Balancing selection in Pattern Recognition Receptor signalling pathways is associated with gene function and pleiotropy in a wild rodent

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.

Complex patterns shape immune genes diversity during invasion of common raccoon in Europe - Selection in action despite genetic drift

Rapid adaptation is common in invasive populations and is crucial to their long-term success. The primary target of selection in the invasive species' new range is standing genetic variation. Therefore, genetic drift and natural selection acting on existing variation are key evolutionary processes through which invaders will evolve over a short timescale. In this study, we used the case of the raccoon Procyon lotor invasion in Europe to identify the forces shaping the diversity of immune genes during invasion. The genes involved in the defence against infection should be under intense selection pressure in the invasive range where novel pathogens are expected to occur. To disentangle the selective and demographic processes shaping the adaptive immune diversity of its invasive and expanding populations, we have developed species-specific single-nucleotide polymorphism markers located in the coding regions of targeted immune-related genes. We characterised the genetic diversity of 110 functionally important immune genes in two invasive and one native raccoon genetic clusters, each presenting a different demographic history. Despite the strong effect of demographic processes in the invasive clusters, we detected a subset of genes exhibiting the diversity pattern suggestive of selection. The most likely process shaping the variation in those genes was balancing selection. The selected genes belong to toll-like receptors and cytokine-related genes. Our results suggest that the prevalence of selection depends on the level of diversity, that is - less genetically diverse invasive population from the Czech Republic displayed fewer signs of selection. Our results highlight the role of standing genetic variation in adapting to new environment. Understanding the evolutionary mechanisms behind invasion success would enable predicting how populations may respond to environmental change.

Selection balancing at innate immune genes: adaptive polymorphism maintenance in Toll-like receptors

Balancing selection is a classic mechanism for maintaining variability in immune genes involved in host–pathogen interactions. However, it remains unclear how widespread the mechanism is across immune genes other than the major histocompatibility complex (MHC). Although occasional reports suggest that balancing selection (heterozygote advantage, negative frequency-dependent selection, and fluctuating selection) may act on other immune genes, the current understanding of the phenomenon in non-MHC immune genes is far from solid. In this review, we focus on Toll-like receptors (TLRs), innate immune genes directly involved in pathogen recognition and immune response activation, as there is a growing body of research testing the assumptions of balancing selection in these genes. After reviewing infection- and fitness-based evidence, along with evidence based on population allelic frequencies and heterozygosity levels, we conclude that balancing selection maintains variation in TLRs, though it tends to occur under specific conditions in certain evolutionary lineages rather than being universal and ubiquitous. Our review also identifies key gaps in current knowledge and proposes promising areas for future research. Improving our understanding of host–pathogen interactions and balancing selection in innate immune genes are increasingly important, particularly regarding threats from emerging zoonotic diseases.

Pathogen-mediated selection favours the maintenance of innate immunity gene polymorphism in a widespread wild ungulate

Toll-like receptors (TLR) play a central role in recognition and host frontline defence against a wide range of pathogens. A number of recent studies have shown that TLR genes (Tlrs) often exhibit large polymorphism in natural populations. Yet, there is little knowledge on how this polymorphism is maintained and how it influences disease susceptibility in the wild. In previous work, we showed that some Tlrs exhibit similarly high levels of genetic diversity as genes of the Major Histocompatibility Complex (MHC), and signatures of contemporary balancing selection in roe deer (Capreolus capreolus), the most abundant cervid species in Europe. Here, we investigated the evolutionary mechanisms by which pathogen-mediated selection could shape this innate immunity genetic diversity by examining the relationships between Tlr (Tlr2, Tlr4 and Tlr5) genotypes (heterozygosity status and presence of specific alleles) and infections with Toxoplasma and Chlamydia, two widespread intracellular pathogens known to cause reproductive failure in ungulates. We showed that Toxoplasma and Chlamydia exposures vary significantly across years and landscape features with few co-infection events detected and that the two pathogens exert antagonistic selection on Tlr2 polymorphism. By contrast, we found limited support for Tlr heterozygote advantage. Our study confirmed the importance of looking beyond Mhc genes in wildlife immunogenetic studies. It also emphasized the necessity to consider multiple pathogen challenges and their spatiotemporal variation to improve our understanding of vertebrate defence evolution against pathogens.

Analysis of natural variation of the rice blast resistance gene Pike and identification of a novel allele Pikg

Plant major resistance (R) genes are effective in detecting pathogen signal molecules and triggering robust defense responses. Investigating the natural variation in R genes will allow identification of the critical amino acid residues determining recognition specificity in R protein and the discovery of novel R alleles. The rice blast resistance gene Pike, comprising of two adjacent CC-NBS-LRR genes, namely, Pike-1 and Pike-2, confers broad-spectrum resistance to Magnaporthe oryzae. Here, we demonstrated that Pike-1 determined Pike-specific resistance through direct interaction with the pathogen signal molecule AvrPik. Analysis of natural variation in 79 Pike-1 variants in the Asian cultivated rice Oryza sativa and its wild relatives revealed that the CC and NBS regions, particularly the CC region of the Pike-1 protein were the most diversified. We also found that balancing selection had occurred in O. sativa and O. rufipogon to maintain the genetic diversity of the Pike-1 alleles. By analysis of amino acid sequences, we identified 40 Pike-1 variants in these rice germplasms. These variants were divided into three major groups that corresponded to their respective clades. A new Pike allele, designated Pikg, that differed from Pike by a single amino acid substitution (D229E) in the Pike-1 CC region of the Pike protein was identified from wild rice relatives. Pathogen assays of Pikg transgenic plants revealed a unique reaction pattern that was different from that of the previously identified Pike alleles, namely, Pik, Pikh, Pikm, Pikp, Piks and Pi1. These findings suggest that minor amino acid residues in Pike-1/Pikg-1 determine pathogen recognition specificity and plant resistance. As a new blast R gene derived from rice wild relatives, Pikg has potential applications in rice breeding.

Divergence in Coding Sequence and Expression of Different Functional Categories of Immune Genes between Two Wild Rodent Species

Differences in immune function between species could be a result of interspecific divergence in coding sequence and/or expression of immune genes. Here, we investigate how the degree of divergence in coding sequence and expression differs between functional categories of immune genes, and if differences between categories occur independently of other factors (expression level, pleiotropy). To this end, we compared spleen transcriptomes of wild-caught yellow-necked mice and bank voles. Immune genes expressed in the spleen were divided into four categories depending on the function of the encoded protein: pattern recognition receptors (PRR); signal transduction proteins; transcription factors; and cyto- and chemokines and their receptors. Genes encoding PRR and cyto-/chemokines had higher sequence divergence than genes encoding signal transduction proteins and transcription factors, even when controlling for potentially confounding factors. Genes encoding PRR also had higher expression divergence than genes encoding signal transduction proteins and transcription factors. There was a positive correlation between expression divergence and coding sequence divergence, in particular for PRR genes. We propose that this is a result of that divergence in PRR coding sequence leads to divergence in PRR expression through positive feedback of PRR ligand binding on PRR expression. When controlling for sequence divergence, expression divergence of PRR genes did not differ from other categories. Taken together, the results indicate that coding sequence divergence of PRR genes is a major cause of differences in immune function between species.

Comparison of spleen transcriptomes of two wild rodent species reveals differences in the immune response against Borrelia afzelii

Different host species often differ considerably in susceptibility to a given pathogen, but the causes of such differences are rarely known. The natural hosts of the tick-transmitted bacterium Borrelia afzelii, which is one of causative agents of Lyme borreliosis in humans, include a variety of small mammals like voles and mice. Previous studies have shown that B. afzelii-infected bank voles (Myodes glareolus) have about ten times higher bacterial load than infected yellow-necked mice (Apodemus flavicollis), indicating that these two species differ in resistance. In this study, we compared the immune response to B. afzelii infection in these host species by using RNA sequencing to quantify gene expression in spleen. Gene set enrichment analysis (GSEA) showed that several immune pathways were down-regulated in infected animals in both bank voles and yellow-necked mice. Moreover, IFNα response was up-regulated in B. afzelii-infected yellow-necked mice, while IL6 signaling and the complement pathway were down-regulated in infected bank voles; differences in regulation of these three pathways between bank voles and yellow-necked mice could thus contribute to the difference in resistance to B. afzelii between the species. This study provides knowledge of gene expression induced by a zoonotic pathogen in its natural host, and possible species-specific regulation of immune responses associated with resistance.