Subsocial Beetle Research Articles

Insect homolog of oxytocin/vasopressin associated with parenting of males but not females in a subsocial beetle.

Parental care is thought to evolve through modification of behavioral precursors, which predicts that the mechanistic changes occur in the genes underlying those traits. The duplicated gene system of oxytocin/vasopressin has been broadly co-opted across vertebrates to influence parenting, from a pre-duplication ancestral role in water balance. It remains unclear whether co-option of these genes for parenting is limited to vertebrates. Here, we experimentally tested for associations between inotocin gene expression and water balance, parental acceptance of offspring, and active parenting in the subsocial beetle Nicrophorus orbicollis, to test whether this single copy homologue of the oxytocin/vasopressin system, has similarly been co-opted for parental care in a species with elaborate parenting. As expected, inotocin was associated with water balance in both sexes. Inotocin expression increased around sexual maturation in both males and females, although more clearly in males. Finally, we found that expression of inotocin was not associated with acceptance of larvae but was associated with a transition to male but not female parenting. Moreover, level of offspring provisioning behavior and gene expression were positively correlated in males but uncorrelated in females. Our results suggest a broad co-option of this system for parenting that may have existed prior to gene duplication, and that inotocin may be associated with flexibility in parenting behavior.

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.

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

Nature Communications 6, Article number: 8449 (2015); Published: 29 September 2015; Updated: 27 May 2016 In the Methods section of this Article, a detail of the analysis involved in producing Fig. 4 and Table 2 was omitted. The section ‘Mapping and detection of differential expression’ should state that the overall magnitude of gene expression change was examined, ignoring the direction (up or downregulation).

The Genome and Methylome of a Beetle with Complex Social Behavior, Nicrophorus vespilloides (Coleoptera: Silphidae).

Testing for conserved and novel mechanisms underlying phenotypic evolution requires a diversity of genomes available for comparison spanning multiple independent lineages. For example, complex social behavior in insects has been investigated primarily with eusocial lineages, nearly all of which are Hymenoptera. If conserved genomic influences on sociality do exist, we need data from a wider range of taxa that also vary in their levels of sociality. Here, we present the assembled and annotated genome of the subsocial beetle Nicrophorus vespilloides, a species long used to investigate evolutionary questions of complex social behavior. We used this genome to address two questions. First, do aspects of life history, such as using a carcass to breed, predict overlap in gene models more strongly than phylogeny? We found that the overlap in gene models was similar between N. vespilloides and all other insect groups regardless of life history. Second, like other insects with highly developed social behavior but unlike other beetles, does N. vespilloides have DNA methylation? We found strong evidence for an active DNA methylation system. The distribution of methylation was similar to other insects with exons having the most methylated CpGs. Methylation status appears highly conserved; 85% of the methylated genes in N. vespilloides are also methylated in the hymentopteran Nasonia vitripennis. The addition of this genome adds a coleopteran resource to answer questions about the evolution and mechanistic basis of sociality and to address questions about the potential role of methylation in social behavior.

Octopaminergic gene expression and flexible social behaviour in the subsocial burying beetle Nicrophorus vespilloides

Flexible behaviour allows organisms to respond appropriately to changing environmental and social conditions. In the subsocial beetle Nicrophorus vespilloides, females tolerate conspecifics when mating, become aggressive when defending resources, and return to social tolerance when transitioning to parenting. Given the association between octopamine and aggression in insects, we hypothesized that genes in the octopaminergic system would be differentially expressed across different social and reproductive contexts. To test this in N. vespilloides, we first obtained the sequences of orthologues of the synthetic enzymes and receptors of the octopaminergic system. We next compared relative gene expression from virgin females, mated females, mated females alone on a resource required for reproduction and mated females on a resource with a male. Expression varied for five receptor genes. The expression of octopamine β receptor 1 and octopamine β receptor 2 was relatively higher in mated females than in other social conditions. Octopamine β receptor 3 was influenced by the presence or absence of a resource and less by social environment. Octopamine α receptor and octopamine/tyramine receptor 1 gene expression was relatively lower in the mated females with a resource and a male. We suggest that in N. vespilloides the octopaminergic system is associated with the expression of resource defence, alternative mating tactics, social tolerance and indirect parental care.

Behavioral Interactions Between Non-Cohabiting Bess Beetles, Odontotaenius disjunctus (Illiger) (Coleoptera: Passalidae)

The bess beetle, Odontotaenius disjunctus (Illiger), is a large subsocial beetle that inhabits decaying logs in which adults build tunnel systems and raise their larvae. Adults are territorial and both males and females protect tunnels from intruders. Though the behavior of many passalid beetle species, including O. disjunctus, has been studied, adult behavior toward conspecifics has not been thoroughly documented and many interactive behaviors have yet to be defined. We describe behaviors exhibited during laboratory interactions between non-cohabitating O. disjunctus adults. Twenty-four O. disjunctus were each tested in two experimental trials, once as a resident and once as an intruder, and in one control trial. We observed 18 behavior patterns and monitored them during trials. There were significant differences between control and experimental trials in some aggression behaviors, and in the total frequency of aggression, but no significant differences in submission, investigation, and neutral behaviors. Our data expand the behavioral repertoire for this species. Adults may use the described mild aggression behaviors in a territorial context, but we found no significant difference in total frequency of mild aggression between residents and intruders.

Social Facilitation and Digging Behavior in the Beetle Odontotaenius floridanus Schuster (Coleoptera: Passalidae)

Laboratory studies assessed the effect of group size on egg-carrying behavior and walking speed of adults of the subsocial beetle, Odontotaenius floridanus Schuster (Coleoptera: Passalidae). Adult males tested solo carried significantly fewer eggs per individual when compared with males tested in groups of 4 and 8 individuals. Similarly, adult males and females in groups of 4 and 8 individuals exhibited significantly faster walking speeds when tested in a linear runway as compared with adults tested alone. There was no significant difference in the number of eggs carried or walking speed between beetles tested in groups of 4 or 8 individuals. This represents the first demonstration of social facilitation in passalid beetles. The adaptive significance of social facilitation in insects is discussed.

Reproductive skew and communal breeding in the subsocial beetle Parastizopus armaticeps

The monogamous beetle Parastizopus armaticeps (Tenebrionidae) constructs breeding burrows in the Kalahari Desert after rainfall, and shows extensive biparental care. However, occasional communal breeding has been reported, where the male shares the burrow with more than one female (ca. 5% of breeding burrows). We induced communal breeding in the laboratory by confining one male with two females in a small breeding cage. In 33% of these trios females stopped being aggressive towards each other and reared offspring communally. Larger females dominate smaller females, so we expected the incidence of communal breeding to be higher, and the reproductive skew to be lower, for equally sized females than for unequally sized females competing for the male. These predictions were not borne out: the incidence of communal breeding was similar in both situations and reproductive skew was independent of the (difference in) body measurements of the two females. The calculated binomial reproductive skew was in close agreement with the distribution expected from random sharing of maternity among the two females. Time spent in parental care per individual and (group) reproductive success of trios were not different from those of monogamous breeding pairs.

Communities of microbes that inhabit the changing hindgut landscape of a subsocial beetle

Microbes that have adopted endosymbiotic life styles not only have evolved to live in specialized habitats within living organisms, but the living habitats also have evolved to accommodate them. The hindgut of the passalid beetle ( Odontotaenius disjunctus) is lined with a cuticle that undergoes dramatic topographic changes during the life cycle of the beetle. This manuscript addresses the changes that have been observed in time and space for the cuticular landscape of the hindgut as well as for the microbial communities within the hindgut. Microbial identity is based on morphology, culture, and extrapolation from previously reported passalid gut inhabitants.

Individual variation and prior experience affect the discrimination of a brood-parasite by its subsocial beetle host

In birds, host experience can modify response to parasites but nothing similar is known for insects. We studied two desert tenebrionid beetles, the subsocial host Parastizopus armaticeps and its obligate and ubiquitous brood-(clepto)parasite Eremostibes opacus, which mimics the host’s odour spectrum. Nearly 10% of host burrows in the field (n=214) remain unparasitised even after introducing E. opacus experimentally (n=22). In discrimination tests, 7% of naive host beetles eject the parasite, showing individual variation in olfactory discrimination ability. To test for effects of prior breeding experience, naive (“first time”) or experienced (“second time”) breeders with or without prior parasite contact were confined with E. opacus at breeding onset, their behaviour recorded and parasite number counted on breeding completion. More first- than second-time breeding pairs completely excluded the parasite and second-time breeders with no prior contact with E. opacus had fewer parasites than any others. These pairs increased burrow-guarding dramatically, mostly during early reproduction, this being the best predictor of parasite number. Prolonged guarding, however, must be traded-off against other brood-care behaviours. Hence, it might pay breeding pairs to take the risk of accepting some parasites breeding within their burrow, and we expect this effect will be more pronounced if poor feeding and digging conditions are taken into account. This might explain why apparently 90% of the P. armaticeps breeding burrows in the field contain at least one parasitic E. opacus.

Reproductive skew and the threat of eviction: a new perspective

Most recent models of the partitioning of reproduction attempt to explain patterns of skew on the assumption that dominant individuals have complete control over breeding opportunities within the group, but may nevertheless concede a share of direct reproduction to subordinates as an incentive to remain peacefully in the association. Although these models may be applicable to some animal societies, we argue that they fail to provide a comprehensive theory of skew. Instead, we suggest that subordinates may often be able to claim unsanctioned reproduction for themselves, but will be forced to exercise a degree of reproductive restraint lest they incite ejection by the dominant. Reproductive skew, in other words, may reflect the threat of ejection (inducing subordinate restraint) rather than the threat of subordinate departure (inducing reproductive concessions by dominants). We present a simple ESS model of reproductive skew under these circumstances, which demonstrates that a shift in emphasis from reproductive concessions by dominants to reproductive restraint on the part of subordinates, radically alters the predictions of skew models. High group productivity, high relatedness and (when group members are related) strong ecological constraints are all expected to lead to reduced skew (the opposite conclusions to those of previous, concession-based analyses). The reason is that these factors reduce the benefits (or increase the costs) of ejection to the dominant, who therefore does best to tolerate more subordinate reproduction.

Female mate choice in a subsocial beetle: male phenotype correlates with helping potential and offspring survival

InParastizopus armaticeps(Tenebrionidae), a nocturnal desert beetle, the males excavate breeding burrows and maintain their moisture level while the females provision the larvae with detritus collected on the surface. The beetles court in small groups on the surface at night after rain. Male size distribution in these groups corresponded to that in the population but more large and fewer small females were present than expected and more large beetles of both sexes bred. Offspring number correlated positively with burrow depth and body length for males but not for females. Since large males dig deeper burrows, which results in higher larval survival rate, females should prefer them. In choice experiments, females selected larger males. Behavioural analyses showed that choice was not dependent on differences in male courtship activity or intermale dominance. When the mass of the smaller male was increased experimentally by a dorsally attached weight, the smaller male was preferred, females estimating male size difference by mass. Partner choice is therefore epigamic for a male phenotypic character which correlates with both parenting ability and greater reproductive success for females.

Biparental investment and reproductive success in a subsocial desert beetle: the role of maternal effort

Parastizopus armaticeps is a nocturnal subsocial detritivorous desert tenebrionid that produces very few offspring per brood. The two environmental factors that constrain reproduction, rapid sand desiccation rate and food scarcity, are countered by biparental effort. Males dig and extend breeding burrows, maintaining their moisture level; females forage on the surface at night for high-quality detritus, the larval food. This was shown to be a scarce and unpredictable resource for which there is high competition. When food was supplemented in a field experiment, offspring number and survivorship doubled and burrow failure due to desiccation dropped from approximately half, the typical failure rate for unsupplemented burrows, to zero. Food supplementation did not, however, increase larval foodstore size and there was no difference in the size of the offspring produced. Supplemented females reallocated their time, foraging less and digging more with the male. This change in maternal behaviour patterns resulted in deeper burrows which remained moist longer, thus extending the larval production period. Female foraging efficiency, particularly food retrieval speed, determined how much time females could allocate to digging, consequently increasing the reproductive success of the pair. Burrow depth and sand moisture level at the burrow base were the major correlates of reproductive success, but the scarcity and unpredictability of high-quality food on the surface and the competition for this resource influenced the number of offspring indirectly through their effect on female behaviour.

Female mate assessment tactics in a subsocial desert beetle: a test of Janetos’ models

The subsocial desert tenebrionid beetle Parastizopus armaticeps is monogamous and nocturnal, with division of labour and female mate selection for large male body size. Mate choice is time-constrained and must take place during the night following sporadic heavy rainfall since pairs can only dig breeding burrows when the sand is moist. Beetles court in small, widely-separated groups. The four tactics proposed by JANETOS (1980), through which females could assess potential mates before coming to a decision, were tested experimentally. Approximately 20% of females adopted the “fixed-threshold” tactic to assess males, all others opting for the “best of n males” tactic. These tactics are considered to be the most advantageous ones when time is limited and the number of prospective mates that can be assessed low, as is the case for P. armaticeps. Females used multiple assessments in “the best of n males tactic”, suggesting that evaluation is constrained by female information retention and integration ability, one of the constraints on sequential comparisons. This tactic also has the highest expected fitness benefits.

Parental care and offspring survival in Copris incertusSay, a sub-social beetle

Abstract. The effects of the lack of maternal care on offspring survival of the sub-social beetle Copris incertusSay (Coleoptera: Scarabaeidae: Scarabaeinae) were tested experimentally by excluding the female and determining the most vulnerable stage of development. Time budget studies were also conducted to determine the amount used by 10 females during the whole cycle from the making of the nest to the emergence of adults. A Kruskal–Wallis test and the subsequent non-parametric multiple comparisons showed a significant difference in survival between nests cared for by the mother and nests where the mother was excluded. Key-factor analysis showed that the absence of parental care affected offspring survival in the transition from the pupal to adult stage. Time budget studies indicated that, whether active or inactive, females were in direct association with the brood balls. Balls that received no care (in experimental exclusions or where the female abandoned the nest) were invaded by the fungi Metarrhizium anisoplaeand Cephalosporiumsp., which covered the surface and the inner part of the ball, considerably affecting pupal survivorship. Results show that sub-social care increases the probability of juveniles reaching adulthood.

Leaving home: predation and the dispersal of larvae from the maternal burrow of Bledius spectabilis, a subsocial intertidal beetle

Females of the intertidal subsocial beetle, Bledius spectabilis Kraatz (Staphylinidae), look after their eggs and young larvae in burrows in saltmarsh mud. Field observations showed that the beetle larvae disperse from the maternal burrow wowards the end of the first instar. The dispersing larvae belonged to a restricted age-group and dispersed only once, at night (between 2200 and 0400 hours). They moved rapidly on the surface of the saltmarsh soil in search of cracks and holes in which to start their own independent burrows. The nocturnal dispersal of the Bledius larvae overlapped with the activity on the soil surface of the predatory carabid beetle Dicheirotrichus gustavi Crotch. The ability of larvae of different ages to survive attack by the carabid was tested in field experiments. Young larvae taken from the maternal burrow were always eaten by the first carabid that attacked them. Older larvae taken from the maternal burrow and the dispersing larvae survived on average at least one attack by the carabid. By delaying dispersal until the latter part of the first instar, the larvae can significantly increase their chance of countering an attack by this predator during this critical phase. The maternal burrow therefore provides not only protection against the tide but a safe place in which the larvae can grow large enough to repulse predator attack.