X-linked variation in immune response in Drosophila melanogaster

Unraveling the intricate link between cell death and neuroinflammation using Drosophila as a model

Protein aggregation is a common pathological occurrence in neurodegenerative diseases. This often leads to neuroinflammation, which exacerbates the aggregation and progression of diseases like Parkinson's and Alzheimer's. Here, we focus on immune responses and neurotoxicity in a Parkinson's disease model in Drosophila. Mutations in the SNCA gene that encodes the alpha (α)-Synuclein protein have been linked to familial Parkinson's disease, disrupting autophagy regulation in neuronal cells and promoting the formation of Lewy bodies, a hallmark of Parkinson's pathology. This results in the loss of dopaminergic neurons, manifesting as movement disorders. α-Synuclein aggregation triggers innate immune responses by activating microglial cells, leading to phagocytic activity and the expression of neuroprotective antimicrobial peptides (AMPs). However, sustained AMP expression or chronic inflammation resulting from inadequate microglial phagocytosis can induce neuronal toxicity and apoptosis, leading to severe dopaminergic neuron loss. This review underscores the mechanistic connection between immune response pathways and α-Synuclein-mediated neurodegeneration using Drosophila models. Furthermore, we extensively explore factors influencing neuroinflammation and key immune signaling pathways implicated in neurodegenerative diseases, particularly Parkinson's disease. Given the limited success of traditional treatments, recent research has focused on therapies targeting inflammatory signaling pathways. Some of these approaches have shown promising results in animal models and clinical trials. We provide an overview of current therapeutic strategies showing potential in treating neurodegenerative diseases, offering new avenues for future research and treatment development.

Pathogen infection alters the gene expression landscape of transposable elements in Drosophila melanogaster

Transposable elements make up substantial proportions of eukaryotic genomes and many are thought to be remnants of ancient viral infections. Current research has begun to highlight the role transposable elements can play in the immune system response to infections. However, most of our knowledge about transposable element expression during infection is limited by the specific host and pathogen factors from each study, making it difficult to compare studies and develop broader patterns regarding the role of transposable elements during infection. Here, we use the tools and resources available in the model, Drosophila melanogaster, to analyze multiple gene expression datasets of flies subject to bacterial, fungal, and viral infections. We analyzed differences in pathogen species, host genotype, host tissue, and sex to understand how these factors impact transposable element expression during infection. Our results highlight both shared and unique transposable element expression patterns between pathogens and suggest a larger effect of pathogen factors over host factors for influencing transposable element expression.

Drosophila melanogaster as a model arthropod carrier for the amphibian chytrid fungus Batrachochytrium dendrobatidis

The fungal pathogen Batrachochytrium dendrobatidis (Bd) causes the disease amphibian chytridiomycosis, which has contributed to population declines in many species of amphibians throughout the world. Previous observational studies have shown that nematodes, waterfowl, lizards, other dipterans, and crayfish have properties which may allow them to harbor and spread Bd; therefore, we sought to determine the carrier capabilities of invertebrates to a further extent in a laboratory setting. We use the insect Drosophila melanogaster as a model organism to quantify the potential relationship between insects and Bd. Our findings show that D. melanogaster can test positive for Bd for up to five days post-exposure and can transmit Bd to conspecifics without suffering mortality. Insects of various types interact with the amphibian habitat and amphibians themselves, making this a potentially important route of transmission between amphibians and of dispersal across the environment.

Sex- and species-associated differences in complement-mediated immunity in humans and rhesus macaques

The complement system can be viewed as a "moderator" of innate immunity, "instructor" of humoral immunity, and "regulator" of adaptive immunity. While sex is known to affect humoral and cellular immune systems, its impact on complement in humans and rhesus macaques, a commonly used non-human primate model system, has not been well studied. To address this knowledge gap, we analyzed serum samples from 90 humans and 72 rhesus macaques for the abundance and activity of the complement system components. While sequences of cascade proteins were highly conserved, dramatically different levels were observed between species. Whereas the low levels detected in rhesus samples raised questions about the suitability of the test for use with macaque samples, differences in levels of complement proteins were observed in male and female humans. Levels of total and antibody-dependent deposition of C1q and C3b on a glycosylated antigen differed between humans and rhesus, suggesting differential recognition of glycans and balance between classical and alternative activation pathways. Functional differences in complement-mediated lysis of antibody-sensitized cells were observed in multiple assays and showed that human females frequently exhibited higher lytic activity than human males or rhesus macaques, which typically did not exhibit such sex-associated differences. Other differences between species and sexes were observed in more narrow contexts-for only certain antibodies, antigens, or assays. Collectively, these results expand knowledge of sex-associated differences in the complement system in humans, identifying differences absent from rhesus macaques.IMPORTANCEThe complement system is a critical part of host defense to many bacterial, fungal, and viral infections. In parallel, rich epidemiological, clinical, and biomedical research evidence demonstrates that sex is an important biological variable in immunity, and many sex-specific differences in immune system are intimately tied with disease outcomes. This study focuses on the intersection of these two factors to define the impact of sex on complement pathway components and activities. This work expands our knowledge of sex-associated differences in the complement system in humans and also identifies the differences that appear to be absent in rhesus macaques, a popular non-human primate model. Whereas differences between species suggest potential limitations in the ability of macaque model to recapitulate human biology, knowledge of sex-based differences in humans has the potential to inform clinical research and practice.

Sex and species associated differences in Complement-mediated immunity in Humans and Rhesus macaques

The complement system can be viewed as a 'moderator' of innate immunity, 'instructor' of humoral immunity, and 'regulator' of adaptive immunity. While sex and aging are known to affect humoral and cellular immune systems, their impact on the complement pathway in humans and rhesus macaques, a commonly used non-human primate model system, have not been well-studied. To address this knowledge gap, we analyzed serum samples from 90 humans and 75 rhesus macaques for the abundance and activity of the complement system components. While sequences of cascade proteins were highly conserved, dramatically different levels were observed between species. Whereas the low levels detected in rhesus samples raised questions about the suitability of the test, differences in levels of complement proteins were observed in male and female humans. Levels of total and antibody-dependent deposition of C1q and C3b on a glycosylated antigen differed between human and rhesus, suggesting differential recognition of glycans. Functional differences in complement-mediated lysis of antibody-sensitized cells were observed in multiple assays and showed that human females frequently exhibited higher lytic activity than human males or rhesus macaques, which typically did not exhibit such sexual dimorphism. Other differences between species and sexes were observed in more narrow contexts-for only certain antibodies, antigens, or assays. Collectively, these results expand our knowledge of sexual dimorphism in the complement system in humans, identifying differences that appear to be absent from rhesus macaques.

Drosophila melanogaster as a model to study age and sex differences in brain injury and neurodegeneration after mild head trauma

Repetitive physical insults to the head, including those that elicit mild traumatic brain injury (mTBI), are a known risk factor for a variety of neurodegenerative conditions including Alzheimer's disease (AD), Parkinson's disease (PD), and chronic traumatic encephalopathy (CTE). Although most individuals who sustain mTBI typically achieve a seemingly full recovery within a few weeks, a subset experience delayed-onset symptoms later in life. As most mTBI research has focused on the acute phase of injury, there is an incomplete understanding of mechanisms related to the late-life emergence of neurodegeneration after early exposure to mild head trauma. The recent adoption of Drosophila-based brain injury models provides several unique advantages over existing preclinical animal models, including a tractable framework amenable to high-throughput assays and short relative lifespan conducive to lifelong mechanistic investigation. The use of flies also provides an opportunity to investigate important risk factors associated with neurodegenerative conditions, specifically age and sex. In this review, we survey current literature that examines age and sex as contributing factors to head trauma-mediated neurodegeneration in humans and preclinical models, including mammalian and Drosophila models. We discuss similarities and disparities between human and fly in aging, sex differences, and pathophysiology. Finally, we highlight Drosophila as an effective tool for investigating mechanisms underlying head trauma-induced neurodegeneration and for identifying therapeutic targets for treatment and recovery.

Genomic architecture and sexually dimorphic expression underlying immunity in the red mason bee, Osmia bicornis

Insect pollinators provide crucial ecosystem services yet face increasing environmental pressures. The challenges posed by novel and reemerging pathogens on bee health means we need to improve our understanding of the immune system, an important barrier to infections and disease. Despite the importance of solitary bees, which are ecologically relevant, our understanding of the genomic basis and molecular mechanisms underlying their immune potential, and how intrinsic and extrinsic factors may influence it is limited. To improve our understanding of the genomic architecture underlying immunity of a key solitary bee pollinator, we characterized putative immune genes of the red mason bee, Osmia bicornis. In addition, we used publicly available RNA-seq datasets to determine how sexes differ in immune gene expression and splicing but also how pesticide exposure may affect immune gene expression in females. Through comparative genomics, we reveal an evolutionarily conserved set of more than 500 putative immune-related genes. We found genome-wide patterns of sex-biased gene expression, with greater enrichment of immune-related processes among genes with higher constitutive expression in males than females. Our results also suggest an up-regulation of immune-related genes in response to exposure to two common neonicotinoids, thiacloprid and imidacloprid. Collectively, our study provides important insights into the gene repertoire, regulation and expression differences in the sexes of O. bicornis, as well as providing additional support for how neonicotinoids can affect immune gene expression, which may affect the capacity of solitary bees to respond to pathogenic threats.

Drosophila melanogaster hosts coevolving with Pseudomonas entomophila pathogen show sex-specific patterns of local adaptation

Background

In spatially structured populations, local adaptation improves organisms’ fitness in their native environment. Hosts and pathogens can rapidly adapt to their local antagonist. Since males and females can differ in their immunocompetence, the patterns of local adaptation can be different between the sexes. However, there is little information about sex differences in local adaptation in host–pathogen systems.

Results

In the current study, we experimentally coevolved four different replicate populations of Drosophila melanogaster (host) and Pseudomonas entomophila (pathogen) along with appropriate controls. We used the four host–pathogen coevolution populations to investigate the occurrence of local adaptation separately in males and females of the coevolving hosts. We also assessed local adaptation in pathogens. We set up a reciprocal infection experiment where we infected each of the four coevolving hosts with their local pathogen or non-local pathogens from the other three replicate populations. We found that overall, male and female hosts had better survivorship when infected with local pathogens, indicating that they were locally adapted. Interestingly, males were more susceptible to non-local pathogens compared to females. In addition, we found no fecundity cost in females infected with either local or non-local pathogens. We found no evidence of local adaptation among the pathogens.

Conclusion

Our study showed sex-specific adaptation in the coevolving hosts where female hosts had a broader response against allopatric coevolving pathogens with no cost in fecundity. Thus, our results might suggest a novel mechanism that can maintain variation in susceptibility in spatially structured populations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02031-8.

Sex Difference Leads to Differential Gene Expression Patterns and Therapeutic Efficacy in Mucopolysaccharidosis IVA Murine Model Receiving AAV8 Gene Therapy

Adeno-associated virus (AAV) vector-based therapies can effectively correct some disease pathology in murine models with mucopolysaccharidoses. However, immunogenicity can limit therapeutic effect as immune responses target capsid proteins, transduced cells, and gene therapy products, ultimately resulting in loss of enzyme activity. Inherent differences in male versus female immune response can significantly impact AAV gene transfer. We aim to investigate sex differences in the immune response to AAV gene therapies in mice with mucopolysaccharidosis IVA (MPS IVA). MPS IVA mice, treated with different AAV vectors expressing human N-acetylgalactosamine 6-sulfate sulfatase (GALNS), demonstrated a more robust antibody response in female mice resulting in subsequent decreased GALNS enzyme activity and less therapeutic efficacy in tissue pathology relative to male mice. Under thyroxine-binding globulin promoter, neutralizing antibody titers in female mice were approximately 4.6-fold higher than in male mice, with GALNS enzyme activity levels approximately 6.8-fold lower. Overall, male mice treated with AAV-based gene therapy showed pathological improvement in the femur and tibial growth plates, ligaments, and articular cartilage as determined by contrasting differences in pathology scores compared to females. Cardiac histology revealed a failure to normalize vacuolation in females, in contrast, to complete correction in male mice. These findings promote the need for further determination of sex-based differences in response to AAV-mediated gene therapy related to developing treatments for MPS IVA.

Female-limited X chromosome evolution reveals that lifespan is mainly modulated by interlocus rather than intralocus sexual conflict

Abstract

Sexual dimorphism in somatic investment may be shaped by two distinct forms of sexual conflict; under intralocus sexual conflict (IASC), males and females have different optimal levels of somatic investment but are constrained from reaching their respective optima by their shared genome, while under interlocus sexual conflict (IRSC), males and females have different optimal sexual strategies, which could have direct or indirect effects on levels of somatic investment. We investigated effects of IASC and IRSC on two aspects of somatic investment, immune defence strategies and longevity, using previously established female-limited experimental evolution lines in Drosophila melanogaster. We found little evidence for any effect of either type of sexual conflict on investment in the immune defence resistance or tolerance. Nor did we find convincing evidence that longevity is subject to IASC in this species. However, we did find evidence that increased female control over mating rate had important and opposite effects on longevity between the sexes. Specifically, females that had adapted to high levels of female control over mating had a longer lifespan when kept in mixed-sex groups, while males had shorter longevity, perhaps due to increased investment in post-copulatory sexual selection. These novel results show that female control over mating rates may have important and unexpected effects on patterns of somatic investment.

Significance statement

Sexual conflict occurs between the two sexes over numerous life history traits, and it is complex to disentangle how these traits interact and affect each other. Here we use a long-term evolution experiment to investigate sexual dimorphism in somatic maintenance. We found no effect of feminising the X chromosome on female immune defence. However, we did find that increased female control over mating rate resulted in longer female lifespan, but reduced male lifespan, and that these effects were dependent on social context (isolated or in mixed-sex groups). Unlike previous studies on the effect of sexual conflict on longevity, our experiment did not manipulate environmental conditions nor the adult sex ratio, which is likely to reduce both pre- and post-copulatory sexual selection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-022-03231-4.

Ecology and the evolution of sex chromosomes

Sex chromosomes are common features of animal genomes, often carrying a sex determination gene responsible for initiating the development of sexually dimorphic traits. The specific chromosome that serves as the sex chromosome differs across taxa as a result of fusions between sex chromosomes and autosomes, along with sex chromosome turnover-autosomes becoming sex chromosomes and sex chromosomes 'reverting' back to autosomes. In addition, the types of genes on sex chromosomes frequently differ from the autosomes, and genes on sex chromosomes often evolve faster than autosomal genes. Sex-specific selection pressures, such as sexual antagonism and sexual selection, are hypothesized to be responsible for sex chromosome turnovers, the unique gene content of sex chromosomes and the accelerated evolutionary rates of genes on sex chromosomes. Sex-specific selection has pronounced effects on sex chromosomes because their sex-biased inheritance can tilt the balance of selection in favour of one sex. Despite the general consensus that sex-specific selection affects sex chromosome evolution, most population genetic models are agnostic as to the specific sources of these sex-specific selection pressures, and many of the details about the effects of sex-specific selection remain unresolved. Here, I review the evidence that ecological factors, including variable selection across heterogeneous environments and conflicts between sexual and natural selection, can be important determinants of sex-specific selection pressures that shape sex chromosome evolution. I also explain how studying the ecology of sex chromosome evolution can help us understand important and unresolved aspects of both sex chromosome evolution and sex-specific selection.

Induction and inhibition of Drosophila X chromosome gene expression are both impeded by the dosage compensation complex

Sex chromosomes frequently differ from the autosomes in the frequencies of genes with sexually dimorphic or tissue-specific expression. Multiple hypotheses have been put forth to explain the unique gene content of the X chromosome, including selection against male-beneficial X-linked alleles, expression limits imposed by the haploid dosage of the X in males, and interference by the dosage compensation complex on expression in males. Here, we investigate these hypotheses by examining differential gene expression in Drosophila melanogaster following several treatments that have widespread transcriptomic effects: bacterial infection, viral infection, and abiotic stress. We found that genes that are induced (upregulated) by these biotic and abiotic treatments are frequently under-represented on the X chromosome, but so are those that are repressed (downregulated) following treatment. We further show that whether a gene is bound by the dosage compensation complex in males can largely explain the paucity of both up- and downregulated genes on the X chromosome. Specifically, genes that are bound by the dosage compensation complex, or close to a dosage compensation complex high-affinity site, are unlikely to be up- or downregulated after treatment. This relationship, however, could partially be explained by a correlation between differential expression and breadth of expression across tissues. Nonetheless, our results suggest that dosage compensation complex binding, or the associated chromatin modifications, inhibit both up- and downregulation of X chromosome gene expression within specific contexts, including tissue-specific expression. We propose multiple possible mechanisms of action for the effect, including a role of Males absent on the first, a component of the dosage compensation complex, as a dampener of gene expression variance in both males and females. This effect could explain why the Drosophila X chromosome is depauperate in genes with tissue-specific or induced expression, while the mammalian X has an excess of genes with tissue-specific expression.

Meta-Analysis of Immune Induced Gene Expression Changes in Diverse Drosophila melanogaster Innate Immune Responses

Simple Summary

Organisms can be infected by a wide range of pathogens, including bacteria, viruses, and parasites. Following infection, the host mounts an immune response to attempt to eliminate the pathogen. These responses are often specific to the type of pathogen and mediated by the expression of specialized genes. We have characterized the expression changes induced in host Drosophila fruit flies following infection by multiple types of pathogens, and identified a small number of genes that show expression changes in each infection. This includes genes that are known to be involved in pathogen resistance, and others that have not been previously studied as immune response genes. These findings provide new insight into transcriptional changes that accompany Drosophila immunity. They may suggest possible roles for the differentially expressed genes in innate immune responses to diverse classes of pathogens, and serve to identify candidate genes for further empirical study of these processes.

Abstract

Organisms are commonly infected by a diverse array of pathogens and mount functionally distinct responses to each of these varied immune challenges. Host immune responses are characterized by the induction of gene expression, however, the extent to which expression changes are shared among responses to distinct pathogens is largely unknown. To examine this, we performed meta-analysis of gene expression data collected from Drosophila melanogaster following infection with a wide array of pathogens. We identified 62 genes that are significantly induced by infection. While many of these infection-induced genes encode known immune response factors, we also identified 21 genes that have not been previously associated with host immunity. Examination of the upstream flanking sequences of the infection-induced genes lead to the identification of two conserved enhancer sites. These sites correspond to conserved binding sites for GATA and nuclear factor κB (NFκB) family transcription factors and are associated with higher levels of transcript induction. We further identified 31 genes with predicted functions in metabolism and organismal development that are significantly downregulated following infection by diverse pathogens. Our study identifies conserved gene expression changes in Drosophila melanogaster following infection with varied pathogens, and transcription factor families that may regulate this immune induction.

Experimental evolution reveals sex-specific dominance for surviving bacterial infection in laboratory populations of Drosophila melanogaster

Males and females are subjected to distinct kinds of selection pressures, often leading to the evolution of sex‐specific genetic architecture, an example being sex‐specific dominance. Sex‐specific dominance reversals (SSDRs), where alleles at sexually antagonistic loci are at least partially dominant in the sex they benefit, have been documented in Atlantic salmon, rainbow trout, and seed beetles. Another interesting feature of many sexually reproducing organisms is the asymmetric inheritance pattern of X chromosomes, which often leads to distinct evolutionary outcomes on X chromosomes compared to autosomes. Examples include the higher efficacy of sexually concordant selection on X chromosomes, and X chromosomes being more conducive to the maintenance of sexually antagonistic polymorphisms under certain conditions. Immunocompetence is a trait that has been extensively investigated for sexual dimorphism with growing evidence for sex‐specific or sexually antagonistic variation. X chromosomes have been shown to harbor substantial immunity‐related genetic variation in the fruit fly, Drosophila melanogaster. Here, using interpopulation crosses and cytogenetic cloning, we investigated sex‐specific dominance and the role of the X chromosome in improved postinfection survivorship of laboratory populations of D. melanogaster selected against pathogenic challenge by Pseudomonas entomophila. We could not detect any contribution of the X chromosome to the evolved immunocompetence of our selected populations, as well as to within‐population variation in immunocompetence. However, we found strong evidence of sex‐specific dominance related to surviving bacterial infection. Our results indicate that alleles that confer a survival advantage to the selected populations are, on average, partially dominant in females but partially recessive in males. This could also imply an SSDR for overall fitness, given the putative evidence for sexually antagonistic selection affecting immunocompetence in Drosophila melanogaster. We also highlight sex‐specific dominance as a potential mechanism of sex differences in immunocompetence, with population‐level sex differences primarily driven by sex differences in heterozygotes.

Transcriptome Analysis Reveals Sexual Disparities between Olfactory and Immune Gene Expression in the Olfactory Epithelium of Megalobrama amblycephala

The olfactory organ is an important chemoreceptor in vertebrates. However, the sexual disparities in gene expression patterns in the olfactory organ in fish remain unstudied. Here, we conducted a transcriptome analysis of the olfactory epithelium (OE) of male and female blunt snout bream (Megalobrama amblycephala) to identify the differences. The histological analysis showed that there were 22 leaf-like olfactory lamellaes on one side of the OE of the adult blunt snout bream. The sensory area of OE is enriched with ciliated receptor cells and microvilli receptor cells. The transcriptome analysis showed that only 10 out of 336 olfactory receptor genes (224 ORs, 5 V1Rs, 55 V2Rs, and 52 TAARs) exhibited significant expression differences between males and females, and most of the differentially expressed genes were related to the immune system. We also validated these results using qPCR: 10 OR genes and 6 immunity-related genes significantly differed between males and females. The FISH analysis results indicated that the ORs were mainly expressed at the edge of the olfactory lamellae. Collectively, our study reveals that gender is not an important factor influencing the expression of olfactory receptors, but the expression of immune genes varies greatly between the genders in blunt snout bream.

Immunological aspects and gender bias during respiratory viral infections including novel Coronavirus disease-19 (COVID-19): A scoping review

The human immune system is not adequately equipped to eliminate new microbes and could result in serious damage on first exposure. This is primarily attributed to the exaggerated immune response (inflammatory disease), which may prove detrimental to the host, as evidenced by SARS-CoV-2 infection. From the experiences of Novel Coronavirus Disease-19 to date, male patients are likely to suffer from high-intensity inflammation and disease severity than the female population. Hormones are considered the significant pillars of sex differences responsible for the discrepancy in immune response exhibited by males and females. Females appear to be better equipped to counter invading respiratory viral pathogens, including the novel SARS-CoV-2, than males. It can be hypothesized that females are more shielded from disease severity, probably owing to the diverse action/influence of estrogen and other sex hormones on both cellular (thymus-derived T lymphocytes) and humoral immunity (antibodies).

Impact of Sex Differences on Mortality in Patients With Sepsis After Trauma: A Nationwide Cohort Study

Objective

Sepsis is the leading cause of death in intensive care units, and sepsis after trauma is associated with increased mortality rates. However, the characteristics of sepsis after trauma remain unknown, and the influence of sex on mortality remains controversial. This study aimed to assess the role of sex in in-hospital mortality in patients with sepsis after trauma.

Methods

We performed a retrospective cohort study involving several emergency hospitals (n=288) in Japan. The data of patients with trauma who developed sepsis after admission from 2004 to 2019 were obtained from the Japan Trauma Data Bank. We divided the patients into two groups according to sex and compared their in-hospital mortality. We also performed subgroup analysis limited to the elderly population (age ≥ 65 years) and evaluated in-hospital mortality between men and women.

Results

A total of 1935 patients met the inclusion criteria during the study period. Of these, 1204 (62.2%) were allocated to the male group and 731 (37.8%) to the female group. Multivariable Cox proportional-hazards analysis showed a significantly lower risk of in-hospital mortality in the female group than in the male group (hazard ratio (HR): 0.74, 95% confidence interval (CI): 0.62–0.89; p=0.001). In the subgroup analysis, multivariable Cox proportional hazards still showed significantly lower risks of in-hospital mortality in the female group than in the male group (HR: 0.72, 95% CI: 0.58–0.88; p=0.002).

Conclusion

The present study shows a significantly increased survival in the female group when compared to that in the male group of patients with sepsis after trauma. The underlying mechanism remains unclear, and further investigations are required.

These Are the Genes You're Looking For: Finding Host Resistance Genes

Humanity's ongoing struggle with new, re-emerging and endemic infectious diseases serves as a frequent reminder of the need to understand host-pathogen interactions. Recent advances in genomics have dramatically advanced our understanding of how genetics contributes to host resistance or susceptibility to bacterial infection. Here we discuss current trends in defining host-bacterial interactions at the genome-wide level, including screens that harness CRISPR/Cas9 genome editing, natural genetic variation, proteomics, and transcriptomics. We report on the merits, limitations, and findings of these innovative screens and discuss their complementary nature. Finally, we speculate on future innovation as we continue to progress through the postgenomic era and towards deeper mechanistic insight and clinical applications.

Drosophila melanogaster Y Chromosome Genes Affect Male Sensitivity to Microbial Infections

The genders of Drosophila melanogaster vary in their sensitivities to microbial pathogens. While many of the immunity-related genes are located on the X chromosome, the polymorphisms within the Y chromosome were also shown to affect the immunity of flies. In this study, we investigated the necessity of individual genes on the Y chromosome (Y-genes) for male sensitivity to microbes. We identified several Y-genes whose genetic inactivation either increases or decreases the sensitivity of males to gastrointestinal infections with fungal Saccharomyces cerevisiae and bacterial Serratia liquefaciens. Specifically, the loss of function mutations in fly kl-5 and Ppr-Y Y-genes lead to increased and decreased sensitivity of males to fungal challenge, respectively, compared to female sensitivity. In contrast, mutations in Drosophila Pp1-Y1, kl-5, kl-3, Ppr-Y, CCY, and FDY Y-genes lead to increased sensitivity of males to bacterial infection, compared to females. Moreover, while these Y-genes are necessary, the Y chromosome is not sufficient for the sensitivity of males to microbes, since the sensitivity of XXY females to fungal and bacterial challenges was not different from the sensitivity of wild-type female flies, compared to males. This study assigns a new immunity-related function to numerous Y-genes in D. melanogaster.

Strength of immune selection in tumors varies with sex and age

Individual MHC genotype constrains the mutational landscape during tumorigenesis. Immune checkpoint inhibition reactivates immunity against tumors that escaped immune surveillance in approximately 30% of cases. Recent studies demonstrated poorer response rates in female and younger patients. Although immune responses differ with sex and age, the role of MHC-based immune selection in this context is unknown. We find that tumors in younger and female individuals accumulate more poorly presented driver mutations than those in older and male patients, despite no differences in MHC genotype. Younger patients show the strongest effects of MHC-based driver mutation selection, with younger females showing compounded effects and nearly twice as much MHC-II based selection. This study presents evidence that strength of immune selection during tumor development varies with sex and age, and may influence the availability of mutant peptides capable of driving effective response to immune checkpoint inhibitor therapy.

Sexual Dimorphisms in Innate Immunity and Responses to Infection in Drosophila melanogaster

The sexes show profound differences in responses to infection and the development of autoimmunity. Dimorphisms in immune responses are ubiquitous across taxa, from arthropods to vertebrates. Drosophila melanogaster shows strong sex dimorphisms in immune system responses at baseline, upon pathogenic challenge, and over aging. We have performed an exhaustive survey of peer-reviewed literature on Drosophila immunity, and present a database of publications indicating the sex(es) analyzed in each study. While we found a growing interest in the community in adult immunity and in reporting both sexes, the main body of work in this field uses only one sex, or does not stratify by sex. We synthesize evidence for sexually dimorphic responses to bacterial, viral, and fungal infections. Dimorphisms may be mediated by distinct immune compartments, and we review work on sex differences in behavioral, epithelial, cellular, and systemic (fat body-mediated) immunity. Emerging work on sexually dimorphic aging of immune tissues, immune senescence, and inflammation are examined. We consider evolutionary drivers for sex differences in immune investment, highlight the features of Drosophila biology that make it particularly amenable to studies of immune dimorphisms, and discuss areas for future exploration.

Why leveraging sex differences in immune trade-offs may illuminate the evolution of senescence

The immune system affects senescence (declines in probabilities of survival or reproduction with age), by shaping late age vulnerability to chronic inflammatory diseases and infections. It is also a dynamic interactive system that must balance competing demands across the life course. Thus, immune system function remains an important frontier in understanding the evolution of senescence.Here, we review our expanding mechanistic understanding of immune function over the life course, in the context of theoretical predictions from life-history evolution. We are especially interested in stage- and sex-dependent costs and benefits of investment in the immune system, given differential life-history priorities of the life stages and sexes.We introduce the costs likely to govern immune allocation across the life course. We then discuss theoretical expectations for differences between the sexes and their likely consequences in terms of how the immune system is both modulated by and may modulate senescence, building on information from life-history theory, experimental immunology and demography.We argue that sex differences in immune function provide a potentially powerful probe of selection pressures on the immune system across the life course. In particular, differences in 'competing' and 'caring' between the sexes have evolved across the tree of life, providing repeated instances of divergent selection pressures on immune function occurring within the same overall bauplan.We conclude by detailing an agenda for future research, including development of theoretical predictions of the differences between the sexes under an array of existing models for sex differences in immunity, and empirical tests of such predictions across the tree of life. A free http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13458/suppinfo can be found within the Supporting Information of this article.

Sexual Dimorphism in Innate Immunity

Sexual dimorphisms account for differences in clinical manifestations or incidence of infectious or autoimmune diseases and malignancy between females and males. Females develop enhanced innate and adaptive immune responses than males and are less susceptible to many infections of bacterial, viral, parasitic, and fungal origin and malignancies but in contrast, they are more prone to develop autoimmune diseases. The higher susceptibility to infections in males is observed from birth to adulthood, suggesting that sex chromosomes and not sex hormones have a major role in sexual dimorphism in innate immunity. Sex-based regulation of immune responses ultimately contributes to age-related disease development and life expectancy. Differences between males and females have been described in the expression of pattern recognition receptors of the innate immune response and in the functional responses of phagocytes and antigen presenting cells. Different factors have been shown to account for the sex-based disparity in immune responses, including genetic factors and hormonal mediators, which contribute independently to dimorphism in the innate immune response. For instance, several genes encoding for innate immune molecules are located on the X chromosome. In addition, estrogen and/or testosterone have been reported to modulate the differentiation, maturation, lifespan, and effector functions of innate immune cells, including neutrophils, macrophages, natural killer cells, and dendritic cells. In this review, we will focus on differences between males and females in innate immunity, which represents the first line of defense against pathogens and plays a fundamental role in the activation, regulation, and orientation of the adaptive immune response.

Metabolic and immunological responses of male and female new Zealand Greenshell™ mussels (Perna canaliculus) infected with Vibrio sp

Massive mortalities due to pathogens are routinely reported in bivalve cultivation that have significant economic consequences for the global aquaculture industry. However, host-pathogen interactions and infection mechanisms that mediate these interactions are poorly understood. In addition, gender-specific immunological responses have been reported for some species, but the reasons for such differences have not been elucidated. In this study, we used a GC/MS-based metabolomics platform and flow cytometry approach to characterize metabolic and immunological responses in haemolymph of male and female mussels (Perna canaliculus) experimentally infected with Vibrio sp. Sex-based differences in immunological responses were identified, with male mussels displaying higher mortality, oxidative stress and apoptosis after pathogen exposure. However, central metabolic processes appeared to be similar between sexes at 24 h post injection with Vibrio sp. DO1. Significant alterations in relative levels of 37 metabolites were detected between infected and uninfected mussels. These metabolites are involved in major perturbations on the host's innate immune system. In addition, there were alterations of seven metabolites in profiles of mussels sampled on the second day and mussels that survived six days after exposure. These metabolites include itaconic acid, isoleucine, phenylalanine, creatinine, malonic acid, glutaric acid and hydroxyproline. Among these, itaconic acid has the potential to be an important biomarker for Vibrio sp. DO1 infection. These findings provide new insights on the mechanistic relationship between a bivalve host and a pathogenic bacterium and highlight the need to consider host sex as a biological variable in future immunological studies.

Nonlinear disease tolerance curves reveal distinct components of host responses to viral infection

The ability to tolerate infection is a key component of host defence and offers potential novel therapeutic approaches for infectious diseases. To yield successful targets for therapeutic intervention, it is important that the analytical tools employed to measure disease tolerance are able to capture distinct host responses to infection. Here, we show that commonly used methods that estimate tolerance as a linear relationship should be complemented with more flexible, nonlinear estimates of this relationship which may reveal variation in distinct components such as host vigour, sensitivity to increases in pathogen loads, and the severity of the infection. To illustrate this, we measured the survival of Drosophila melanogaster carrying either a functional or non-functional regulator of the JAK-STAT immune pathway (G9a) when challenged with a range of concentrations of Drosophila C virus (DCV). While classical linear model analyses indicated that G9a affected tolerance only in females, a more powerful nonlinear logistic model showed that G9a mediates viral tolerance to different extents in both sexes. This analysis also revealed that G9a acts by changing the sensitivity to increasing pathogen burdens, but does not reduce the ultimate severity of disease. These results indicate that fitting nonlinear models to host health-pathogen burden relationships may offer better and more detailed estimates of disease tolerance.

The route of infection determines Wolbachia antibacterial protection in Drosophila

Bacterial symbionts are widespread among metazoans and provide a range of beneficial functions. Wolbachia-mediated protection against viral infection has been extensively demonstrated in Drosophila. In mosquitoes that are artificially transinfected with Drosophila melanogaster Wolbachia (wMel), protection from both viral and bacterial infections has been demonstrated. However, no evidence for Wolbachia-mediated antibacterial protection has been demonstrated in Drosophila to date. Here, we show that the route of infection is key for Wolbachia-mediated antibacterial protection. Drosophila melanogaster carrying Wolbachia showed reduced mortality during enteric-but not systemic-infection with the opportunist pathogen Pseudomonas aeruginosaWolbachia-mediated protection was more pronounced in male flies and is associated with increased early expression of the antimicrobial peptide Attacin A, and also increased expression of a reactive oxygen species detoxification gene (Gst D8). These results highlight that the route of infection is important for symbiont-mediated protection from infection, that Wolbachia can protect hosts by eliciting a combination of resistance and disease tolerance mechanisms, and that these effects are sexually dimorphic. We discuss the importance of using ecologically relevant routes of infection to gain a better understanding of symbiont-mediated protection.

Sex differences in immune responses

Males and females differ in their immunological responses to foreign and self-antigens and show distinctions in innate and adaptive immune responses. Certain immunological sex differences are present throughout life, whereas others are only apparent after puberty and before reproductive senescence, suggesting that both genes and hormones are involved. Furthermore, early environmental exposures influence the microbiome and have sex-dependent effects on immune function. Importantly, these sex-based immunological differences contribute to variations in the incidence of autoimmune diseases and malignancies, susceptibility to infectious diseases and responses to vaccines in males and females. Here, we discuss these differences and emphasize that sex is a biological variable that should be considered in immunological studies.

Host adaptation is contingent upon the infection route taken by pathogens

Evolution of pathogen virulence is affected by the route of infection. Also, alternate infection routes trigger different physiological responses on hosts, impinging on host adaptation and on its interaction with pathogens. Yet, how route of infection may shape adaptation to pathogens has not received much attention at the experimental level. We addressed this question through the experimental evolution of an outbred Drosophila melanogaster population infected by two different routes (oral and systemic) with Pseudomonas entomophila. The two selection regimes led to markedly different evolutionary trajectories. Adaptation to infection through one route did not protect from infection through the alternate route, indicating distinct genetic bases. Finally, relatively to the control population, evolved flies were not more resistant to bacteria other than Pseudomonas and showed higher susceptibility to viral infections. These specificities and trade-offs may contribute to the maintenance of genetic variation for resistance in natural populations. Our data shows that the infection route affects host adaptation and thus, must be considered in studies of host-pathogen interaction.

Genome-wide patterns of natural variation reveal strong selective sweeps and ongoing genomic conflict in Drosophila mauritiana

Although it is well understood that selection shapes the polymorphism pattern in Drosophila, signatures of classic selective sweeps are scarce. Here, we focus on Drosophila mauritiana, an island endemic, which is closely related to Drosophila melanogaster. Based on a new, annotated genome sequence, we characterized the genome-wide polymorphism by sequencing pooled individuals (Pool-seq). We show that the interplay between selection and recombination results in a genome-wide polymorphism pattern characteristic for D. mauritiana. Two large genomic regions (>500 kb) showed the signature of almost complete selective sweeps. We propose that the absence of population structure and limited geographic distribution could explain why such pronounced sweep patterns are restricted to D. mauritiana. Further evidence for strong adaptive evolution was detected for several nucleoporin genes, some of which were not previously identified as genes involved in genomic conflict. Since this adaptive evolution is continuing after the split of D. mauritiana and Drosophila simulans, we conclude that genomic conflict is not restricted to short episodes, but rather an ongoing process in Drosophila.

Quantitative measurement of the immune response and sleep in Drosophila

A complex interaction between the immune response and host behavior has been described in a wide range of species. Excess sleep, in particular, is known to occur as a response to infection in mammals (1) and has also recently been described in Drosophila melanogaster(2). It is generally accepted that sleep is beneficial to the host during an infection and that it is important for the maintenance of a robust immune system(3,4). However, experimental evidence that supports this hypothesis is limited(4), and the function of excess sleep during an immune response remains unclear. We have used a multidisciplinary approach to address this complex problem, and have conducted studies in the simple genetic model system, the fruitfly Drosophila melanogaster. We use a standard assay for measuring locomotor behavior and sleep in flies, and demonstrate how this assay is used to measure behavior in flies infected with a pathogenic strain of bacteria. This assay is also useful for monitoring the duration of survival in individual flies during an infection. Additional measures of immune function include the ability of flies to clear an infection and the activation of NFκB, a key transcription factor that is central to the innate immune response in Drosophila. Both survival outcome and bacterial clearance during infection together are indicators of resistance and tolerance to infection. Resistance refers to the ability of flies to clear an infection, while tolerance is defined as the ability of the host to limit damage from an infection and thereby survive despite high levels of pathogen within the system(5). Real-time monitoring of NFκB activity during infection provides insight into a molecular mechanism of survival during infection. The use of Drosophila in these straightforward assays facilitates the genetic and molecular analyses of sleep and the immune response and how these two complex systems are reciprocally influenced.

Investigating natural variation in Drosophila courtship song by the evolve and resequence approach

A primary goal of population genetics is to determine the genetic basis of natural trait variation. We could significantly advance this goal by developing comprehensive genome-wide approaches to link genotype and phenotype in model organisms. Here we combine artificial selection with population-based resequencing to investigate the genetic basis of variation in the interpulse interval (IPI) of Drosophila melanogaster courtship song. We performed divergent selection on replicate populations for only 14 generations, but had considerable power to differentiate alleles that evolved due to selection from those that evolved stochastically. We identified a large number of variants that changed frequency in response to selection for this simple behavior, and they are highly underrepresented on the X chromosome. Though our power was adequate using this experimental technique, the ability to differentiate causal variants from those affected by linked selection requires further development.

Associations between Variation in X Chromosome Male Reproductive Genes and Sperm Competitive Ability in Drosophila melanogaster

Variation in reproductive success has long been thought to be mediated in part by genes encoding seminal proteins. Here we explore the effect on male reproductive phenotypes of X-linked polymorphisms, a chromosome that is depauperate in genes encoding seminal proteins. Using 57 X chromosome substitution lines, sperm competition was tested both when the males from the wild-extracted line were the first to mate (“defense” crosses), followed by a tester male, and when extracted-line males were the second to mate, after a tester male (“offfense” crosses). We scored the proportion of progeny sired by each male, the fecundity, the remating rate and refractoriness to remating, and tested the significance of variation among lines. Eleven candidate genes were chosen based on previous studies, and portions of these genes were sequenced in all 57 lines. A total of 131 polymorphisms were tested for associations with the reproductive phenotypes using linear models. Nine polymorphisms in 4 genes were found to show significant associations (at a 5% FDR). Overall, it appears that the X chromosomes harbor abundant variation in sperm competition, especially considering the paucity of seminal protein genes. This suggests that much of the male reproductive variation lies outside of genes that encode seminal proteins.

The genetic basis for male x female interactions underlying variation in reproductive phenotypes of Drosophila

In Drosophila, where females mate multiply, sperm competition contributes strongly to fitness variability among males. Males transfer "Acp" seminal proteins to females during mating, and these proteins influence the outcome of sperm competition. Because Acps function within the female, male proteins can directly interact with female molecules in a manner that affects reproductive fitness. Here we begin to dissect the genetic architecture of male×female interactions underlying reproductive phenotypes important to sperm competition. By utilizing chromosome extraction lines, we demonstrate that the third and X chromosomes each have large effects on fertility phenotypes, female remating rate, and the sperm competition parameter, P1. Strikingly, the third and X chromosomes harbor genetic variation that gives rise to strong male×female interactions that modulate female remating rate and P1. Encoded on these chromosomes are, respectively, sex peptide (SP) and sex peptide receptor (SPR), the only pair of physically interacting male Acp and female receptor known. We identified several intriguing allelic interactions between SP and SPR. The results of this study begin to elucidate the complex genetic architecture of reproductive and sperm competition phenotypes and have significant implications for the evolution of male and female characters.

Genotype and gene expression associations with immune function in Drosophila

It is now well established that natural populations of Drosophila melanogaster harbor substantial genetic variation associated with physiological measures of immune function. In no case, however, have intermediate measures of immune function, such as transcriptional activity of immune-related genes, been tested as mediators of phenotypic variation in immunity. In this study, we measured bacterial load sustained after infection of D. melanogaster with Serratia marcescens, Providencia rettgeri, Enterococcus faecalis, and Lactococcus lactis in a panel of 94 third-chromosome substitution lines. We also measured transcriptional levels of 329 immune-related genes eight hours after infection with E. faecalis and S. marcescens in lines from the phenotypic tails of the test panel. We genotyped the substitution lines at 137 polymorphic markers distributed across 25 genes in order to test for statistical associations among genotype, bacterial load, and transcriptional dynamics. We find that genetic polymorphisms in the pathogen recognition genes (and particularly in PGRP-LC, GNBP1, and GNBP2) are most significantly associated with variation in bacterial load. We also find that overall transcriptional induction of effector proteins is a significant predictor of bacterial load after infection with E. faecalis, and that a marker upstream of the recognition gene PGRP-SD is statistically associated with variation in both bacterial load and transcriptional induction of effector proteins. These results show that polymorphism in genes near the top of the immune system signaling cascade can have a disproportionate effect on organismal phenotype due to the amplification of minor effects through the cascade.

Genetic variation for resistance to infection is widespread among insects and other organisms. However, the extent to which this variation in resistance is mediated by changes in infection-induced gene expression is not known. In this study, we assayed expression of immune system genes and bacterial load after infection in a genotyped panel of lines of the model insect Drosophila melanogaster. We find that statistical associations between genetic variants and bacterial load tend to cluster in genes encoding proteins involved in microbial recognition. Variation in suppression of bacterial growth is also determined in part by genetic variation in the expression of downstream components of the immune system that function to directly kill bacteria, despite finding no genetic variation in any single of these effector gene significantly associated with phenotype. Instead, it appears that activity differences in upstream components of the pathway have a cascading effect that results in larger variation in the expression of coordinately regulated downstream effector genes. These results imply that the interactions among genes need to be taken into account when assessing the phenotypic consequences of genetic variation, as signaling cascades such as those in the immune response have the potential to amplify the phenotypic effects of minor genetic variation in individual genes.