Transcriptomes of parents identify parenting strategies and sexual conflict in a subsocial beetle

Revisiting the ecology and evolution of burying beetle behavior (Staphylinidae: Silphinae)

Investigating fundamental processes in biology requires the ability to ground broad questions in species-specific natural history. This is particularly true in the study of behavior because an organism's experience of the environment will influence the expression of behavior and the opportunity for selection. Here, we provide a review of the natural history and behavior of burying beetles of the genus Nicrophorus to provide the groundwork for comparative work that showcases their remarkable behavioral and ecological diversity. Burying beetles have long fascinated scientists because of their well-developed parenting behavior, exhibiting extended post-hatching care of offspring that varies extensively within and across taxa. Despite the burgeoning success of burying beetles as a model system for the study of behavioral evolution, there has not been a review of their behavior, ecology, and evolution in over 25 years. To address this gap, we leverage a developing community of researchers who have contributed to a detailed knowledge of burying beetles to highlight the utility of Nicrophorus for investigating the causes and consequences of social and behavioral evolution.

Gene regulation supporting sociality shared across lineages and variation in complexity

Across evolutionary lineages, insects vary in social complexity, from those that exhibit extended parental care to those with elaborate divisions of labor. Here, we synthesize the sociogenomic resources from hundreds of species to describe common gene regulatory mechanisms in insects that regulate social organization across phylogeny and levels of social complexity. Different social phenotypes expressed by insects can be linked to the organization of co-expressing gene networks and features of the epigenetic landscape. Insect sociality also stems from processes like the emergence of parental care and the decoupling of ancestral genetic programs. One underexplored avenue is how variation in a group's social environment affects the gene expression of individuals. Additionally, an experimental reduction of gene expression would demonstrate how the activity of specific genes contributes to insect social phenotypes. While tissue specificity provides greater localization of the gene expression underlying social complexity, emerging transcriptomic analysis of insect brains at the cellular level provides even greater resolution to understand the molecular basis of social insect evolution.

Sex-specific behavioral and physiological changes during single parenting in a biparental species, Columba livia

Many species exhibit biparental care to maximize fitness. When a partner is lost, the surviving partner may alter their behavior to compensate offspring. Whether both sexes use the same physiological mechanisms to manifest their change in behavior remains elusive. We investigated behaviors and mechanisms associated with the alteration of parental care post-partner removal in a biparental avian species, the rock dove (Columba livia). We hypothesized that rock dove single parents experience sex-biased changes in neural genomic transcription and reproductive behaviors, and these changes are related to chick development. We manipulated parental partner presence and measured parental attendance, offspring growth, gene expression of glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in the pituitary, and GR, MR, and estrogen receptor beta (ER-β) in the hypothalamus. We also measured circulating plasma concentrations of the stress-associated hormone corticosterone and the parental care-associated hormone prolactin. We also quantified prolactin gene (PRL) expression changes in the pituitary, as well as prolactin receptor (PRLR) expression in the hypothalamus and pituitary. We found that single mothers and fathers maintained similar provisioning levels as paired parents, but spent less cumulative time brooding chicks. Chicks of single parents were smaller than paired-parented chicks after three days post-hatch. Mothers in both treatment groups experienced higher expression of hypothalamic GR as compared to fathers. Single parents experienced lower PRL gene expression in the pituitary as compared to paired parents. No significant differences were found for the circulating hormones or other genes listed.

takeout gene expression is associated with temporal kin recognition

A key component of parental care is avoiding killing and eating one's own offspring. Many organisms commit infanticide but switch to parental care when their own offspring are expected, known as temporal kin recognition. It is unclear why such types of indirect kin recognition are so common across taxa. One possibility is that temporal kin recognition may evolve through alteration of simple mechanisms, such as co-opting mechanisms that influence the regulation of timing and feeding in other contexts. Here, we determine whether takeout, a gene implicated in coordinating feeding, influences temporal kin recognition in Nicrophorus orbicollis. We found that takeout expression was not associated with non-parental feeding changes resulting from hunger, or a general transition to the full parental care repertoire. However, beetles that accepted and provided care to their offspring had a higher takeout expression than beetles that committed infanticide. Together, these data support the idea that the evolution of temporal kin recognition may be enabled by co-option of mechanisms that integrate feeding behaviour in other contexts.

Brood size, food availability, and body size affects male care decisions and offspring performance

Parental care strategies do not only vary greatly across species, but also within species there can be substantial between- and within-individual variation in parental care behavior. To better understand the evolution of care strategies, it is crucial to determine how and when parents modify their behavior in response to internal as well as environmental factors. Here, we investigated the effect of brood size, resource size and an individual's quality on care strategies of uniparental males and examined the downstream consequences on offspring performance in the burying beetle Nicrophorus vespilloides. Burying beetles breed on small vertebrate cadavers and, on average, males invest much less in care than females. Nevertheless, we found that uniparentally caring males were responsive to their social and non-social environment and adjusted the amount as well as the type of care to the size of the brood, the size of the cadaver and their own body size. Additionally, we show that the care strategies affected offspring performance. Specifically, males that cared longer had larger and more surviving larvae. Our results add to our understanding of plastic parenting strategies by showing that even the sex that provides less care can evolve a very flexible care behavior.

Gene expression underlying parenting and being parented shows limited plasticity in response to different ambient temperatures

Flexible interactions between parents and offspring are essential for buffering families against variable, unpredictable, and challenging environmental conditions. In the subsocial carrion beetle, Nicrophorus orbicollis, mid-summer temperatures impose steep fitness costs on parents and offspring but do not elicit behavioural plasticity in parents. Here, we ask if plasticity of gene expression underpins this behavioural stability or facilitates independent compensation by larvae. To test this, we characterized gene expression of parents and offspring before and during active parenting under benign (20°C) and stressful (24°C) temperatures to identify genes of parents and offspring associated with thermal response, parenting/being parented, and gene expression plasticity associated with behavioural stability of parental care. The main effects of thermal and social condition each shaped patterns of gene expression in females, males, and larvae. In addition, we implicated 79 genes in females as "buffering" parental behaviour across environments. The majority of these underwent significant changes in expression in actively parenting mothers at the benign temperature, but not at the stressful temperature. Our results suggest that neither genetic programmes for parenting nor their effects on offspring gene expression are fundamentally different under stressful conditions, and that behavioural stability is associated primarily with the maintenance of existing genetic programmes rather than replacement or supplementation. Thus, while selection for compensatory gene expression could expand the range of thermal conditions parents will tolerate, without expanding the toolkit of genes involved selection is unlikely to lead to adaptive changes of function.

Parent-offspring conflict and its outcome under uni-and biparental care

Conflicts over parental investment are predicted to be common among family members, especially between parents and their offspring. Parent-offspring conflict has been studied in many brood-caring organisms, but whether its outcome is closer to the parental or offspring optimum is usually unknown, as is whether the presence of a second parent, a caring male partner, can affect the outcome. Here, we manipulated the initial brood size of single and paired female burying beetles to examine how many offspring are necessary to maintain parental care in the current brood. We found that mothers continued to invest in small broods even if their reproductive output would have been higher if they had discontinued their care and produced a second brood instead. Consequently, our data suggests that the offspring have the upper hand in the conflict. However, our results further show that paired females laid a second egg clutch more often and produced more offspring than single females, suggesting that the presence of a male partner shifts the conflict outcome towards the parental optimum. This latter result not only is a novel aspect of parent-offspring theory, but also represents an additional factor that might explain the evolution of biparental care.

Sex-specific influence of communal breeding experience on parenting performance and fitness in a burying beetle

Communal breeding, wherein multiple conspecifics live and reproduce together, may generate short-term benefits in terms of defence and reproduction. However, its carry-over effects remain unclear. We experimentally tested the effects of communal breeding on parental care and reproduction in burying beetles (Nicrophorus vespilloides), which use carcasses as breeding resources and provide parental care to offspring. We subjected individuals to communal or non-communal breeding (i.e. pair breeding) during their first breeding event and to non-communal breeding during their second breeding event. We measured the parental care of individuals and of groups and the reproductive success of groups during both breeding events. In communal groups, large individuals became dominant and largely monopolized the carcass, whereas small individuals (i.e. subordinates) had restricted access to the carcass. At the first breeding event, large males in communal groups spent more time providing care than large males in non-communal groups, whereas such an effect was not observed for large females and small individuals. Reproductive successes were similar in communal and non-communal groups, indicating no short-term benefits of communal breeding in terms of reproduction. Compared with males from non-communal groups, males originating from communal groups produced a larger size of brood during their second breeding event, whereas such an effect was not observed for females. Our results demonstrate the sex-specific effects of communal breeding experience on parenting performance and fitness.

Phenotypic Plasticity: What Has DNA Methylation Got to Do with It?

How does one genome give rise to multiple, often markedly different, phenotypes in response to an environmental cue? This phenomenon, known as phenotypic plasticity, is common amongst plants and animals, but arguably the most striking examples are seen in insects. Well-known insect examples include seasonal morphs of butterfly wing patterns, sexual and asexual reproduction in aphids, and queen and worker castes of eusocial insects. Ultimately, we need to understand how phenotypic plasticity works at a mechanistic level; how do environmental signals alter gene expression, and how are changes in gene expression translated into novel morphology, physiology and behaviour? Understanding how plasticity works is of major interest in evolutionary-developmental biology and may have implications for understanding how insects respond to global change. It has been proposed that epigenetic mechanisms, specifically DNA methylation, are the key link between environmental cues and changes in gene expression. Here, we review the available evidence on the function of DNA methylation of insects, the possible role(s) for DNA methylation in phenotypic plasticity and also highlight key outstanding questions in this field as well as new experimental approaches to address these questions.

Evolutionary change in the construction of the nursery environment when parents are prevented from caring for their young directly

Parental care can be partitioned into traits that involve direct engagement with offspring and traits that are expressed as an extended phenotype and influence the developmental environment, such as constructing a nursery. Here, we use experimental evolution to test whether parents can evolve modifications in nursery construction when they are experimentally prevented from supplying care directly to offspring. We exposed replicate experimental populations of burying beetles (Nicrophorus vespilloides) to different regimes of posthatching care by allowing larvae to develop in the presence (Full Care) or absence of parents (No Care). After only 13 generations of experimental evolution, we found an adaptive evolutionary increase in the pace at which parents in the No Care populations converted a dead body into a carrion nest for larvae. Cross-fostering experiments further revealed that No Care larvae performed better on a carrion nest prepared by No Care parents than did Full Care larvae. We conclude that parents construct the nursery environment in relation to their effectiveness at supplying care directly, after offspring are born. When direct care is prevented entirely, they evolve to make compensatory adjustments to the nursery in which their young will develop. The rapid evolutionary change observed in our experiments suggests there is considerable standing genetic variation for parental care traits in natural burying beetle populations-for reasons that remain unclear.

Parental Care System and Brood Size Drive Sex Difference in Reproductive Allocation: An Experimental Study on Burying Beetles

Life-history theory predicts that increased resource allocation in current reproduction comes at the cost of survival and future reproductive fitness. In taxa with biparental care, each parent can adjust investment on current reproduction according to changes in their partner’s effort, but these adjustments may be different for males and females as they may have different reproductive strategies. Numerous theoretical and empirical studies have proposed the mechanism underlying such adjustments. In addition, the value of the brood or litter (brood size) has also been suggested to affect the amount of care through manipulation of brood size. While the two conditions have been studied independently, the impact of their interplay on potential sex-dependent future reproductive performance remains largely unknown. In this study, we simultaneously manipulated both care system (removal of either parent vs. no removal) and brood size in a burying beetle (Nicrophorus vespilloides) to understand their joint effect on reproductive allocation and trade-off between current and future reproduction. Our results show that males compensated for mate loss by significantly increasing the level of care regardless of brood size, while females exhibited such compensation only for small brood size. Additionally, with an increase in allocation to current reproduction, males showed decreased parental investment during the subsequent breeding event as a pair. These findings imply a dual influence of parental care system and brood size on allocation in current reproduction. Moreover, the impact of such adjustments on sex-dependent differences in future reproduction (parental care, larvae number, and average larval mass at dispersal) is also demonstrated. Our findings enhance the understanding of sex roles in parental investment and highlight their importance as drivers of reproductive allocation.

Survey of neurotransmitter receptor gene expression into and out of parental care in the burying beetle Nicrophorus vespilloides

Understanding the genetic influences of traits of nonmodel organisms is crucial to understanding how novel traits arise. Do new traits require new genes or are old genes repurposed? How predictable is this process? Here, we examine this question for gene expression influencing parenting behavior in a beetle, Nicrophorus vespilloides. Parental care, produced from many individual behaviors, should be influenced by changes of expression of multiple genes, and one suggestion is that the genes can be predicted based on knowledge of behavior expected to be precursors to parental care, such as aggression, resource defense, and mating on a resource. Thus, testing gene expression during parental care allows us to test expectations of this "precursor hypothesis" for multiple genes and traits. We tested for changes of the expression of serotonin, octopamine/tyramine, and dopamine receptors, as well as one glutamate receptor, predicting that these gene families would be differentially expressed during social interactions with offspring and associated resource defense. We found that serotonin receptors were strongly associated with social and aggression behavioral transitions. Octopamine receptors produced a complex picture of gene expression over a reproductive cycle. Dopamine was not associated with the behavioral transitions sampled here, while the glutamate receptor was most consistent with a behavioral change of resource defense/aggression. Our results generate new hypotheses, refine candidate lists for further studies, and inform the genetic mechanisms that are co-opted during the evolution of parent-offspring interactions, a likely evolutionary path for many lineages that become fully social. The precursor hypothesis, while not perfect, does provide a starting point for identifying candidate genes.

Effect of food source availability in the salivary gland transcriptome of the unique burying beetle Nicrophorus pustulatus (Coleoptera: Silphidae)

Nicrophorus is a genus of beetles that bury and transform small vertebrate carcasses into a brood ball coated with their oral and anal secretions to prevent decay and that will serve as a food source for their young. Nicrophorus pustulatus is an unusual species with the ability to overtake brood of other burying beetles and whose secretions, unlike other Nicrophorus species, has been reported not to exhibit antimicrobial properties. This work aims to better understand how the presence or absence of a food source influences the expression of genes involved in the feeding process of N. pustulatus. To achieve that, total RNA was extracted from pooled samples of salivary gland tissue from N. pustulatus and sequenced using an Illumina platform. The resulting reads were used to assemble a de novo transcriptome using Trinity. Duplicates with more than 95% similarity were removed to obtain a "unigene" set. Annotation of the unigene set was done using the Trinotate pipeline. Transcript abundance was determined using Kallisto and differential gene expression analysis was performed using edgeR. A total of 651 genes were found to be differentially expressed, including 390 upregulated and 261 downregulated genes in fed insects compared to starved. Several genes upregulated in fed beetles are associated with the insect immune response and detoxification processes with only one transcript encoding for the antimicrobial peptide (AMP) defensin. These results confirm that N. pustulatus does not upregulate the production of genes encoding AMPs during feeding. This study provides a snapshot of the changes in gene expression in the salivary glands of N. pustulatus following feeding while providing a well described transcriptome for the further analysis of this unique burying beetle.

Back to the basics? Transcriptomics offers integrative insights into the role of space, time and the environment for gene expression and behaviour

Fuelled by the ongoing genomic revolution, broadscale RNA expression surveys are fast replacing studies targeting one or a few genes to understand the molecular basis of behaviour. Yet, the timescale of RNA-sequencing experiments and the dynamics of neural gene activation are insufficient to drive real-time switches between behavioural states. Moreover, the spatial, functional and transcriptional complexity of the brain (the most commonly targeted tissue in studies of behaviour) further complicates inference. We argue that a Central Dogma-like 'back-to-basics' assumption that gene expression changes cause behaviour leaves some of the most important aspects of gene-behaviour relationships unexplored, including the roles of environmental influences, timing and feedback from behaviour-and the environmental shifts it causes-to neural gene expression. No perfect experimental solutions exist but we advocate that explicit consideration, exploration and discussion of these factors will pave the way toward a richer understanding of the complicated relationships between genes, environments, brain gene expression and behaviour over developmental and evolutionary timescales.

Carry on caring: infected females maintain their parental care despite high mortality

Parental care is a key component of an organism's reproductive strategy that is thought to trade-off with allocation toward immunity. Yet, it is unclear how caring parents respond to pathogens: do infected parents reduce care as a sickness behavior or simply from being ill or do they prioritize their offspring by maintaining high levels of care? To address this issue, we investigated the consequences of infection by the pathogen Serratia marcescens on mortality, time spent providing care, reproductive output, and expression of immune genes of female parents in the burying beetle Nicrophorus vespilloides. We compared untreated control females with infected females that were inoculated with live bacteria, immune-challenged females that were inoculated with heat-killed bacteria, and injured females that were injected with buffer. We found that infected and immune-challenged females changed their immune gene expression and that infected females suffered increased mortality. Nevertheless, infected and immune-challenged females maintained their normal level of care and reproductive output. There was thus no evidence that infection led to either a decrease or an increase in parental care or reproductive output. Our results show that parental care, which is generally highly flexible, can remain remarkably robust and consistent despite the elevated mortality caused by infection by pathogens. Overall, these findings suggest that infected females maintain a high level of parental care, a strategy that may ensure that offspring receive the necessary amount of care but that might be detrimental to the parents' own survival or that may even facilitate disease transmission to offspring.

Constrained flexibility of parental cooperation limits adaptive responses to harsh conditions

Parental care is predicted to evolve to mitigate harsh environments, thus adaptive plasticity of care may be an important response to our climate crisis. In biparental species, fitness costs may be reduced by resolving conflict and enhancing cooperation among partners. We investigated this prediction with the burying beetle, Nicrophorus orbicollis, by exposing them to contrasting benign and harsh thermal environments. Despite measurable fitness costs under the harsh environment, sexual conflict persisted in the form of sex-specific social plasticity. That is, females provided equivalent care with or without males, whereas males with partners deserted earlier and reduced provisioning effort. The interaction of social condition and thermal environment did not explain variation in individual behavior, failing to support a temperature-mediated shift from conflict to cooperation. Examining selection gradients and splines on cumulative care revealed a likely explanation for these patterns. Contrary to predictions, increased care did not enhance offspring performance under stress. Rather, different components of care were under different selection regimes, with optimization constrained due to lack of coordination between parents. We suggest that the potential for parenting to ameliorate the effects of our climate crisis may depend on the sex-specific evolutionary drivers of parental care, and that this may be best reflected in components of care.

Adaptation to monogamy influences parental care but not mating behavior in the burying beetle, Nicrophorus vespilloides

The mating system is expected to have an important influence on the evolution of mating and parenting behaviors. Although many studies have used experimental evolution to examine how mating behaviors evolve under different mating systems, this approach has seldom been used to study the evolution of parental care. We used experimental evolution to test whether adaptation to different mating systems involves changes in mating and parenting behaviors in populations of the burying beetle, Nicrophorus vespilloides. We maintained populations under monogamy or promiscuity for six generations. This manipulation had an immediate impact on reproductive performance and adult survival. Compared to monogamy, promiscuity reduced brood size and adult (particularly male) survival during breeding. After six generations of experimental evolution, there was no divergence between monogamous and promiscuous populations in mating behaviors. Parents from the promiscuous populations (especially males) displayed less care than parents from the monogamous populations. Our results are consistent with the hypothesis that male care will increase with the certainty of paternity. However, it appears that this change is not associated with a concurrent change in mating behaviors.

Functional genomics of parental care of insects

Parental care was likely the first step most lineages made towards sociality. However, the molecular mechanisms that generate parental care are not broadly characterized. Insects are important as an evolutionary independent group from classic models of parental care, such as, house mice. They provide an opportunity to test the generality of our understanding. With this review, I survey the functional genomics of parental care of insects, summarize several recent advances in the broader framework for studying and understanding parental care, and finish with suggested priorities for further research. Although there are too few studies to draw definitive conclusions, I argue that natural selection appears to be rewiring existing gene networks to produce parental care, that the epigenetic mechanisms influencing parental care are not well understood, and, as an interesting early consensus, that genes strongly associated with carer/offspring interactions appear biased towards proteins that are secreted. I summarize the studies that have functionally validate candidate genes and highlight the increasing need to perform this work. I finish with arguments for both conceptual and practical changes moving forward. I argue that future work can increase the use of predictive frameworks, broaden its definition of conservation of mechanism to gene networks rather than single genes, and increase the use of more established comparative methods. I further highlight the practical considerations of standardizing analyses and reporting, increasing the sampling of both carers and offspring, better characterizing gene regulatory networks, better characterizing taxonomically restricted genes and any consistent role they have underpinning parental care, and using factorial designs to disentangle the influence of multiple variables on the expression of parental care.

Parental Care, Destabilizing Selection, and the Evolution of Tetrapod Endothermy

Parental care has evolved convergently an extraordinary number of times among tetrapods that reproduce terrestrially, suggesting strong positive selection for this behavior in the terrestrial environment. This review speculates that destabilizing selection on parental care, and especially embryo incubation, drove the convergent evolution of many tetrapod traits, including endothermy.

Offspring reverse transcriptome responses to maternal deprivation when reared with pathogens in an insect with facultative family life

Offspring of species with facultative family life are able to live with and without parents (i.e. to adjust to extreme changes in their social environment). While these adjustments are well understood on a phenotypic level, their genetic underpinnings remain surprisingly understudied. Investigating gene expression changes in response to parental absence may elucidate the genetic constraints driving evolutionary transitions between solitary and family life. Here, we manipulated maternal presence to observe gene expression changes in the fat body of juvenile European earwigs, an insect with facultative family life. Because parents typically protect offspring against pathogens, expression changes were recorded in pathogen-free and pathogen-exposed environments. We found that manipulating maternal presence changed the expression of 154 genes, including several metabolism and growth-related genes, and that this change depended on pathogen presence. Specifically, localization and cell transporter genes were downregulated in maternal absence without pathogens but upregulated with pathogens. At least one immunity gene (pathogenesis-related protein 5) was affected by pathogen exposure regardless of maternal presence. Overall, our findings explicate how offspring adjust to parental deprivation on a molecular level and reveal that such adjustments heavily depend on pathogens in the environment. This emphasizes the central role of pathogens in family life evolution.

Contribution of males to brood care can compensate for their food consumption from a shared resource

The sharing of the same food source among parents and offspring can be a driver of the evolution of family life and parental care. However, if all family members desire the same meal, competitive situations can arise, especially if resource depletion is likely. When food is shared for reproduction and the raising of offspring, parents have to decide whether they should invest in self-maintenance or in their offspring and it is not entirely clear how these two strategies are balanced. In the burying beetle Nicrophorus vespilloides, parents care for their offspring either bi- or uniparentally at a vertebrate carcass as the sole food source. The question of whether biparental care in this species offers the offspring a better environment for development compared with uniparental care has been the subject of some debate. We tested the hypothesis that male contribution to biparental brood care has a beneficial effect on offspring fitness but that this effect can be masked because the male also feeds from the shared resource. We show that a mouse carcass prepared by two Nicrophorus beetles is lighter compared with a carcass prepared by a single female beetle at the start of larval hatching and provisioning. This difference in carcass mass can influence offspring fitness when food availability is limited, supporting our hypothesis. Our results provide new insights into the possible evolutionary pathway of biparental care in this species of burying beetles.

From the host's point of view: Effects of variation in burying beetle brood care and brood size on the interaction with parasitic mites

The fitness and virulence of parasites is often determined by how many resources they can wrangle out of their hosts. Host defenses that help to keep resources from the parasites will then reduce virulence and parasite fitness. Here, we study whether host brood care and brood size regulation can protect host fitness and harm a parasite. We use the biparental brood-caring burying beetle Nicrophorus vespilloides and its phoretic Poecilochirus carabi mites as a model. Since paternal brood care does not seem to benefit the offspring in a clean laboratory setting, the male presence has been suggested to strengthen the defense against parasites. We manipulated male presence and found no effect on the fitness of the parasitic mites or the beetle offspring. We further manipulated beetle brood size and found larger broods to reduce parasite fitness. The specific pattern we observed suggests that beetle larvae are strong competitors and consume the carrion resource before all parasites develop. They thus starve the parasites. These results shed new light on the observation that the parasites appear to reduce host brood size early on–potentially to avert later competition their offspring might have to face.

The genetic mechanism of selfishness and altruism in parent-offspring coadaptation

The social bond between parents and offspring is characterized by coadaptation and balance between altruistic and selfish tendencies. However, its underlying genetic mechanism remains poorly understood. Using transcriptomic screens in the subsocial European earwig, Forficula auricularia, we found the expression of more than 1600 genes associated with experimentally manipulated parenting. We identified two genes, Th and PebIII, each showing evidence of differential coexpression between treatments in mothers and their offspring. In vivo RNAi experiments confirmed direct and indirect genetic effects of Th and PebIII on behavior and fitness, including maternal food provisioning and reproduction, and offspring development and survival. The direction of the effects consistently indicated a reciprocally altruistic function for Th and a reciprocally selfish function for PebIII. Further metabolic pathway analyses suggested roles for Th-restricted endogenous dopaminergic reward, PebIII-mediated chemical communication and a link to insulin signaling, juvenile hormone, and vitellogenin in parent-offspring coadaptation and social evolution.

Neurogenomic insights into paternal care and its relation to territorial aggression

Motherhood is characterized by dramatic changes in brain and behavior, but less is known about fatherhood. Here we report that male sticklebacks—a small fish in which fathers provide care—experience dramatic changes in neurogenomic state as they become fathers. Some genes are unique to different stages of paternal care, some genes are shared across stages, and some genes are added to the previously acquired neurogenomic state. Comparative genomic analysis suggests that some of these neurogenomic dynamics resemble changes associated with pregnancy and reproduction in mammalian mothers. Moreover, gene regulatory analysis identifies transcription factors that are regulated in opposite directions in response to a territorial challenge versus during paternal care. Altogether these results show that some of the molecular mechanisms of parental care might be deeply conserved and might not be sex-specific, and suggest that tradeoffs between opposing social behaviors are managed at the gene regulatory level.

Compared to motherhood, the molecular changes associated with fatherhood are less understood. Here, the authors investigate gene expression changes associated with paternal care in male stickleback fish, and compare them with patterns in territorial aggression.

From phenotype to genotype: the precursor hypothesis predicts genetic influences that facilitate transitions in social behavior

Parental care is expected to be one of the key evolutionary precursors to advanced social behavior. This suggests that there could be common genetic underpinnings to both parental care and sociality. However, little is known of the genetics underlying care. Here, we suggest that ethological predictions of behavioral precursors to care along with a genetic toolkit for behavior provide testable hypotheses and a defined approach to investigating genetics of sociality. We call this the 'precursor hypothesis'.

Enduring rules of care within pairs - how blue tit parents resume provisioning behaviour after experimental disturbance

Sexual conflict over parental investment can result in suboptimal reproductive output. A recent hypothesis suggests that equality in investment, and hence conflict resolution, may be reached via coordination of parental activities like alternating nest visits. However, how robust patterns of care within couples are against temporal disturbances that create asymmetries in parental investment remains as yet to be shown. We here experimentally created such a social disturbance in a wild population of biparental blue tits (Cyanistes caeruleus) when provisioning their nestlings. We randomly caught and subsequently released one of the parents when nestlings were 6 and 12 days old respectively. On average, the parent that was caught did not resume care for nearly two hours. We then compared the levels of individual investment and within-pair coordination before, during and after the absence of the disturbed parent. We show that the remaining parent partially compensated by increasing its provisioning rate, but this compensatory response was strongest in females when nestlings were 6 days old. Once the caught parent returned to feed its nestlings, both parents resumed provisioning at a similar rate as before the disturbance. Likewise, the within-pair alternation level quickly resembled the pre-manipulated level, independent of nestling age or which sex was caught. Thus our experiment highlights the resilience of parental behaviour against temporal disturbances of individual parents.

Evolution of Personal and Social Immunity in the Context of Parental Care

Social immunity moderates the spread of pathogens in social groups and is especially likely in groups structured by genetic relatedness. The extent to which specific immune pathways are used is unknown. Here, we investigate the expression and social role of three functionally separate immune genes (pgrp-sc2, thaumatin, and defensin) during parental care in the beetle Nicrophorus vespilloides. These genes reside in different immune pathways, allowing us to test whether specific components of the immune system are targeted for social immunity. To test for the evolution of specificity, we manipulated the influence of social context and timing on gene expression and quantified the covariance of maternal immune gene expression and offspring fitness. Larvae reduced expression of all three genes in the presence of parents. Parental pgrp-sc2 and thaumatin increased during direct parenting, while defensin was upregulated before larvae arrived. Parental expression of pgrp-sc2 and thaumatin responded similarly to experimental manipulation of timing and presence of larvae, which differed from the response of defensin. We found a positive covariance between maternal expression and offspring fitness for pgrp-sc2 and thaumatin but not defensin. We suggest that social immunity can involve specific genes and pathways, reflecting evolution as an interacting phenotype during parenting.

The other facets of family life and their role in the evolution of animal sociality

Family life forms an integral part of the life history of species across the animal kingdom and plays a crucial role in the evolution of animal sociality. Our current understanding of family life, however, is almost exclusively based on studies that (i) focus on parental care and associated family interactions (such as those arising from sibling rivalry and parent-offspring conflict), and (ii) investigate these phenomena in the advanced family systems of mammals, birds, and eusocial insects. Here, we argue that these historical biases have fostered the neglect of key processes shaping social life in ancestral family systems, and thus profoundly hamper our understanding of the (early) evolution of family life. Based on a comprehensive survey of the literature, we first illustrate that the strong focus on parental care in advanced social systems has deflected scrutiny of other important social processes such as sibling cooperation, parent-offspring competition and offspring assistance. We then show that accounting for these neglected processes - and their changing role over time - could profoundly alter our understanding of the origin and subsequent evolution of family life. Finally, we outline how this 'diachronic' perspective on the evolution of family living provides novel insights into general processes driving the evolution of animal sociality. Overall, we infer that the explicit consideration of thus-far neglected facets of family life, together with their study across the whole diversity of family systems, are crucial to advance our understanding of the processes that shape the evolution of social life.

Changes of gene expression but not cytosine methylation are associated with male parental care reflecting behavioural state, social context and individual flexibility

Behaviour is often a front line response to changing environments. Recent studies show behavioural changes are associated with changes of gene expression; however, these studies have primarily focused on discrete behavioural states. We build on these studies by addressing additional contexts that produce qualitatively similar behavioural changes. We measured levels of gene expression and cytosine methylation, which is hypothesized to regulate the transcriptional architecture of behavioural transitions, within the brain during male parental care of the burying beetle Nicrophorus vespilloides in a factorial design. Male parenting is a suitably plastic behaviour because although male N. vespilloides typically do not provide direct care (i.e. feed offspring) when females are present, levels of feeding by a male equivalent to the female can be induced by removing the female. We examined three different factors: behavioural state (caring versus non-caring), social context (with or without a female mate) and individual flexibility (if a male switched to direct care after his mate was removed). The greatest number of differentially expressed genes were associated with behavioural state, followed by social context and individual flexibility. Cytosine methylation was not associated with changes of gene expression in any of the factors. Our results suggest a hierarchical association between gene expression and the different factors, but that this process is not controlled by cytosine methylation. Our results further suggest that the extent a behaviour is transient plays an underappreciated role in determining its underpinning molecular mechanisms.

Predictable gene expression related to behavioral variation in parenting

Differential gene expression has been associated with transitions between behavioral states for a wide variety of organisms and behaviors. Heterochrony, genetic toolkits, and predictable pathways underlying behavioral transitions have been hypothesized to explain the relationship between transcription and behavioral changes. Less studied is how variation in transcription is related to variation within a behavior, and if the genes that are associated with this variation are predictable. Here, we adopt an evolutionary systems biology perspective to address 2 hypotheses relating differential expression to changes within and between behavior. We predicted fewer genes will be associated with variation within a behavior than with transitions between states, and the genes underlying variation within a behavior will represent a narrower set of biological functions. We tested for associations with parenting variation within a state with a set of genes known a priori to be differentially expressed (DE) between parenting states in the burying beetle Nicrophorus vespilloides. As predicted, we found that far fewer genes are DE related to variation within parenting. Moreover, these were not randomly distributed among categories or pathways in the gene set we tested and primarily involved genes associated with neurotransmission. We suggest that this means candidate genes will be easier to identify for associations within a behavior, as descriptions of behavioral state may include more than a single phenotype.

A sustained change in the supply of parental care causes adaptive evolution of offspring morphology

Although cooperative social interactions within species are considered an important driver of evolutionary change, few studies have experimentally demonstrated that they cause adaptive evolution. Here we address this problem by studying the burying beetle Nicrophorus vespilloides. In this species, parents and larvae work together to obtain nourishment for larvae from the carrion breeding resource: parents feed larvae and larvae also self-feed. We established experimentally evolving populations in which we varied the assistance that parents provided for their offspring and investigated how offspring evolved in response. We show that in populations where parents predictably supplied more care, larval mandibles evolved to be smaller in relation to larval mass, and larvae were correspondingly less self-sufficient. Previous work has shown that antagonistic social interactions can generate escalating evolutionary arms races. Our study shows that cooperative interactions can yield the opposite evolutionary outcome: when one party invests more, the other evolves to invest less.

The amount and predictability of parental care may influence the evolution of offspring traits. Here, the authors experimentally evolve burying beetles at different levels of parental care and find smaller mandibles and lower self-sufficiency in populations with more care.

Back to the roots: the importance of using simple insect societies to understand the molecular basis of complex social life

The evolutionary trajectories toward insect eusociality come in two broad forms. In species like wasps, bees, and ants, the first helpers remained at the nest primarily to help with brood care. In species like aphids and termites, on the other hand, nest defense was initially the primary ecological driving force. To better understand the molecular basis of these two alternative evolutionary trajectories, it is therefore important to study the mechanistic basis of brood care and nest defense behavior. So far, most studies have compared morphologically distinct castes in advanced eusocial species of ants, bees, wasps, and termites. However, the interpretation of such comparisons is limited by multiple confounding factors and the fact that castes are typically fixed and cannot be manipulated at the adult stage. In this review, we argue that conducting molecular studies of brood care and nest defense in simpler, more flexible insect societies may complement studies of advanced eusocial insects and provide avenues toward more functional analyses. We review the available literature and propose candidate study systems for future molecular investigations of brood care and nest defense in social insects.

Juvenile hormone and parental care in subsocial insects: implications for the role of juvenile hormone in the evolution of sociality

Juvenile hormone (JH) has both gonadotropic and non-gonadotropic roles in eusocial insects. There is controversy over whether the non-gonadotropic role is novel, related specifically to social evolution, or is a second conserved role. Study of subsocial insects suggests that when JH is non-gonadotropic, the specific associations of high JH, low vitellogenin, suppressed ovarian development and elevated metabolism may parallel those in workers of eusocial insects. This suggests that a more fundamental understanding of JH in insect biology is required.

Genome sequencing of Rhinorhipus Lawrence exposes an early branch of the Coleoptera

Background

Rhinorhipidae Lawrence, 1988 is an enigmatic beetle family represented by a single species, Rhinorhipus tamborinensis Lawrence, 1988, from Australia, with poorly established affinities near the superfamily Elateroidea (click beetles, soldier beetles and fireflies) or the more inclusive series (infraorder) Elateriformia. Its evolutionary position may inform the basal relationships of the suborder Polyphaga, the largest clade of Coleoptera.

Results

We analyzed four densely sampled DNA datasets of major coleopteran lineages for mitogenomes, rRNA genes and single copy nuclear genes. Additionally, genome sequencing was used for incorporation of R. tamborinensis into a set of 4220 orthologs for 24 terminals representing 12 polyphagan superfamilies. Topologies differed to various degrees, but all consistently refute the proposed placement of Rhinorhipidae in Elateroidea and instead indicate either sister relationships with other Elateriformia, frequently together with Nosodendridae, another divergent small family hitherto placed in Derodontoidea, or in an isolated position among the deepest lineages of Polyphaga. The phylogenomic analyses recovered Rhinorhipus in a sister position to all other Elateriformia composed of five superfamilies. Therefore, we erect the new superfamily Rhinorhipoidea Lawrence, 1988, stat. Nov., with the type-family Rhinorhipidae. The origins of the Rhinorhipidae were dated to the Upper Triassic/Lower Jurassic at the very early phase of polyphagan diversification.

Conclusions

Thus, Rhinorhipidae adds another example to several recently recognized ancient relict lineages which are interspersed within contemporaneous hugely species-rich lineages of Coleoptera.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0262-0) contains supplementary material, which is available to authorized users.

De novo Assembly of the Burying Beetle Nicrophorus orbicollis (Coleoptera: Silphidae) Transcriptome Across Developmental Stages with Identification of Key Immune Transcripts

Burying beetles (Nicrophorus spp.) are among the relatively few insects that provide parental care while not belonging to the eusocial insects such as ants or bees. This behavior incurs energy costs as evidenced by immune deficits and shorter life-spans in reproducing beetles. In the absence of an assembled transcriptome, relatively little is known concerning the molecular biology of these beetles. This work details the assembly and analysis of the Nicrophorus orbicollis transcriptome at multiple developmental stages. RNA-Seq reads were obtained by next-generation sequencing and the transcriptome was assembled using the Trinity assembler. Validation of the assembly was performed by functional characterization using Gene Ontology (GO), Eukaryotic Orthologous Groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. Differential expression analysis highlights developmental stage-specific expression patterns, and immunity-related transcripts are discussed. The data presented provides a valuable molecular resource to aid further investigation into immunocompetence throughout this organism's sexual development.

Differential gene expression is not required for facultative sex allocation: a transcriptome analysis of brain tissue in the parasitoid wasp Nasonia vitripennis

Whole-transcriptome technologies have been widely used in behavioural genetics to identify genes associated with the performance of a behaviour and provide clues to its mechanistic basis. Here, we consider the genetic basis of sex allocation behaviour in the parasitoid wasp Nasonia vitripennis. Female Nasonia facultatively vary their offspring sex ratio in line with Hamilton's theory of local mate competition (LMC). A single female or 'foundress' laying eggs on a patch will lay just enough sons to fertilize her daughters. As the number of 'foundresses' laying eggs on a patch increases (and LMC declines), females produce increasingly male-biased sex ratios. Phenotypic studies have revealed the cues females use to estimate the level of LMC their sons will experience, but our understanding of the genetics underlying sex allocation is limited. Here, we exposed females to three foundress number conditions, i.e. three LMC conditions, and allowed them to oviposit. mRNA was extracted from only the heads of these females to target the brain tissue. The subsequent RNA-seq experiment confirmed that differential gene expression is not associated with the response to sex allocation cues and that we must instead turn to the underlying neuroscience to reveal the underpinnings of this impressive behavioural plasticity.

Duplication and Sub/Neofunctionalization of Malvolio, an Insect Homolog of Nramp, in the Subsocial Beetle Nicrophorus vespilloides

With growing numbers of sequenced genomes, increasing numbers of duplicate genes are being uncovered. Here we examine Malvolio, a gene in the natural resistance-associated macrophage protein (Nramp) family, that has been duplicated in the subsocial beetle, Nicrophorus vespilloides, which exhibits advanced parental behavior. There is only one copy of Mvl in honey bees and Drosophila, whereas in vertebrates there are two copies that are subfunctionalized. We first compared amino acid sequences for Drosophila, beetles, mice, and humans. We found a high level of conservation between the different species, although there was greater variation in the C-terminal regions. A phylogenetic analysis across multiple insect orders suggested that Mvl has undergone several independent duplications. To examine the potential for different functions where it has been duplicated, we quantified expression levels of Mvl1 and Mvl2 in eight tissues in N. vespilloides We found that while Mvl1 was expressed ubiquitously, albeit at varying levels, expression of Mvl2 was limited to brain and midgut. Because Mvl has been implicated in behavior, we examined expression during different behavioral states that reflected differences in opportunity for social interactions and expression of parental care behaviors. We found differing expression patterns for the two copies, with Mvl1 increasing in expression during resource preparation and feeding offspring, and Mvl2 decreasing in these same states. Given these patterns of expression, along with the protein analysis, we suggest that Mvl in N. vespilloides has experienced sub/neofunctionalization following its duplication, and may be evolving differing and tissue-specific roles in behavior and physiology.

Modification of feeding circuits in the evolution of social behavior

Adaptive trade-offs between foraging and social behavior intuitively explain many aspects of individual decision-making. Given the intimate connection between social behavior and feeding/foraging at the behavioral level, we propose that social behaviors are linked to foraging on a mechanistic level, and that modifications of feeding circuits are crucial in the evolution of complex social behaviors. In this Review, we first highlight the overlap between mechanisms underlying foraging and parental care and then expand this argument to consider the manipulation of feeding-related pathways in the evolution of other complex social behaviors. We include examples from diverse taxa to highlight that the independent evolution of complex social behaviors is a variation on the theme of feeding circuit modification.

Relating quantitative variation within a behavior to variation in transcription

Many studies have shown that variation in transcription is associated with changes in behavioral state, or with variation within a state, but little has been done to address if the same genes are involved in both. Here, we investigate the transcriptional basis of variation in parental provisioning using two species of burying beetle, Nicrophorus orbicollis and Nicrophorus vespilloides. We used RNA-seq to compare transcription in parents that provided high amounts of provisioning behavior versus low amounts in males and females of each species. We found no overarching transcriptional patterns distinguishing high from low caring parents, and no informative transcripts that displayed particularly large expression differences in either sex. However, we did find subtler gene expression differences between high and low provisioning parents that are consistent across both sexes and species. Furthermore, we show that transcripts previously implicated in transitioning into parental care in N. vespilloides had high variance in the levels of transcription and were unusually likely to display differential expression between high and low provisioning parents. Thus, quantitative behavioral variation appears to reflect many transcriptional differences of small effect. Furthermore, the same transcripts required for the transition between behavioral states are also related to variation within a behavioral state.

Development and application of 14 microsatellite markers in the burying beetle Nicrophorus vespilloides reveals population genetic differentiation at local spatial scales

Burying beetles (genus Nicrophorus) are relatively rare among insects in providing sophisticated parental care. Consequently, they have become model species in research analysing social evolution, the evolution of parental care and mating systems. We used the recently published N. vespilloides genome and transcriptome to develop microsatellite markers. Specifically, we developed 14 polymorphic markers with five to 13 alleles per locus and used them to investigate levels of genetic differentiation in four south Cambridgeshire (UK) populations of N. vespilloides, separated by 21 km at most. The markers revealed significant genetic structuring among populations (global F_ST = 0.023) with all but one of the pairwise comparisons among populations being significant. The single exception was the comparison between the two closest populations, which are approximately 2.5 km apart. In general, the microsatellite markers showed lower observed heterozygosity than expected. We infer that there is limited dispersal between populations and potentially also some inbreeding within them and suggest that this may be due to habitat fragmentation. We discuss these results in the context of recent laboratory experiments on inbreeding and beetle flight.

The role of neuropeptide F in a transition to parental care

The genetics of complex social behaviour can be dissected by examining the genetic influences of component pathways, which can be predicted based on expected evolutionary precursors. Here, we examine how gene expression in a pathway that influences the motivation to eat is altered during parental care that involves direct feeding of larvae. We examine the expression of neuropeptide F, and its receptor, in the burying beetle Nicrophorus vespilloides, which feeds pre-digested carrion to its begging larvae. We found that the npf receptor was greatly reduced during active care. Our research provides evidence that feeding behaviour was a likely target during the evolution of parental care in N. vespilloides Moreover, dissecting complex behaviours into ethologically distinct sub-behaviours is a productive way to begin to target the genetic mechanisms involved in the evolution of complex behaviours.