Effects of immune challenge on expression of life-history and immune trait expression in sexually reproducing metazoans-a meta-analysis

BACKGROUND

Life-history theory predicts a trade-off between investment into immune defence and other fitness-related traits. Accordingly, individuals are expected to upregulate their immune response when subjected to immune challenge. However, this is predicted to come at the expense of investment into a range of other traits that are costly to maintain, such as growth, reproduction and survival. Currently, it remains unclear whether the magnitude of such costs, and trade-offs involving immune investment and other traits, manifests consistently across species and sexes. To address this, we conducted a meta-analysis to investigate how changes in sex, ontogenetic stage and environmental factors shape phenotypic trait expression following an immune challenge.

RESULTS

We explored the effects of immune challenge on three types of traits across sexually reproducing metazoans: life-history, morphological and proximate immune traits (235 effect sizes, 53 studies, 37 species [21 invertebrates vs. 16 vertebrates]). We report a general negative effect of immune challenge on survival and reproduction, a positive effect on immune trait expression, but no effect on morphology or development time. The negative effects of immune challenge on reproductive traits and survival were larger in females than males. We also report a pronounced effect of the immune treatment agent used (e.g. whether the treatment involved a live pathogen or not) on the host response to immune challenge, and find an effect of mating status on the host response in invertebrates.

CONCLUSION

These results suggest that costs associated with immune deployment following an immune challenge are context-dependent and differ consistently in their magnitude across the sexes of diverse taxonomic lineages. We synthesise and discuss the outcomes in the context of evolutionary theory on sex differences in life-history and highlight the need for future studies to carefully consider the design of experiments aimed at disentangling the costs of immune deployment.

Public attitudes towards novel reproductive technologies: a citizens' jury on mitochondrial donation

STUDY QUESTION

Does an informed group of citizens endorse the clinical use of mitochondrial donation in a country where this is not currently permitted?

SUMMARY ANSWER

After hearing balanced expert evidence and having opportunity for deliberation, a majority (11/14) of participants in a citizens’ jury believed that children should be able to be born using mitochondrial donation.

WHAT IS KNOWN ALREADY

Research suggests that patients, oocyte donors and health professionals support mitochondrial donation to prevent transmission of mitochondrial disease. Less is known about public acceptability of this novel reproductive technology, especially from evidence using deliberative methods.

STUDY DESIGN, SIZE, DURATION

This study comprised a citizens’ jury, an established method for determining the views of a well-informed group of community members. The jury had 14 participants, and ran over one and a half days in 2017.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Jurors were members of the public with no experience of mitochondrial disease. They heard and engaged with relevant evidence and were asked to answer the question: ‘Should Australia allow children to be born following mitochondrial donation?’

MAIN RESULTS AND THE ROLE OF CHANCE

Eleven jurors decided that Australia should allow children to be born following mitochondrial donation; 7 of whom added conditions such as the need to limit who can access the intervention. Three jurors decided that children should not (or not yet) be born using this intervention. All jurors were particularly interested in the reliability of evidence, licensing/regulatory mechanisms and the rights of children to access information about their oocyte donors.

LIMITATIONS, REASONS FOR CAUTION

Jurors’ views were well informed and reflected critical deliberation and discussion, but are not intended to be representative of the whole population.

WIDER IMPLICATIONS OF THE FINDINGS

When presented with high quality evidence, combined with opportunities to undertake structured deliberation of novel reproductive technologies, members of the public are able to engage in detailed discussions. This is the first study to use an established deliberative method to gauge public views towards mitochondrial donation.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by a University of Sydney Industry and Community Collaboration Seed Award (2017), which was awarded contingent on additional funding from the Mito Foundation. Additional funding was provided by the Mito Foundation. The Foundation was not involved in jury facilitation or deliberation, nor analysis of research data.

TRIAL REGISTRATION NUMBER

Not applicable.

Maternal inheritance of mitochondria: implications for male fertility?

Evolutionary theory predicts maternal inheritance of the mitochondria will lead to the accumulation of mutations in the mitochondrial DNA (mtDNA) that impair male fertility, but leave females unaffected. The hypothesis has been referred to as 'Mother's Curse'. There are many examples of mtDNA mutations or haplotypes, in humans and other metazoans, associated with decreases in sperm performance, but seemingly few reports of associations involving female reproductive traits; an observation that has been used to support the Mother's Curse hypothesis. However, it is unclear whether apparent signatures of male bias in mitochondrial genetic effects on fertility reflect an underlying biological bias or a technical bias resulting from a lack of studies to have screened for female effects. Here, we conduct a systematic literature search of studies reporting mitochondrial genetic effects on fertility-related traits in gonochoristic metazoans (animals with two distinct sexes). Studies of female reproductive outcomes were sparse, reflecting a large technical sex bias across the literature. We were only able to make a valid assessment of sex specificity of mitochondrial genetic effects in 30% of cases. However, in most of these cases, the effects were male biased, including examples of male bias associated with mtDNA mutations in humans. These results are therefore consistent with the hypothesis that maternal inheritance has enriched mtDNA sequences with mutations that specifically impair male fertility. However, future research that redresses the technical imbalance in studies conducted per sex will be key to enabling researchers to fully assess the wider implications of the Mother's Curse hypothesis to male reproductive biology.

No effect of mitochondrial genotype on reproductive plasticity following exposure to a non-infectious pathogen challenge in female or male Drosophila

Mitochondrial genetic variation shapes the expression of life-history traits associated with reproduction, development and survival, and has also been associated with the prevalence and progression of infectious bacteria and viruses in humans. The breadth of these effects on multifaceted components of health, and their link to disease susceptibility, led us to test whether variation across mitochondrial haplotypes affected reproductive success following an immune challenge in the form of a non-infectious pathogen. We test this, by challenging male and female fruit flies (Drosophila melanogaster), harbouring each of three distinct mitochondrial haplotypes in an otherwise standardized genetic background, to either a mix of heat-killed bacteria, or a procedural control, prior to measuring their subsequent reproductive performance. The effect of the pathogen challenge on reproductive success did not differ across mitochondrial haplotypes; thus there was no evidence that patterns of reproductive plasticity were modified by the mitochondrial genotype following a non-infectious pathogen exposure. We discuss the implications of our data, and suggest future research avenues based on these results.

Reproductive activity triggers accelerated male mortality and decreases lifespan: genetic and gene expression determinants in Drosophila

Reproduction and aging evolved to be intimately associated. Experimental selection for early-life reproduction drives the evolution of decreased longevity in Drosophila whereas experimental selection for increased longevity leads to changes in reproduction. Although life history theory offers hypotheses to explain these relationships, the genetic architecture and molecular mechanisms underlying reproduction-longevity associations remain a matter of debate. Here we show that mating triggers accelerated mortality in males and identify hundreds of genes that are modulated upon mating in the fruit fly Drosophila melanogaster. Interrogation of genome-wide gene expression in virgin and recently mated males revealed coherent responses, with biological processes that are upregulated (testis-specific gene expression) or downregulated (metabolism and mitochondria-related functions) upon mating. Furthermore, using a panel of genotypes from the Drosophila Synthetic Population Resource (DSPR) as a source of naturally occurring genetic perturbation, we uncover abundant variation in longevity and reproduction-induced mortality among genotypes. Genotypes displayed more than fourfold variation in longevity and reproduction-induced mortality that can be traced to variation in specific segments of the genome. The data reveal individual variation in sensitivity to reproduction and physiological processes that are enhanced and suppressed upon mating. These results raise the prospect that variation in longevity and age-related traits could be traced to processes that coordinate germline and somatic function.

Transgenerational plasticity following a dual pathogen and stress challenge in fruit flies

BACKGROUND

Phenotypic plasticity operates across generations, when the parental environment affects phenotypic expression in the offspring. Recent studies in invertebrates have reported transgenerational plasticity in phenotypic responses of offspring when the mothers had been previously exposed to either live or heat-killed pathogens. Understanding whether this plasticity is adaptive requires a factorial design in which both mothers and their offspring are subjected to either the pathogen challenge or a control, in experimentally matched and mismatched combinations. Most prior studies exploring the capacity for pathogen-mediated transgenerational plasticity have, however, failed to adopt such a design. Furthermore, it is currently poorly understood whether the magnitude or direction of pathogen-mediated transgenerational responses will be sensitive to environmental heterogeneity. Here, we explored the transgenerational consequences of a dual pathogen and stress challenge administered in the maternal generation in the fruit fly, Drosophila melanogaster. Prospective mothers were assigned to a non-infectious pathogen treatment consisting of an injection with heat-killed bacteria or a procedural control, and a stress treatment consisting of sleep deprivation or control. Their daughters and sons were similarly assigned to the same pathogen treatment, prior to measurement of their reproductive success.

RESULTS

We observed transgenerational interactions involving pathogen treatments of mothers and their offspring, on the reproductive success of daughters but not sons. These interactions were unaffected by sleep deprivation.

CONCLUSIONS

The direction of the transgenerational effects was not consistent with that predicted under a scenario of adaptive transgenerational plasticity. Instead, they were indicative of expectations based on terminal investment.

Evolutionary implications of mitochondrial genetic variation: mitochondrial genetic effects on OXPHOS respiration and mitochondrial quantity change with age and sex in fruit flies

The ancient acquisition of the mitochondrion into the ancestor of modern-day eukaryotes is thought to have been pivotal in facilitating the evolution of complex life. Mitochondria retain their own diminutive genome, with mitochondrial genes encoding core subunits involved in oxidative phosphorylation. Traditionally, it was assumed that there was little scope for genetic variation to accumulate and be maintained within the mitochondrial genome. However, in the past decade, mitochondrial genetic variation has been routinely tied to the expression of life-history traits such as fertility, development and longevity. To examine whether these broad-scale effects on life-history trait expression might ultimately find their root in mitochondrially mediated effects on core bioenergetic function, we measured the effects of genetic variation across twelve different mitochondrial haplotypes on respiratory capacity and mitochondrial quantity in the fruit fly, Drosophila melanogaster. We used strains of flies that differed only in their mitochondrial haplotype, and tested each sex separately at two different adult ages. Mitochondrial haplotypes affected both respiratory capacity and mitochondrial quantity. However, these effects were highly context-dependent, with the genetic effects contingent on both the sex and the age of the flies. These sex- and age-specific genetic effects are likely to resonate across the entire organismal life-history, providing insights into how mitochondrial genetic variation may contribute to sex-specific trajectories of life-history evolution.

Transgenerational interactions involving parental age and immune status affect female reproductive success in Drosophila melanogaster

It is well established that the parental phenotype can influence offspring phenotypic expression, independent of the effects of the offspring's own genotype. Nonetheless, the evolutionary implications of such parental effects remain unclear, partly because previous studies have generally overlooked the potential for interactions between parental sources of non-genetic variance to influence patterns of offspring phenotypic expression. We tested for such interactions, subjecting male and female Drosophila melanogaster of two different age classes to an immune activation challenge or a control treatment. Flies were then crossed in all age and immune status combinations, and the reproductive success of their immune- and control-treated daughters measured. We found that daughters produced by two younger parents exhibited reduced reproductive success relative to those of other parental age combinations. Furthermore, immune-challenged daughters exhibited higher reproductive success when produced by immune-challenged relative to control-treated mothers, a pattern consistent with transgenerational immune priming. Finally, a complex interplay between paternal age and parental immune statuses influenced daughter's reproductive success. These findings demonstrate the dynamic nature of age- and immune-mediated parental effects, traceable to both parents, and regulated by interactions between parents and between parents and offspring.

Dose-dependent effects of an immune challenge at both ultimate and proximate levels in Drosophila melanogaster

Immune responses are highly dynamic. The magnitude and efficiency of an immune response to a pathogen can change markedly across individuals, and such changes may be influenced by variance in a range of intrinsic (e.g. age, genotype, sex) and external (e.g. abiotic stress, pathogen identity, strain) factors. Life history theory predicts that up-regulation of the immune system will come at a physiological cost, and studies have confirmed that increased investment in immunity can reduce reproductive output and survival. Furthermore, males and females often have divergent reproductive strategies, and this might drive the evolution of sex-specific life history trade-offs involving immunity, and sexual dimorphism in immune responses per se. Here, we employ an experiment design to elucidate dose-dependent and sex-specific responses to exposure to a nonpathogenic immune elicitor at two scales--the 'ultimate' life history and the underlying 'proximate' immune level in Drosophila melanogaster. We found dose-dependent effects of immune challenges on both male and female components of reproductive success, but not on survival, as well as a response in antimicrobial activity. These results indicate that even in the absence of the direct pathogenic effects that are associated with actual disease, individual life histories respond to a perceived immune challenge--but with the magnitude of this response being contingent on the initial dose of exposure. Furthermore, the results indicate that immune responses at the ultimate life history level may indeed reflect underlying processes that occur at the proximate level.

Effects of cytoplasmic genes on sperm viability and sperm morphology in a seed beetle: implications for sperm competition theory?

Sperm competition theory predicts that sperm traits influencing male fertilizing ability will evolve adaptively. However, it has been suggested that some sperm traits may be at least partly encoded by mitochondrial genes. If true, this may constrain the adaptive evolution of such traits because mitochondrial DNA (mtDNA) is maternally inherited and there is thus no selection on mtDNA in males. Phenotypic variation in such traits may nevertheless be high because mutations in mtDNA that have deleterious effects on male traits, but neutral or beneficial effects in females, may be maintained by random processes or selection in females. We used backcrossing to create introgression lines of seed beetles (Callosobruchus maculatus), carrying orthogonal combinations of distinct lineages of cytoplasmic and nuclear genes, and then assayed sperm viability and sperm length in all lines. We found sizeable cytoplasmic effects on both sperm traits and our analyses also suggested that the cytoplasmic effects varied across nuclear genetic backgrounds. We discuss some potential implications of these findings for sperm competition theory.

Combined influence of maternal and paternal quality on sex allocation in red-capped robins

Sex allocation theory predicts females will adaptively manipulate sex ratios to maximize their progeny's reproductive value. Recently, the generality of biased sex allocation in birds has been questioned by meta-analytic reviews, which demonstrate that many previously reported significant results may simply reflect sampling error. Here, we utilize a robust sample size and powerful statistical approach to determine whether parental quality is correlated with biased sex allocation in red-capped robins. Indices of maternal quality (including interactive effects of age and condition) were strongly related to sex allocation. These relationships were in the predicted directions, with larger effect sizes than those of previous studies in this field. There were also paternal correlates, involving age and the source of paternity. We propose that biased sex allocation occurs in this species, and is maintained by differing production costs of each sex and genetic benefits to females of producing sons when fertilized by high-quality males.