Trans‐generational immune priming is constrained by the maternal immune response in an insect

Immune‐challenged vertebrate and invertebrate females can transfer immunity to their offspring. This trans‐generational immune priming (TGIP) is beneficial for the offspring if the maternal infection risk persists across generations. However, because immunity is costly, fitness consequences of TGIP have been found in primed offspring. Furthermore, transferring immunity to offspring may be costly for immune‐challenged females who are also carrying the costs of their immune response. A negative relationship between levels of immunity between mothers and offspring might therefore be expected. Consistent with this hypothesis, we show that in the insect, Tenebrio molitor, the magnitude of antibacterial immune response of immune‐challenged females negatively correlates with levels of antibacterial activity of their eggs. This negative relationship was only present in small females that are inherently of lower quality. Furthermore, female body size did not affect immune responsiveness to the challenge, indicating that small females favoured their immunity at the expenses of that of their eggs.     

The effect of maternal and paternal immune challenge on offspring immunity and reproduction in a cricket

Trans‐generational immune priming is the transmission of enhanced immunity to offspring following a parental immune challenge. Although within‐generation increased investment into immunity demonstrates clear costs on reproductive investment in a number of taxa, the potential for immune priming to impact on offspring reproductive investment has not been thoroughly investigated. We explored the reproductive costs of immune priming in a field cricket, Teleogryllus oceanicus. To assess the relative importance of maternal and paternal immune status, mothers and fathers were immune‐challenged with live bacteria or a control solution and assigned to one of four treatments in which one parent, neither or both parents were immune‐challenged. Families of offspring were reared to adulthood under a food‐restricted diet, and approximately 10 offspring in each family were assayed for two measures of immunocompetence. We additionally quantified offspring reproductive investment using sperm viability for males and ovary mass for females. We demonstrate that parental immune challenge has significant consequences for the immunocompetence and, in turn, reproductive investment of their male offspring. A complex interaction between maternal and paternal immune status increased the antibacterial immune response of male offspring. This increased immune response was associated with a reduction in son's sperm viability, implicating a trans‐generational resource trade‐off between investment into immunocompetence and reproduction. Our data also show that these costs are sexually dimorphic, as daughters did not demonstrate a similar increase in immunity, despite showing a reduction in ovary mass.     

No maternal effects after stimulation of the melanization response in the yellow fever mosquito Aedes aegypti

The costs and benefits of activating the immune system can reach across generations. Thus, in vertebrates and in several invertebrates, stimulating the immune system of a female can enhance immunity of her offspring or decrease offspring fitness. We evaluated the potential maternally transmitted costs and benefits of the melanization response, an innate immune response of insects that helps to protect mosquitoes from malaria parasites. We manipulated the maternal melanization response of the yellow fever mosquito Aedes aegypti by inoculating female mosquitoes with negatively charged sephadex beads or with immunologically inert glass beads; a control group was not inoculated. In the next generation, we assayed the melanization response and measured three other life-history traits: survival up to emergence, the age at emergence, and body size (estimated as wing length). We found no evidence of fitness costs or benefits for the offspring. A retrospective power analysis found that our experiment would have detected an effect size that is three times smaller than the maternal immune priming effects that have been reported in the literature. We did find a strong correlation between offspring wing length and melanization response. Overall, our findings indicate that trans-generational immune priming in invertebrates cannot be generalized, and that it may depend on the species, the immune challenge, and the environmental conditions.     

Differential expression and costs between maternally and paternally derived immune priming for offspring in an insect

1.?When parasitized, both vertebrates and invertebrates can enhance the immune defence of their offspring, although this transfer of immunity is achieved by different mechanisms. In some insects, immune-challenged males can also initiate trans-generational immune priming (TGIP), but its expressions appear qualitatively different from the one induced by females similarly challenged. 2.?The existence of male TGIP challenges the traditional view of the parental investment theory, which predicts that females should invest more into their progeny than males. However, sexual dimorphism in life-history strategies and the potential costs associated with TGIP may nevertheless lead to dissymmetric investment between males and females into the immune protection of the offspring. 3.?Using the yellow mealworm beetle, Tenebrio molitor, we show that after parental exposure to a bacterial-like infection, maternal and paternal TGIP are associated with the enhancement of different immune effectors and different fitness costs in the offspring. While all the offspring produced by challenged mothers had enhanced immune defence, only those from early reproductive episodes were immune primed by challenged fathers. 4.?Despite the fact that males and females may share a common interest in providing their offspring with an immune protection from the current pathogenic threat, they seem to have evolved different strategies concerning this investment.     

The indirect benefits of mating with attractive males outweigh the direct costs

The fitness consequences of mate choice are a source of ongoing debate in evolutionary biology. Recent theory predicts that indirect benefits of female choice due to offspring inheriting superior genes are likely to be negated when there are direct costs associated with choice, including any costs of mating with attractive males. To estimate the fitness consequences of mating with males of varying attractiveness, we housed female house crickets, Acheta domesticus, with either attractive or unattractive males and measured a variety of direct and indirect fitness components. These fitness components were combined to give relative estimates of the number of grandchildren produced and the intrinsic rate of increase (relative net fitness). We found that females mated to attractive males incur a substantial survival cost. However, these costs are cancelled out and may be outweighed by the benefits of having offspring with elevated fitness. This benefit is due predominantly, but not exclusively, to the effect of an increase in sons' attractiveness. Our results suggest that the direct costs that females experience when mating with attractive males can be outweighed by indirect benefits. They also reveal the value of estimating the net fitness consequences of a mating strategy by including measures of offspring quality in estimates of fitness.     

Direct benefits of choosing a high‐fitness mate can offset the indirect costs associated with intralocus sexual conflict

Intralocus sexual conflict generates a cost to mate choice: high‐fitness partners transmit genetic variation that confers lower fitness to offspring of the opposite sex. Our earlier work in the fruit fly, Drosophila melanogaster, revealed that these indirect genetic costs were sufficient to reverse potential “good genes” benefits of sexual selection. However, mate choice can also confer direct fitness benefits by inducing larger numbers of progeny. Here, we consider whether direct benefits through enhanced fertility could offset the costs associated with intralocus sexual conflict in D. melanogaster. Using hemiclonal analysis, we found that females mated to high‐fitness males produced 11% more offspring compared to those mated to low‐fitness males, and high‐fitness females produced 34% more offspring than low‐fitness females. These direct benefits more than offset the reduction in offspring fitness caused by intralocus sexual conflict, creating a net fitness benefit for each sex to pairing with a high‐fitness partner. Our findings highlight the need to consider both direct and indirect effects when investigating the fitness impacts of mate choice. Direct fitness benefits may shelter sexually antagonistic alleles from selection, suggesting a novel mechanism for the maintenance of fitness variation.     

Offspring survival is negatively related to maternal response to sheep red blood cells in zebra finches

The immune system is an important player in individual trade-offs, but what has rarely been explored is how different strategies of investment in immune response may affect reproductive decisions. We examined the relationship between the strength of maternal immune response and offspring viability and immune response in captive zebra finches Taeniopygia guttata. In three independent experiments, the females and subsequently their adult offspring were challenged with sheep red blood cells, and their responses were measured. There was no relationship between offspring immune response and that of their mothers. However, we found offspring survival until adulthood to be negatively related to maternal antibody titers. That effect was consistent among all experiments and apparent despite the fact that we partially cross-fostered newly hatched nestlings between nests of different females. This suggests that the observed effects of maternal immune response is not mediated by potentially altered female rearing abilities. To our knowledge, this is the first study showing the relationship between the strength of the immune response and between-generational fitness costs in birds.     

Polyandry produces sexy sons at the cost of daughters in red flour beetles

Female mating with multiple males within a single fertile period is a common phenomenon in the animal kingdom. Female insects are particularly promiscuous. It is not clear why females mate with multiple partners despite several potential costs, such as expenditure of time and energy, reduced lifespan, risk of predation and contracting sexually transmitted diseases. Female red flour beetles (Tribolium castaneum) obtain sufficient sperm from a single insemination to retain fertility for several months. Nonetheless they copulate repeatedly within minutes with different males despite no direct fitness benefits from this behaviour. One hypothesis is that females mate with multiple partners to provide indirect benefits via enhanced offspring fitness. To test this hypothesis, we compared the relative fitness of F(1) offspring from females mated with single males and multiple males (2, 4, 8, or 16 partners), under the condition of relatively high intraspecific competition. We found that a female mating with 16 males enhanced the relative fitness of F(1) males (in two out of three trials) but reduced F(1) females' fitness (in two independent trials) in comparison with singly mated females. We also determined whether several important fitness correlates were affected by polyandry. We found that F(1) males from mothers with 16 partners inseminated more females than F(1) males from mothers with a single partner. The viability of the eggs sired or produced by F(1) males and females from highly polyandrous mothers was also increased under conditions of low intra-specific competition. Thus, the effects of polyandry on F(1) offspring fitness depend on environmental conditions. Our results demonstrated a fitness trade-off between male and female offspring from polyandrous mothers in a competitive environment. The mechanisms and biological significance of this unique phenomenon are discussed.     

COSTLY INFIDELITY: LOW LIFETIME FITNESS OF EXTRA‐PAIR OFFSPRING IN A PASSERINE BIRD

Extra‐pair copulation (EPC) is widespread in socially monogamous species, but its evolutionary benefits remain controversial. Indirect genetic benefit hypotheses postulate that females engage in EPC to produce higher quality extra‐pair offspring (EPO) than within‐pair offspring (WPO). In contrast, the sexual conflict hypothesis posits that EPC is beneficial to males but not to females. Thus, under the sexual conflict hypothesis, EPO are predicted to be no fitter than WPO. We tested these two hypotheses in a 12‐year dataset with complete life‐history and pedigree information from an isolated island population of house sparrows (Passer domesticus). We compared fitness components of EPO and two types of WPO: (1) WPO from genetically polyandrous “unfaithful” mothers, and (2) WPO from genetically monogamous mothers. We found that all three groups of offspring had similar probabilities of hatching and nestling survival. Unexpectedly, EPO had the lowest probability of recruiting into the breeding population and the lowest lifetime reproductive output. Our results indicate that EPO incurred indirect genetic costs, rather than benefits, which is contrary to indirect benefit models. Importantly, the indirect costs we observed are also underappreciated in current sexual conflict models. Our results call for improved theoretical frameworks that incorporate indirect costs by extending current sexual conflict models.     

Sexy sons from re-mating do not recoup the direct costs of harmful male interactions in the Drosophila melanogaster laboratory model system

The empirical foundation for sexual conflict theory is the data from many different taxa demonstrating that females are harmed while interacting with males. However, the interpretation of this keystone evidence has been challenged because females may more than counterbalance the direct costs of interacting with males by the indirect benefits of obtaining higher quality genes for their offspring. A quantification of this trade-off is critical to resolve the controversy and is presented here. A multi-generation fitness assay in the Drosophila melanogaster laboratory model system was used to quantify both the direct costs to females due to interactions with males and indirect benefits via sexy sons. We specifically focus on the interactions that occur between males and nonvirgin females. In the laboratory environment of our base population, females mate soon after eclosion and store sufficient sperm for their entire lifetime, yet males persistently court these nonvirgin females and frequently succeed in re-mating them. Females may benefit from these interactions despite direct costs to their lifetime fecundity if re-mating allows them to trade-up to mates of higher genetic quality and thereby secure indirect benefits for their offspring. We found that direct costs of interactions between males and nonvirgin females substantially exceeded indirect benefits through sexy sons. These data, in combination with past studies of the good genes route of indirect benefits, demonstrate that inter-sexual interactions drive sexually antagonistic co-evolution in this model system.     

Attractive males do not sire superior daughters

Much of the recent work on the evolution of female choice has focused on the relative influence of direct and indirect benefits, and particularly whether direct costs can be offset by indirect benefits. Studies investigating whether attractive males benefit females by increasing the viability of their offspring often report mating advantages to sons consistent with the Fisher process, while detecting no or weak viability benefits. One potential reason for this is that sons may trade-off viability benefits with investment in costly traits that enhance mating success, leading to the suggestion that viability benefits may be better detected by examining daughters’ fitness. Here we investigate the relationship between male attractiveness and daughters’ fitness in Drosophila simulans. We measured daughter (and dam) lifetime reproductive success and longevity. We found no evidence that attractive males sire high fitness daughters. Additionally, neither daughters nor dams gained direct benefits from mating with attractive males. However, aspects of daughters’ fitness were related to dam characters.     

Male‐induced costs of mating for females compensated by offspring viability benefits in an insect

Sexual conflict facilitates the evolution of traits that increase the reproductive success of males at the expense of components of female fitness. Theory suggests that indirect benefits are unlikely to offset the direct costs to females from antagonistic male adaptations, but empirical studies examining the net fitness pay‐offs of the interaction between the sexes are scarce. Here, we investigate whether matings with males that invest intrinsically more into accessory gland tissue undermine female lifetime reproductive success (LRS) in the cricket Teleogryllus oceanicus. We found that females incur a longevity cost of mating that is proportional to the partner’s absolute investment into the production of accessory gland products. However, male accessory gland weight positively influences embryo survival, and harmful ejaculate‐induced effects are cancelled out when these are put in the context of female LRS. The direct costs of mating with males that sire offspring with higher viability are thus compensated by direct and possibly indirect genetic benefits in this species.     

Maternal pathogen exposure causes diet‐ and pathogen‐specific transgenerational costs

Transgenerational effects, whereby the environment experienced by a parent leads to an altered offspring phenotype, have now been described in a variety of taxa. In invertebrates, much of the research on these effects has concentrated on the role of parental exposure to pathogens or immune elicitors in determining offspring immune investment or disease resistance. To date, however, studies of transgenerational effects in invertebrates have generally been restricted to single infections or immune elicitors in ideal laboratory environments. Animals in field situations will commonly experience sub‐optimal environments and co‐infection by multiple species of parasites and pathogens, leading to increased relative costs of immune investment and changing fitness benefits from offspring responses to the parental environment. Here we investigate a more ecologically realistic scenario involving both multiple infections and resource limitation, using the Indian meal moth Plodia interpunctella as a model host, challenged with the entomopathogenic bacterium Bacillus thuringiensis and fungus Beauveria bassiana. Mothers were exposed to low doses of one or both pathogens, or a control. Offspring from each family were reared on either good‐ or poor‐quality food and then exposed to one or both pathogens. Maternal exposure to pathogens led to reduced pathogen resistance in offspring, depending on the combination of maternal and offspring pathogen‐specific infections and resource limitation in the offspring generation. Much research to date has focussed on trans‐generational immune priming, in which parental exposure to pathogens or immune elicitors leads to upregulated immune reactivity in their offspring. The lack of any such effects in our system suggests that the production of less resistant offspring following parental exposure to pathogens might be an important alternative, driven by costs of resistance rather than adaptive benefits.     

DOES GENETIC DIVERSITY REDUCE SIBLING COMPETITION?

An enduring hypothesis for the proximal benefits of sex is that recombination increases the genetic variation among offspring and that this genetic variation increases offspring performance. A corollary of this hypothesis is that mothers that mate multiply increase genetic variation within a clutch and gain benefits due to genetic diversity alone. Many studies have demonstrated that multiple mating can increase offspring performance, but most attribute this increase to sexual selection and the role of genetic diversity has received less attention. Here, we used a breeding design to generate populations of full-siblings, half-siblings, and unrelated individuals of the solitary ascidian Ciona intestinalis. Importantly, we preclude the potentially confounding influences of maternal effects and sexual selection. We found that individuals in populations with greater genetic diversity had greater performance (metamorphic success, postmetamorphic survival, and postmetamorphic size) than individuals in populations with lower genetic diversity. Furthermore, we show that by mating with multiple males and thereby increasing genetic variation within a single clutch of offspring, females gain indirect fitness benefits in the absence of mate-choice. Our results show that when siblings are likely to interact, genetic variation among individuals can decrease competition for resources and generate substantial fitness benefits within a single generation.     

Relationship between maternal transfer of immunity and mother fecundity in an insect

Trans-generational immune priming (TGIP) corresponds to the plastic adjustment of offspring immunity as a result of maternal immune experience. TGIP is expected to improve mother's fitness by improving offspring individual performance in an environment where parasitism becomes more prevalent. However, it was recently demonstrated that maternal transfer of immunity to the offspring is costly for immune-challenged female insects. Thus, these females might not provide immune protection to all their offspring because of the inherent cost of other fitness-related traits. Females are therefore expected to adjust their investment to individual offspring immune protection in ways that maximize their fitness. In this study, we investigated how bacterially immune-challenged females of the mealworm beetle, Tenebrio molitor, provision their eggs with immune protection according to egg production. We found that immune-challenged females provide a variable number of their eggs with internal antibacterial activity along egg-laying bouts. Furthermore, within the first immune-protected egg-laying bout (2-4 days after the maternal immune challenge), the number of eggs protected was strongly dependent on the number of eggs produced. Immune-challenged females might therefore adjust their investment into TGIP and fecundity according of their individual perception of the risk of dying from the infection and the expected parasitic conditions for the offspring.     

Social and genetic benefits of parental investment suggest sex differences in selection pressures

Provisioning behavior in altricial birds is often used to measure parental investment and is assumed to have fitness consequences to the parents providing it, with the benefits outweighing the costs. Here we investigate the fitness costs and benefits (parent survival and offspring recruitment) of provisioning behavior in wild house sparrows Passer domesticus, using long‐term data from a pedigreed isolated population. We disentangled the long‐term fitness consequences in terms of number of recruits, of provisioning behavior from those of other parental investments and individual quality through a cross‐foster design. We accounted for extra‐pair offspring in all analyses. Provisioning behavior confers social fitness benefits in terms of the number of recruits to both parents. Only in females we detected an influence individual quality: female sparrows with high provisioning frequencies were associated with more genetic recruits than those who provided food less frequently to their young, even though foster parents reared the offspring. We detected a relationship between annual survival probability and provisioning behavior only in males, but not in females. This finding, together with indirect benefits differing by sex, indicates that different selection pressures are acting on the sexes. Our study can show that it is justified to use provisioning behavior as a form of parental investment sensu Trivers, since we show that this behavior is costly to parents and that the genetic fitness benefits exceed the costs.     

Different effects of paternal trans-generational immune priming on survival and immunity in step and genetic offspring

Paternal trans-generational immune priming, whereby fathers provide immune protection to offspring, has been demonstrated in the red flour beetle Tribolium castaneum exposed to the insect pathogen Bacillus thuringiensis. It is currently unclear how such protection is transferred, as in contrast to mothers, fathers do not directly provide offspring with a large amount of substances. In addition to sperm, male flour beetles transfer seminal fluids in a spermatophore to females during copulation. Depending on whether paternal trans-generational immune priming is mediated by sperm or seminal fluids, it is expected to either affect only the genetic offspring of a male, or also their step offspring that are sired by another male. We therefore conducted a double-mating experiment and found that only the genetic offspring of an immune primed male show enhanced survival upon bacterial challenge, while phenoloxidase activity, an important insect immune trait, and the expression of the immune receptor PGRP were increased in all offspring. This indicates that information leading to enhanced survival upon pathogen exposure is transferred via sperm, and thus potentially constitutes an epigenetic effect, whereas substances transferred with the seminal fluid could have an additional influence on offspring immune traits and immunological alertness.     

Assessing putative interlocus sexual conflict in Drosophila melanogaster using experimental evolution

The theoretical foundation of sexually antagonistic coevolution is that females suffer a net fitness cost through their interactions with males. The empirical prediction is that direct costs to female lifetime fecundity will exceed indirect benefits despite a possible increase in the genetic quality of offspring. Although direct costs of males have been repeatedly shown, to date no study has comprehensively tested whether females are compensated for this direct harm through indirect benefits. Here we use experimental evolution to show that a mutation giving Drosophila melanogaster females nearly complete resistance to the direct costs of male courtship and remating, but which also excluded almost all indirect benefits, is strongly favoured by selection. We estimated the selection coefficient favouring the resistance allele to be +20%. These results demonstrate that any indirect benefits that females accrued were not sufficient to counter-balance the direct costs of males, and reinforce a large body of past studies by verifying interlocus sexual conflict in this model system.     

Transgenerational priming of immunity: maternal exposure to a bacterial antigen enhances offspring humoral immunity

Young vertebrates have limited capacity to synthesize antibodies and are dependent on the protection of maternally transmitted antibodies for humoral disease resistance early in life. However, mothers may enhance fitness by priming their offspring's immune systems to elevate disease resistance. Transgenerational induced defences have been documented in plants and invertebrates, but maternal priming of offspring immunity in vertebrates has been essentially neglected. To test the ability of mothers to stimulate the immune systems of offspring, we manipulated maternal and offspring antigen exposure in a wild population of birds, pied flycatchers (Ficedula hypoleuca). We show that immunization of the mother before egg laying apparently stimulates a transgenerational defence against pathogens by elevating endogenous offspring antibody production. If the disease environments encountered by mothers and offspring are similar, this transgenerational immune priming may allow young to better cope with the local pathogen fauna.