were used from a previous work, which models the antibacterial activity as a function of solvation potential energy, using molecular dynamics data. Regarding only the antibacterial predictions, for HD5, no biological differences between wild-type and its variants were observed; while for HBD1, the results suggest that the R29G, K31T, Y28* and C05* variants could be less active than the wild-type one. The data here reported could lead to a substantial improvement in knowledge about the impact of missense SNPs in human defensins and their world distribution. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 633-644, 2016.

Understanding rare and common diseases in the context of human evolution

The wealth of available genetic information is allowing the reconstruction of human demographic and adaptive history. Demography and purifying selection affect the purge of rare, deleterious mutations from the human population, whereas positive and balancing selection can increase the frequency of advantageous variants, improving survival and reproduction in specific environmental conditions. In this review, I discuss how theoretical and empirical population genetics studies, using both modern and ancient DNA data, are a powerful tool for obtaining new insight into the genetic basis of severe disorders and complex disease phenotypes, rare and common, focusing particularly on infectious disease risk.

Immune genes and divergent antimicrobial peptides in flies of the subgenus Drosophila

Background

Drosophila is an important model for studying the evolution of animal immunity, due to the powerful genetic tools developed for D. melanogaster. However, Drosophila is an incredibly speciose lineage with a wide range of ecologies, natural histories, and diverse natural enemies. Surprisingly little functional work has been done on immune systems of species other than D. melanogaster. In this study, we examine the evolution of immune genes in the speciose subgenus Drosophila, which diverged from the subgenus Sophophora (that includes D. melanogaster) approximately 25–40 Mya. We focus on D. neotestacea, a woodland species used to study interactions between insects and parasitic nematodes, and combine recent transcriptomic data with infection experiments to elucidate aspects of host immunity.

Results

We found that the vast majority of genes involved in the D. melanogaster immune response are conserved in D. neotestacea, with a few interesting exceptions, particularly in antimicrobial peptides (AMPs); until recently, AMPs were not thought to evolve rapidly in Drosophila. Unexpectedly, we found a distinct diptericin in subgenus Drosophila flies that appears to have evolved under diversifying (positive) selection. We also describe the presence of the AMP drosocin, which was previously thought to be restricted to the subgenus Sophophora, in the subgenus Drosophila. We challenged two subgenus Drosophila species, D. neotestacea and D. virilis with bacterial and fungal pathogens and quantified AMP expression.

Conclusions

While diptericin in D. virilis was induced by exposure to gram-negative bacteria, it was not induced in D. neotestacea, showing that conservation of immune genes does not necessarily imply conservation of the realized immune response. Our study lends support to the idea that invertebrate AMPs evolve rapidly, and that Drosophila harbor a diverse repertoire of AMPs with potentially important functional consequences.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0805-y) contains supplementary material, which is available to authorized users.

The potential for adaptive maintenance of diversity in insect antimicrobial peptides

Genes involved in immune defence are among the fastest evolving in the genomes of many species. Interestingly, however, genes encoding antimicrobial peptides (AMPs) have shown little evidence for adaptive divergence in arthropods, despite the centrality of these peptides in direct killing of microbial pathogens. This observation, coupled with a failure to detect phenotypic consequence of genetic variation in AMPs, has led to the hypothesis that individual AMPs make minor contributions to overall immune defence and that AMPs instead act as a collective cocktail. Recent data, however, have suggested an alternative explanation for the apparent lack of adaptive divergence in AMP genes. Molecular evolutionary and phenotypic data have begun to suggest that variant AMP alleles may be maintained through balancing selection in invertebrates, a pattern similar to that observed in several vertebrate AMPs. Signatures of balancing selection include high rates of non-synonymous polymorphism, trans-species amino acid polymorphisms, and convergence of amino acid states across the phylogeny. In this review, we revisit published literature on insect AMP genes and analyse newly available population genomic datasets in Drosophila, finding enrichment for patterns consistent with adaptive maintenance of polymorphism.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

Detecting Genomic Signatures of Natural Selection with Principal Component Analysis: Application to the 1000 Genomes Data

To characterize natural selection, various analytical methods for detecting candidate genomic regions have been developed. We propose to perform genome-wide scans of natural selection using principal component analysis (PCA). We show that the common FST index of genetic differentiation between populations can be viewed as the proportion of variance explained by the principal components. Considering the correlations between genetic variants and each principal component provides a conceptual framework to detect genetic variants involved in local adaptation without any prior definition of populations. To validate the PCA-based approach, we consider the 1000 Genomes data (phase 1) considering 850 individuals coming from Africa, Asia, and Europe. The number of genetic variants is of the order of 36 millions obtained with a low-coverage sequencing depth (3×). The correlations between genetic variation and each principal component provide well-known targets for positive selection (EDAR, SLC24A5, SLC45A2, DARC), and also new candidate genes (APPBPP2, TP1A1, RTTN, KCNMA, MYO5C) and noncoding RNAs. In addition to identifying genes involved in biological adaptation, we identify two biological pathways involved in polygenic adaptation that are related to the innate immune system (beta defensins) and to lipid metabolism (fatty acid omega oxidation). An additional analysis of European data shows that a genome scan based on PCA retrieves classical examples of local adaptation even when there are no well-defined populations. PCA-based statistics, implemented in the PCAdapt R package and the PCAdapt fast open-source software, retrieve well-known signals of human adaptation, which is encouraging for future whole-genome sequencing project, especially when defining populations is difficult.

Convergent Balancing Selection on an Antimicrobial Peptide in Drosophila

Genes of the immune system often evolve rapidly and adaptively, presumably driven by antagonistic interactions with pathogens [1-4]. Those genes encoding secreted antimicrobial peptides (AMPs), however, have failed to exhibit conventional signatures of strong adaptive evolution, especially in arthropods (e.g., [5, 6]) and often segregate for null alleles and gene deletions [3, 4, 7, 8]. Furthermore, quantitative genetic studies have failed to associate naturally occurring polymorphism in AMP genes with variation in resistance to infection [9-11]. Both the lack of signatures of positive selection in AMPs and lack of association between genotype and immune phenotypes have yielded an interpretation that AMP genes evolve under relaxed evolutionary constraint, with enough functional redundancy that variation in, or even loss of, any particular peptide would have little effect on overall resistance [12, 13]. In stark contrast to the current paradigm, we identified a naturally occurring amino acid polymorphism in the AMP Diptericin that is highly predictive of resistance to bacterial infection in Drosophila melanogaster [13]. The identical amino acid polymorphism arose in parallel in the sister species D. simulans, by independent mutation with equivalent phenotypic effect. Convergent substitutions at the same amino acid residue have evolved at least five times across the Drosophila genus. We hypothesize that the alternative alleles are maintained by balancing selection through context-dependent or fluctuating selection. This pattern of evolution appears to be common in AMPs but is invisible to conventional screens for adaptive evolution that are predicated on elevated rates of amino acid divergence.

Genomic Signatures of Selective Pressures and Introgression from Archaic Hominins at Human Innate Immunity Genes

Human genes governing innate immunity provide a valuable tool for the study of the selective pressure imposed by microorganisms on host genomes. A comprehensive, genome-wide study of how selective constraints and adaptations have driven the evolution of innate immunity genes is missing. Using full-genome sequence variation from the 1000 Genomes Project, we first show that innate immunity genes have globally evolved under stronger purifying selection than the remainder of protein-coding genes. We identify a gene set under the strongest selective constraints, mutations in which are likely to predispose individuals to life-threatening disease, as illustrated by STAT1 and TRAF3. We then evaluate the occurrence of local adaptation and detect 57 high-scoring signals of positive selection at innate immunity genes, variation in which has been associated with susceptibility to common infectious or autoimmune diseases. Furthermore, we show that most adaptations targeting coding variation have occurred in the last 6,000-13,000 years, the period at which populations shifted from hunting and gathering to farming. Finally, we show that innate immunity genes present higher Neandertal introgression than the remainder of the coding genome. Notably, among the genes presenting the highest Neandertal ancestry, we find the TLR6-TLR1-TLR10 cluster, which also contains functional adaptive variation in Europeans. This study identifies highly constrained genes that fulfill essential, non-redundant functions in host survival and reveals others that are more permissive to change-containing variation acquired from archaic hominins or adaptive variants in specific populations-improving our understanding of the relative biological importance of innate immunity pathways in natural conditions.

Trans-Species Polymorphism in Immune Genes: General Pattern or MHC-Restricted Phenomenon?

Immunity exhibits extraordinarily high levels of variation. Evolution of the immune system in response to host-pathogen interactions in particular ecological contexts appears to be frequently associated with diversifying selection increasing the genetic variability. Many studies have documented that immunologically relevant polymorphism observed today may be tens of millions years old and may predate the emergence of present species. This pattern can be explained by the concept of trans-species polymorphism (TSP) predicting the maintenance and sharing of favourable functionally important alleles of immune-related genes between species due to ongoing balancing selection. Despite the generality of this concept explaining the long-lasting adaptive variation inherited from ancestors, current research in TSP has vastly focused only on major histocompatibility complex (MHC). In this review we summarise the evidence available on TSP in human and animal immune genes to reveal that TSP is not a MHC-specific evolutionary pattern. Further research should clearly pay more attention to the investigation of TSP in innate immune genes and especially pattern recognition receptors which are promising candidates for this type of evolution. More effort should also be made to distinguish TSP from convergent evolution and adaptive introgression. Identification of balanced TSP variants may represent an accurate approach in evolutionary medicine to recognise disease-resistance alleles.

IL-22 Up-Regulates β-Defensin-2 Expression in Human Alveolar Epithelium via STAT3 but Not NF-κB Signaling Pathway

Human β-defensin-2(HBD-2) is one of the two major vertebrate antimicrobial peptide families (α and β), which is highly expressed by proinflammatory induction in the lung and exhibit broad-spectrum antimicrobial activity. We observed that IL-22 receptors high expressed on the membrane of A549 cells; HBD-2 mRNA was expressed in a time- and concentration-dependent manners in A549 cells when treated with IL-22; further studies demonstrated that HBD-2 expression was attenuated by AG490, but to JSH-23, inhibitors of p-STAT3 DNA binding and NF-κB/p65 subunit nuclear translocation, respectively. These results support that IL-22-mediated signalling pathway of HBD-2 gene expression involved STAT3 but not NF-κB in human alveolar epithelium. These findings provide a new insight into how IL-22 may play an important link between innate and adaptive immunity, thereby anti-infection locally in the alveolar epithelium.

Evidence of convergent evolution in humans and macaques supports an adaptive role for copy number variation of the β-defensin-2 gene

β-defensins are a family of important peptides of innate immunity, involved in host defense, immunomodulation, reproduction, and pigmentation. Genes encoding β-defensins show evidence of birth-and-death evolution, adaptation by amino acid sequence changes, and extensive copy number variation (CNV) within humans and other species. The role of CNV in the adaptation of β-defensins to new functions remains unclear, as does the adaptive role of CNV in general. Here, we fine-map CNV of a cluster of β-defensins in humans and rhesus macaques. Remarkably, we found that the structure of the CNV is different between primates, with distinct mutational origins and CNV boundaries defined by retroviral long terminal repeat elements. Although the human β-defensin CNV region is 322 kb and encompasses several genes, including β-defensins, a long noncoding RNA gene, and testes-specific zinc-finger transcription factors, the orthologous region in the rhesus macaque shows CNV of a 20-kb region, containing only a single gene, the ortholog of the human β-defensin-2 gene. Despite its independent origins, the range of gene copy numbers in the rhesus macaque is similar to humans. In addition, the rhesus macaque gene has been subject to divergent positive selection at the amino acid level following its initial duplication event between 3 and 9.5 Ma, suggesting adaptation of this gene as the macaque successfully colonized novel environments outside Africa. Therefore, the molecular phenotype of β-defensin-2 CNV has undergone convergent evolution, and this gene shows evidence of adaptation at the amino acid level in rhesus macaques.

Advantageous diversity maintained by balancing selection in humans

Most human polymorphisms are neutral or slightly deleterious, but some genetic variation is advantageous and maintained in populations by balancing selection. Considered a rarity and overlooked for years, balanced polymorphisms have recently received renewed attention with several lines of evidence showing their relevance in human evolution. From theoretical work on its role in adaptation to empirical studies that identify its targets, recent developments have showed that balancing selection is more prevalent than previously thought. Here we review these developments and discuss their implications in our understanding of the influence of balancing selection in human evolution. We also review existing evidence on the biological functions that benefit most from advantageous diversity, and the functional consequences of these variants. Overall, we argue that balancing selection must be considered an important selective force in human evolution.

A novel subclass of bovine β-defensins links reproduction and immunology

β-defensins are effector molecules of the innate immune system, found in many diverse species. Their presence in invertebrates as well as vertebrates suggests highly conserved functional roles. Most β-defensins are believed to act as antimicrobial agents at epithelial surfaces, although additional functions have also been described, including immune regulatory activity, wound repair and a role in coat-colour determination. High expression of β-defensins have been found in testis and epididymidal epithelium as well as in the seminal fluid of humans, macaque, rat, mouse and cow. Human and macaque β-defensins have recently been shown to affect sperm motility while a mutation in β-defensin 126 is associated with reduced fertility in men. Genetic variation in bovine defensin genes may explain the increased incidence of low fertility in cattle. Here, we present a summary of the known functions of β-defensins as well as their emerging role in reproduction and their potential to improve fertility in cattle.

Bovine β-defensin gene family: opportunities to improve animal health?

Recent analysis of the bovine genome revealed an expanded suite of β-defensin genes that encode what are referred to as antimicrobial or host defense peptides (HDPs). Whereas primate genomes also encode α- and θ-defensins, the bovine genome contains only the β-defensin subfamily of HDPs. β-Defensins perform diverse functions that are critical to protection against pathogens but also in regulation of the immune response and reproduction. As the most comprehensively studied subclass of HDPs, β-defensins possess the widest taxonomic distribution, found in invertebrates as well as plants, indicating an ancient point of origin. Cross-species comparison of the genomic arrangement of β-defensin gene repertoire revealed them to vary in number among species presumably due to differences in pathogenic selective pressures but also genetic drift. β-Defensin genes exist in a single cluster in birds, but four gene clusters exist in dog, rat, mouse, and cow. In humans and chimpanzees, one of these clusters is split in two as a result of a primate-specific pericentric inversion producing five gene clusters. A cluster of β-defensin genes on bovine chromosome 13 has been recently characterized, and full genome sequencing has identified extensive gene copy number variation on chromosome 27. As a result, cattle have the most diverse repertoire of β-defensin genes so far identified, where four clusters contain at least 57 genes. This expansion of β-defensin HDPs may hold significant potential for combating infectious diseases and provides opportunities to harness their immunological and reproductive functions in commercial cattle populations.

Signatures of selection in the Iberian honey bee (Apis mellifera iberiensis) revealed by a genome scan analysis of single nucleotide polymorphisms

Understanding the genetic mechanisms of adaptive population divergence is one of the most fundamental endeavours in evolutionary biology and is becoming increasingly important as it will allow predictions about how organisms will respond to global environmental crisis. This is particularly important for the honey bee, a species of unquestionable ecological and economical importance that has been exposed to increasing human-mediated selection pressures. Here, we conducted a single nucleotide polymorphism (SNP)-based genome scan in honey bees collected across an environmental gradient in Iberia and used four FST -based outlier tests to identify genomic regions exhibiting signatures of selection. Additionally, we analysed associations between genetic and environmental data for the identification of factors that might be correlated or act as selective pressures. With these approaches, 4.4% (17 of 383) of outlier loci were cross-validated by four FST -based methods, and 8.9% (34 of 383) were cross-validated by at least three methods. Of the 34 outliers, 15 were found to be strongly associated with one or more environmental variables. Further support for selection, provided by functional genomic information, was particularly compelling for SNP outliers mapped to different genes putatively involved in the same function such as vision, xenobiotic detoxification and innate immune response. This study enabled a more rigorous consideration of selection as the underlying cause of diversity patterns in Iberian honey bees, representing an important first step towards the identification of polymorphisms implicated in local adaptation and possibly in response to recent human-mediated environmental changes.

Complete haplotype sequence of the human immunoglobulin heavy-chain variable, diversity, and joining genes and characterization of allelic and copy-number variation

The immunoglobulin heavy-chain locus (IGH) encodes variable (IGHV), diversity (IGHD), joining (IGHJ), and constant (IGHC) genes and is responsible for antibody heavy-chain biosynthesis, which is vital to the adaptive immune response. Programmed V-(D)-J somatic rearrangement and the complex duplicated nature of the locus have impeded attempts to reconcile its genomic organization based on traditional B-lymphocyte derived genetic material. As a result, sequence descriptions of germline variation within IGHV are lacking, haplotype inference using traditional linkage disequilibrium methods has been difficult, and the human genome reference assembly is missing several expressed IGHV genes. By using a hydatidiform mole BAC clone resource, we present the most complete haplotype of IGHV, IGHD, and IGHJ gene regions derived from a single chromosome, representing an alternate assembly of ∼1 Mbp of high-quality finished sequence. From this we add 101 kbp of previously uncharacterized sequence, including functional IGHV genes, and characterize four large germline copy-number variants (CNVs). In addition to this germline reference, we identify and characterize eight CNV-containing haplotypes from a panel of nine diploid genomes of diverse ethnic origin, discovering previously unmapped IGHV genes and an additional 121 kbp of insertion sequence. We genotype four of these CNVs by using PCR in 425 individuals from nine human populations. We find that all four are highly polymorphic and show considerable evidence of stratification (Fst = 0.3-0.5), with the greatest differences observed between African and Asian populations. These CNVs exhibit weak linkage disequilibrium with SNPs from two commercial arrays in most of the populations tested.

Population genetic tools for dissecting innate immunity in humans

Innate immunity involves direct interactions between the host and microorganisms, both pathogenic and symbiotic, so natural selection is expected to strongly influence genes involved in these processes. Population genetics investigates the impact of past natural selection events on the genome of present-day human populations, and it complements immunological as well as clinical and epidemiological genetic studies. Recent data show that the impact of selection on the different families of innate immune receptors and their downstream signalling molecules varies considerably. This Review discusses these findings and highlights how they help to delineate the relative functional importance of innate immune pathways, which can range from being essential to being redundant.

CRS-peptides: unique defense peptides of mouse Paneth cells

The intestine is the most densely colonized site in both mice and man. Recent data suggest that the intestinal flora is, in part, controlled by antimicrobial substances secreted by the intestinal epithelium. The defense system of the small intestine includes a protective mucus layer, a high turnover of epithelial cells, and a regulated secretion of effector molecules, notably antimicrobial peptides. Human and mouse small intestines share many similarities in their intestinal defense micro-organization, including the secretion of the well-known α-defensins. Mice, however, produce an additional unique antimicrobial peptide family, the CRS (cryptdin-related sequences)-peptides, not found in man. This review comprises a detailed presentation of the peptide-based defense of the gut, with specific emphasis on the CRS-peptide family. The first part presents the current knowledge of the CRS-peptide family's biochemical characteristics and nomenclature, and the second part is devoted to the possible role of this family in the homeostasis of the gut.

Lineage-specific evolution of T-cell immunoglobulin and mucin domain 1 gene in the primates

T-cell immunoglobulin domain and mucin domain containing protein 1 (TIM1), also known as a cellular receptor for hepatitis A virus (HAVCR1) or a molecule induced by ischemic injury in the kidney (KIM1), is involved in the regulation of immune responses. We investigated a natural selection history of TIM1 by comparative sequencing analysis in 24 different primates. It was found that TIM1 had become a pseudogene in multiple lineages of the New World monkey. We also investigated T cell lines originated from four different New World monkey species and confirmed that TIM1 was not expressed at the mRNA level. On the other hand, there were ten amino acid sites in the Ig domain of TIM1 in the other primates, which were suggested to be under positive natural selection. In addition, mucin domain of TIM1 was highly polymorphic in the Old World monkeys, which might be under balanced selection. These data suggested that TIM1 underwent a lineage-specific evolutionary pathway in the primates.

A common mutation in the defensin DEFB126 causes impaired sperm function and subfertility

A glycosylated polypeptide, β-defensin 126 (DEFB126), derived from the epididymis and adsorbed onto the sperm surface, has been implicated in immunoprotection and efficient movement of sperm in mucosal fluids of the female reproductive tract. Here, we report a sequence variant in DEFB126 that has a two-nucleotide deletion in the open reading frame, which generates an abnormal mRNA. The allele frequency of this variant sequence was high in both a European (0.47) and a Chinese (0.45) population cohort. Binding of the Agaricus bisporus lectin to the sperm surface glycocalyx was significantly lower in men with the homozygous variant (del/del) genotype than in those with either a del/wt or a wt/wt genotype, suggesting an altered sperm glycocalyx with fewer O-linked oligosaccharides in del/del men. Moreover, sperm from del/del carriers exhibited an 84% reduction in the rate of penetration of a hyaluronic acid gel, a surrogate for cervical mucus, compared to the other genotypes. This reduction in sperm performance in hyaluronic acid gels was not a result of decreased progressive motility (average curvilinear velocity) or morphological deficits. Nevertheless, DEFB126 genotype and lectin binding were correlated with sperm performance in the penetration assays. In a prospective cohort study of newly married couples who were trying to conceive by natural means, couples were less likely to become pregnant and took longer to achieve a live birth if the male partner was homozygous for the variant sequence. This common sequence variation in DEFB126, and its apparent effect of impaired reproductive function, will allow a better understanding, clinical evaluation, and possibly treatment of human infertility.

A signature of balancing selection in the region upstream to the human UGT2B4 gene and implications for breast cancer risk

UDP-glucuronosyltransferase 2 family, polypeptide B4 (UGT2B4) is an important metabolizing enzyme involved in the clearance of many xenobiotics and endogenous substrates, especially steroid hormones and bile acids. The HapMap data show that numerous SNPs upstream of UGT2B4 are in near-perfect linkage disequilibrium with each other and occur at intermediate frequency, indicating that this region might contain a target of natural selection. To investigate this possibility, we chose three regions (4.8 kb in total) for resequencing and observed a striking excess of intermediate-frequency alleles that define two major haplotypes separated by many mutation events and with little differentiation across populations, thus suggesting that the variation pattern upstream UGT2B4 is highly unusual and may be the result of balancing selection. We propose that this pattern is due to the maintenance of a regulatory polymorphism involved in the fine tuning of UGT2B4 expression so that heterozygous genotypes result in optimal enzyme levels. Considering the important role of steroid hormones in breast cancer susceptibility, we hypothesized that variation in this region could predispose to breast cancer. To test this hypothesis, we genotyped tag SNP rs13129471 in 1,261 patients and 825 normal women of African ancestry from three populations. The frequency comparison indicated that rs13129471 was significantly associated with breast cancer after adjusting for ethnicity [P = 0.003; heterozygous odds ratio (OR) 1.02, 95% confidence interval (CI) 0.81-1.28; homozygous OR 1.50, 95% CI 1.15-1.95]. Our results provide new insights into UGT2B4 sequence variation and indicate that a signal of natural selection may lead to the identification of disease susceptibility variants.

Candidate targets of balancing selection in the genome of Staphylococcus aureus

Signatures of balancing selection can highlight polymorphisms and functions that are important to the long-term fitness of a species. We performed a first genome-wide scan for balancing selection in a bacterial species, Staphylococcus aureus, which is a common cause of serious antimicrobial-resistant infections of humans. Using a sliding window approach, the genomes of 16 strains of S. aureus, including 5 new genome sequences presented here, and 1 outgroup strain of S. epidermidis were scanned for signatures of balancing selection. A total of 195 short windows were investigated based on their extreme values of both Tajima's D (>2.03) and π/K ratios (>0.12) relative to the rest of the genome. To test the unusualness of these windows, an Approximate Bayesian Computation framework was used to select a null demographic model that better accounted for the observed data than did the standard neutral model. A total of 186 windows were demonstrated to be unusual under the null model and, thus, represented candidate loci under balancing selection. These 186 candidate windows were located within 99 candidate genes that were spread across 62 different loci. Nearly all the signal (97.2%) was located within coding sequences; balancing selection on gene regulation apparently occurs through the targeting of global regulators such as agr and gra/aps. The agr locus had some of the strongest signatures of balancing selection, which provides new insight into the causes of diversity at this locus. The list of candidate genes included multiple virulence-associated genes and was significantly enriched for functions in amino acid and inorganic ion transport and metabolism and in defense mechanisms against innate immunity and antimicrobials, highlighting these particular functions as important to the fitness of this pathogen.

Refinement of primate copy number variation hotspots identifies candidate genomic regions evolving under positive selection

Background

Copy number variants (CNVs), defined as losses and gains of segments of genomic DNA, are a major source of genomic variation.

Results

In this study, we identified over 2,000 human CNVs that overlap with orthologous chimpanzee or orthologous macaque CNVs. Of these, 170 CNVs overlap with both chimpanzee and macaque CNVs, and these were collapsed into 34 hotspot regions of CNV formation. Many of these hotspot regions of CNV formation are functionally relevant, with a bias toward genes involved in immune function, some of which were previously shown to evolve under balancing selection in humans. The genes in these primate CNV formation hotspots have significant differential expression levels between species and show evidence for positive selection, indicating that they have evolved under species-specific, directional selection.

Conclusions

These hotspots of primate CNV formation provide a novel perspective on divergence and selective pressures acting on these genomic regions.

A worldwide analysis of beta-defensin copy number variation suggests recent selection of a high-expressing DEFB103 gene copy in East Asia

Beta-defensins are a family of multifunctional genes with roles in defense against pathogens, reproduction, and pigmentation. In humans, six beta-defensin genes are clustered in a repeated region which is copy-number variable (CNV) as a block, with a diploid copy number between 1 and 12. The role in host defense makes the evolutionary history of this CNV particularly interesting, because morbidity due to infectious disease is likely to have been an important selective force in human evolution, and to have varied between geographical locations. Here, we show CNV of the beta-defensin region in chimpanzees, and identify a beta-defensin block in the human lineage that contains rapidly evolving noncoding regulatory sequences. We also show that variation at one of these rapidly evolving sequences affects expression levels and cytokine responsiveness of DEFB103, a key inhibitor of influenza virus fusion at the cell surface. A worldwide analysis of beta-defensin CNV in 67 populations shows an unusually high frequency of high-DEFB103-expressing copies in East Asia, the geographical origin of historical and modern influenza epidemics, possibly as a result of selection for increased resistance to influenza in this region. Hum Mutat 32:743–750, 2011. © 2011 Wiley-Liss, Inc.

Haplotype variation in the ACE gene in global populations, with special reference to India, and an alternative model of evolution of haplotypes

Angiotensin-I-converting enzyme (ACE) is known to be associated with human cardiovascular and psychiatric pathophysiology. We have undertaken a global survey of the haplotypes in ACE gene to study diversity and to draw inferences on the nature of selective forces that may be operating on this gene. We have investigated the haplotype profiles reconstructed using polymorphisms in the regulatory (rs4277405, rs4459609, rs1800764, rs4292, rs4291), exonic (rs4309, rs4331, rs4343), and intronic (rs4340; Alu [I/D]) regions covering 17.8 kb of the ACE gene. We genotyped these polymorphisms in a large number of individuals drawn from 15 Indian ethnic groups and estimated haplotype frequencies. We compared the Indian data with available data from other global populations. Globally, five major haplotypes were observed. High-frequency haplotypes comprising mismatching alleles at the loci considered were seen in all populations. The three most frequent haplotypes among Africans were distinct from the major haplotypes of other world populations. We have studied the evolution of the two major haplotypes (TATATTGIA and CCCTCCADG), one of which contains an Alu insertion (I) and the other a deletion (D), seen most frequently among Caucasians (68%), non-African HapMap populations (65-88%), and Indian populations (70-95%) in detail. The two major haplotypes among Caucasians are reported to represent two distinct clades A and B. Earlier studies have postulated that a third clade C (represented by the haplotypes TACATCADG and TACATCADA) arose from an ancestral recombination event between A and B. We find that a more parsimonious explanation is that clades A and B have arisen by recombination between haplotypes belonging to clade C and a high-frequency African haplotype CCCTTCGIA. The haplotypes, which according to our hypothesis are the putative non-recombinants (PuNR), are uncommon in all non-African populations (frequency range 0-12%). Conversely, the frequencies of the putative recombinant haplotypes (PuR) are very low in the Africans populations (2-8%), indicating that the recombination event is likely to be ancient and arose before, perhaps shortly prior to, the global dispersal of modern humans. The global frequency spectrum of the PuR and the PuNR is difficult to explain only by drift. It appears likely that the ACE gene has been undergoing a combination of different selective pressures.

ELECTRONIC SUPPLEMENTARY MATERIAL

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Using human demographic history to infer natural selection reveals contrasting patterns on different families of immune genes

Detecting regions of the human genome that are, or have been, influenced by natural selection remains an important goal for geneticists. Many methods are used to infer selection, but there is a general reliance on an accurate understanding of how mutation and recombination events are distributed, and the well-known link between these processes and their evolutionary transience introduces uncertainty into inferences. Here, we present and apply two new, independent approaches; one based on single nucleotide polymorphisms (SNPs) that exploits geographical patterns in how humans lost variability as we colonized the world, the other based on the relationship between microsatellite repeat number and heterozygosity. We show that the two methods give concordant results. Of these, the SNP-based method is both widely applicable and detects selection over a well-defined time interval, the last 50 000 years. Analysis of all human genes by their Gene Ontology codes reveals how accelerated and decelerated loss of variability are both preferentially associated with immune genes. Applied to 168 immune genes used as the focus of a previous study, we show that members of the same gene family tend to yield similar indices of selection, even when located on different chromosomes. We hope our approach will provide a useful tool with which to infer where selection has acted to shape the human genome.

Comparative genomics and evolution of the alpha-defensin multigene family in primates

Defensin genes encode small cationic antimicrobial peptides that form an important part of the innate immune system. They are divided into three families, alpha (α), beta (β), and theta (), according to arrangement of the disulfide bonding pattern between cysteine residues. Considering the functional importance of defensins, investigators have studied the evolution and the genomic organization of defensin genes. However, these studies have been restricted mainly to β-defensins. To understand the evolutionary dynamics of α-defensin genes among primates, we identified the α-defensin repertoires in human, chimpanzee, orangutan, macaque, and marmoset. The α-defensin genes in primates can be classified into three phylogenetic classes (class I, II, and III). The presence of all three classes in the marmoset indicates that their divergence occurred before the separation of New World and Old World monkeys. Comparative analysis of the α-defensin genomic clusters suggests that the makeup of the α-defensin gene repertoires between primates is quite different, as their genes have undergone dramatic birth-and-death evolution. Analysis of the encoded peptides of the α-defensin genes indicates that despite the overall high level of sequence divergence, certain amino acid residues or motifs are conserved within and between the three phylogenetic classes. The evolution of α-defensins in primates, therefore, appears to be governed by two opposing evolutionary forces. One force stabilizes specific amino acid residues and motifs to preserve the functional and structural integrity of the molecules and the other diversifies the sequences generating molecules with a wide range of activities against a large number of pathogens.

Molecular diversity of antimicrobial effectors in the oyster Crassostrea gigas

Background

To gain insight into the molecular diversity of antimicrobial peptides and proteins in the oyster Crassostrea gigas, we characterized and compared the sequence polymorphism of the antimicrobial peptides (AMPs), Cg-Defensins (Cg-Defs) and Cg-Proline Rich peptide (Cg-Prp), and of the bactericidal permeability increasing protein, Cg-BPI. For that, we analyzed genomic and transcript sequences obtained by specific PCR amplification and in silico searches.

Results

High diversification among the three antimicrobial effectors was evidenced by this polymorphism survey. On the basis of sequence phylogenies, each AMP aggregates into clearly defined groups of variants and is the product of a multigenic family displaying a variety of gene structures. In contrast, Cg-bpi forms a single group and is encoded by a single gene copy. Moreover, we identified for both AMPs several genetic mechanisms of diversification such as recombination, parallel mutations leading to phylogenetic homoplasy and indel events. In addition, the non synonymous to synonymous substitutions ratio by codon (dN/dS) revealed several negatively and positively selected sites for both AMPs, suggesting that directional selection pressures have shaped their sequence variations.

Conclusions

This study shows for the first time in a mollusc that antimicrobial peptides and proteins have been subject to distinct patterns of diversification and we evidence the existence of different evolutionary routes leading to such sequence variability.

Role of antimicrobial peptides (AMP) and pattern recognition receptors (PRR) in the intestinal mucosa homeostasis

Homeostasis and integrity of bowel mucosa is assured by well controlled mechanical, biochemical and immunological mechanisms. First line of defense is presented by the antimicrobial peptides (AMP), which form a continuous layer on the bowel surface, produced by intestinal specific (Paneth) and non-specific epithelial cells. AMPs have a significant antimicrobial, antifungal and antiviral, as well as immunomodulatory effects. Next line of defense is the pattern recognition receptors (PRR), which allows identifying conservative molecular patterns of different pathogens, and starts antimicrobial and inflammatory mechanisms through gene-expression induction. We review the most recent knowledge and studies concerning these mechanisms.

Homozygosity and risk of childhood death due to invasive bacterial disease

Background

Genetic heterozygosity is increasingly being shown to be a key predictor of fitness in natural populations, both through inbreeding depression, inbred individuals having low heterozygosity, and also through chance linkage between a marker and a gene under balancing selection. One important component of fitness that is often highlighted is resistance to parasites and other pathogens. However, the significance of equivalent loci in human populations remains unclear. Consequently, we performed a case-control study of fatal invasive bacterial disease in Kenyan children using a genome-wide screen with microsatellite markers.

Methods

148 cases, comprising children aged <13 years who died of invasive bacterial disease, (variously, bacteraemia, bacterial meningitis or neonatal sepsis) and 137 age-matched, healthy children were sampled in a prospective study conducted at Kilifi District Hospital, Kenya. Samples were genotyped for 134 microsatellite markers using the ABI LD20 marker set and analysed for an association between homozygosity and mortality.

Results

At five markers homozygosity was strongly associated with mortality (odds ratio range 4.7 – 12.2) with evidence of interactions between some markers. Mortality was associated with different non-overlapping marker groups in Gram positive and Gram negative bacterial disease. Homozygosity at susceptibility markers was common (prevalence 19–49%) and, with the large effect sizes, this suggests that bacterial disease mortality may be strongly genetically determined.

Conclusion

Balanced polymorphisms appear to be more widespread in humans than previously appreciated and play a critical role in modulating susceptibility to infectious disease. The effect sizes we report, coupled with the stochasticity of exposure to pathogens suggests that infection and mortality are far from random due to a strong genetic basis.

Selection on an antimicrobial peptide defensin in ants

Ants live in crowded nests with interacting individuals, which makes them particularly prone to infectious diseases. The question is, how do ants cope with the increased risk of pathogen transmission due to sociality? We have studied the molecular evolution of defensin, a gene encoding an antimicrobial protein, in ants. Defensin sequences from several ant species were analyzed with maximum likelihood models of codon substitution to infer selection. Positive selection was detected in the mature region of defensin, whereas the signal and pro regions seem to be evolving neutrally. We also found a significantly higher rate of nonsynonymous substitutions in some phylogenetic lineages, as well as dN/dS >1, suggesting varying selection pressures in different lineages. Earlier studies on the molecular evolution of insect antimicrobial peptide genes have focused on termites and dipteran species, and detected positive selection only in duplicated termicin genes in termites. These findings, together with our present results, provide an indication that the immune systems of social insects (ants and termites) and dipteran insects may have responded differently to the selection pressure caused by microbial pathogens.

Copy number variation in the human genome and its implication in autoimmunity

The causes of autoimmune disease remain poorly defined. However, it is known that genetic factors contribute to disease susceptibility. Hitherto, studies have focused upon single nucleotide polymorphisms as both tools for mapping and as probable causal variants. Recent studies, using genome-wide analytical techniques, have revealed that, in the genome, segments of DNA ranging in size from kilobases to megabases can vary in copy number. These changes of DNA copy number represent an important element of genomic polymorphism in humans and in other species and may therefore make a substantial contribution to phenotypic variation and population differentiation. Furthermore, copy number variation (CNV) in genomic regions harbouring dosage-sensitive genes may cause or predispose to a variety of human genetic diseases. Several recent studies have reported an association between CNV and autoimmunity in humans such as systemic lupus, psoriasis, Crohn's disease, rheumatoid arthritis and type 1 diabetes. The use of novel analytical techniques facilitates the study of complex human genomic structures such as CNV, and allows new susceptibility loci for autoimmunity to be found that are not readily mappable by single nucleotide polymorphism-based association analyses alone.

Defensins and the dynamic genome: what we can learn from structural variation at human chromosome band 8p23.1

Over the past four years, genome-wide studies have uncovered numerous examples of structural variation in the human genome. This includes structural variation that changes copy number, such as deletion and duplication, and structural variation that does not change copy number, such as orientation and positional polymorphism. One region that contains all these types of variation spans the chromosome band 8p23.1. This region has been studied in some depth, and the focus of this review is to examine our current understanding of the variation of this region. We also consider whether this region is a good model for other structurally variable regions in the genome and what the implications of this variation are for clinical studies. Finally, we discuss the bioinformatics challenges raised, discuss the evolution of the region, and suggest some future priorities for structural variation research.

Balancing selection at a frog antimicrobial peptide locus: fluctuating immune effector alleles?

Balancing selection is common on many defense genes, but it has rarely been reported for immune effector proteins such as antimicrobial peptides (AMPs). We describe genetic diversity at a brevinin-1 AMP locus in three species of leopard frogs (Rana pipiens, Rana blairi, and Rana palustris). Several highly divergent allelic lineages are segregating at this locus. That this unusual pattern results from balancing selection is demonstrated by multiple lines of evidence, including a ratio of nonsynonymous/synonymous polymorphism significantly higher than 1, the ZnS test, incongruence between the number of segregating sites and haplotype diversity, and significant Tajima's D values. Our data are more consistent with a model of fluctuating selection in which alleles change frequencies over time than with a model of stable balancing selection such as overdominance. Evidence for fluctuating selection includes skewed allele frequencies, low levels of synonymous variation, nonneutral values of Tajima's D within allelic lineages, an inverse relationship between the frequency of an allelic lineage and its degree of polymorphism, and divergent allele frequencies among populations. AMP loci could be important sites of adaptive genetic diversity, with consequences for host–pathogen coevolution and the ability of species to resist disease epidemics.