The evolution of male traits in social insects

The dual coding of a single sex pheromone receptor in Asian honeybee Apis cerana

In Asian honeybees, virgin queens typically only mate during a single nuptial flight before founding a colony. This behavior is controlled by the queen-released mandibular pheromone (QMP). 9-oxo-(E)-2-decenoic acid (9-ODA), a key QMP component, acts as sex pheromone and attracts drones. However, how the queens prevent additional mating remains elusive. Here, we show that the secondary QMP component methyl p-hydroxybenzoate (HOB) released by mated queens inhibits male attraction to 9-ODA. Results from electrophysiology and in situ hybridization assay indicated that HOB alone significantly reduces the spontaneous spike activity of 9-ODA-sensitive neurons, and AcerOr11 is specifically expressed in sensilla placodea from the drone's antennae, which are the sensilla that narrowly respond to both 9-ODA and HOB. Deorphanization of AcerOr11 in Xenopus oocyte system showed 9-ODA induces robust inward (regular) currents, while HOB induces inverse currents in a dose-dependent manner. This suggests that HOB potentially acts as an inverse agonist against AcerOr11.

Mating- and oviposition-dependent changes of the spermatheca and colleterial glands in the pest termite Cryptotermes brevis (Blattaria, Isoptera, Kalotermitidae)

The spermatheca and colleterial glands of female insects are organs associated with the reproductive system, responsible for sperm storage and secretion of egg coverings, respectively. Here we compared the development, secretory activity, and chemical nature of the secretion in the spermatheca and colleterial glands of different-aged females of the drywood termite Cryptotermes brevis. We also provide the ultrastructure of these organs in alate females. These structures have been poorly investigated in termites when compared to other eusocial insects (Hymenoptera) and termite-related dictyopterans (mantises and cockroaches). The spermatheca of C. brevis comprises a cone-shaped structure, connected to the genital chamber by a short duct. The colleterial glands, in turn, are divided into anterior and posterior tubules, each showing a basal trunk, and join into a common duct. Histological and histochemical analyses showed that the secretion of proteins and polysaccharides by the spermatheca takes place before pairing, but increases as females mate and store sperm. Colleterial glands of alates showed non-synchronous secretory activity, but the synthesis of products increased in egg-laying queens, together with the epithelium height. Ultrastructure of the spermatheca and colleterial glands revealed epithelia composed of class III secretory cells. Richness of mitochondria and electron-dense secretion in the spermatheca indicates synthesis and transport of content. Presence and absence of colleterial gland secretion in different individuals may reflect variable maturation stages of the females and secretory cells. Assuming that termites are iteroparous, the development and secretion of the spermatheca and colleterial glands play a crucial role for C. brevis queens.

Sexual dimorphism in the proventriculus of the buff-tailed bumblebee Bombus terrestris (L. 1758) (Hymenoptera: Apidae)

Research on eusocial bee species like Bombus terrestris is primarily focused on the worker caste, which is why their morphology and anatomy are already well described. This includes the alimentary tract, which is adapted for feeding on nectar and pollen. Located at the transition between crop and ventriculus is a highly specialised compartment, the proventriculus. In female workers of B. terrestris, the proventriculus is surrounded by muscles and consists of four anterior lips. A detailed description, however, is only provided for B. terrestis worker bees while studies on the proventriculus of the male reproductive caste are absent. Here, we provide a detailed analysis of the differences between the proventriculus of the B. terrestris males and females through morphometrics, histology and scanning electron microscopy imaging, and unravel a distinct sexual dimorphism. The male proventriculus is wider resulting in a greater volume than the female proventriculus. Histological analysis revealed 4 distinctive chambers of the male proventriculus, which are completely covered with hairs on the inside. In contrast, those chambers in the proventriculus of female B. terrestris, are only rudimentarily present forming only small pouches with hairs in the junctions between the proventricular folds inside the proventriculus. The morphological differences in the proventriculus may be based on different modi vivendi, as males do not return to the colony and fly longer distances. This and the synthesis of sperm and mating plug might require higher energy reserves, leading to the necessity of higher food storage capacities.

Proteomic differences in seminal fluid of social insects whose sperm differ in heat tolerance

In the coming years, climate change is likely to increase the frequency and intensity of heatwaves. In many organisms, heat stress provokes physiological perturbations and can lead to decreased male fertility. Bumblebees are endo-heterothermic but display interspecific differences in thermotolerance that could have conservation implications. For the species of concern Bombus magnus, exposure to high temperatures can severely reduce sperm quality and, consequently, reproductive success. Such is not the case for B. terrestris, a ubiquitous species. To decipher the mechanisms at play, we characterized the seminal fluid proteomes of the two species. We quantified 1121 proteins, of which 522 were differentially expressed between B. terrestris and B. magnus. Several proteins with protective functions, such as proteases, antioxidant proteins and various heat-shock proteins, were present at higher levels in B. terrestris than in B. magnus under both control and heat-stress conditions. The same was true for proteins involved in cellular homeostasis, immunity, lipid/sugar metabolism and thermotolerance. Furthermore, proteins involved in the capture and elimination of reactive oxygen species also occurred at much high levels in B. terrestris. Overall, these results clearly indicate differences in the seminal proteome of the more thermotolerant B. terrestris versus B. magnus. The differences may contribute to explaining interspecific differences in sperm survival.

Trade-offs between immunity and competitive ability in fighting ant males

BACKGROUND

Fighting disease while fighting rivals exposes males to constraints and trade-offs during male-male competition. We here tested how both the stage and intensity of infection with the fungal pathogen Metarhizium robertsii interfere with fighting success in Cardiocondyla obscurior ant males. Males of this species have evolved long lifespans during which they can gain many matings with the young queens of the colony, if successful in male-male competition. Since male fights occur inside the colony, the outcome of male-male competition can further be biased by interference of the colony's worker force.

RESULTS

We found that severe, but not yet mild, infection strongly impaired male fighting success. In late-stage infection, this could be attributed to worker aggression directed towards the infected rather than the healthy male and an already very high male morbidity even in the absence of fighting. Shortly after pathogen exposure, however, male mortality was particularly increased during combat. Since these males mounted a strong immune response, their reduced fighting success suggests a trade-off between immune investment and competitive ability already early in the infection. Even if the males themselves showed no difference in the number of attacks they raised against their healthy rivals across infection stages and levels, severely infected males were thus losing in male-male competition from an early stage of infection on.

CONCLUSIONS

Males of the ant C. obscurior have a well-developed immune system that raises a strong immune response very fast after fungal exposure. This allows them to cope with mild pathogen exposures without compromising their success in male-male competition, and hence to gain multiple mating opportunities with the emerging virgin queens of the colony. Under severe infection, however, they are weak fighters and rarely survive a combat already at early infection when raising an immune response, as well as at progressed infection, when they are morbid and preferentially targeted by worker aggression. Workers thereby remove males that pose a future disease threat by biasing male-male competition. Our study thus reveals a novel social immunity mechanism how social insect workers protect the colony against disease risk.

Positive selection in cytochrome P450 genes is associated with gonad phenotype and mating strategy in social bees

The honey bee, Apis mellifera differs from all other social bees in its gonad phenotype and mating strategy. Honey bee queens and drones have tremendously enlarged gonads, and virgin queens mate with several males. In contrast, in all the other bees, the male and female gonads are small, and the females mate with only one or very few males, thus, suggesting an evolutionary and developmental link between gonad phenotype and mating strategy. RNA-seq comparisons of A. mellifera larval gonads revealed 870 genes as differentially expressed in queens versus workers and drones. Based on Gene Ontology enrichment we selected 45 genes for comparing the expression levels of their orthologs in the larval gonads of the bumble bee Bombus terrestris and the stingless bee, Melipona quadrifasciata, which revealed 24 genes as differentially represented. An evolutionary analysis of their orthologs in 13 solitary and social bee genomes revealed four genes with evidence of positive selection. Two of these encode cytochrome P450 proteins, and their gene trees indicated a lineage-specific evolution in the genus Apis, indicating that cytochrome P450 genes may be involved in the evolutionary association of polyandry and the exaggerated gonad phenotype in social bees.

Long-term sperm storage in eusocial Hymenoptera

In internally fertilizing species, sperm transfer is not always immediately followed by egg fertilization, and female sperm storage (FSS) may occur. FSS is a phenomenon in which females store sperm in a specialized organ for periods lasting from a few hours to several years, depending on the species. Eusocial hymenopterans (ants, social bees, and social wasps) hold the record for FSS duration. In these species, mating takes place during a single nuptial flight that occurs early in adult life for both sexes; they never mate again. Males die quickly after copulation but survive posthumously as sperm stored in their mates' spermathecae. Reproductive females, also known as queens, have a much longer life expectancy, up to 20 years in some species. Here, we review what is currently known about the molecular adaptations underlying the remarkable FSS capacities in eusocial hymenopterans. Because sperm quality is crucial to the reproductive success of both sexes, we also discuss the mechanisms involved in sperm storage and preservation in the male seminal vesicles prior to ejaculation. Finally, we propose future research directions that should broaden our understanding of this unique biological phenomenon.

Sperm competition increases sperm production and quality in Cataglyphis desert ants

Sperm competition is a pervasive evolutionary force that shapes sperm traits to maximize fertilization success. Indeed, it has been shown to increase sperm production in both vertebrates and invertebrates. However, sperm production is energetically costly, which may result in trade-offs among sperm traits. In eusocial hymenopterans, such as ants, mating dynamics impose unique selective pressures on ejaculate. Males are sperm limited: they enter adulthood with a fixed amount of sperm that will not be renewed. We explored whether sperm competition intensity was associated with sperm quantity and quality (i.e. sperm viability and DNA fragmentation) in nine Cataglyphis desert ants. Our results provide phylogenetically robust evidence that sperm competition is positively correlated with sperm production and sperm viability. However, it was unrelated to sperm DNA integrity, indicating the absence of a trade-off involving this trait. These findings underscore that sperm competition may strongly mould sperm traits and drive reproductive performance in eusocial Hymenoptera.

The Evolution of Tyramides in Male Fungus-Growing Ants (Formicidae: Myrmicinae: Attini: Attina)

Ants use a variety of semiochemicals for essential activities and have been a source for many novel natural products. While ant taxa produce a wide variety of chemicals, the chemistry and ecology of male ants have remained understudied. Tyramides are a class of compounds that have been found only in males of the Myrmicinae ant subfamily. Tyramides found in the fire ant Solenopsis invicta are transferred to gynes during mating where they are converted to tyramine, leading to rapid reproductive development. To further understand the evolution of tyramide production in male ants, we determined the tyramide composition in males of 15 fungus-growing ant species (Formicidae: Myrmicinae: Attini: Attina) and a Megalomyrmex species (Formicidae: Myrmicinae: Solenopsidini). Thirteen tyramides were identified, four for the first time in natural sources, and their percent composition was mapped to the fungus-growing ant phylogeny.

A single-cell transcriptomic atlas tracking the neural basis of division of labour in an ant superorganism

Ant colonies with permanent division of labour between castes and highly distinct roles of the sexes have been conceptualized to be superorganisms, but the cellular and molecular mechanisms that mediate caste/sex-specific behavioural specialization have remained obscure. Here we characterized the brain cell repertoire of queens, gynes (virgin queens), workers and males of Monomorium pharaonis by obtaining 206,367 single-nucleus transcriptomes. In contrast to Drosophila, the mushroom body Kenyon cells are abundant in ants and display a high diversity with most subtypes being enriched in worker brains, the evolutionarily derived caste. Male brains are as specialized as worker brains but with opposite trends in cell composition with higher abundances of all optic lobe neuronal subtypes, while the composition of gyne and queen brains remained generalized, reminiscent of solitary ancestors. Role differentiation from virgin gynes to inseminated queens induces abundance changes in roughly 35% of cell types, indicating active neurogenesis and/or programmed cell death during this transition. We also identified insemination-induced cell changes probably associated with the longevity and fecundity of the reproductive caste, including increases of ensheathing glia and a population of dopamine-regulated Dh31-expressing neurons. We conclude that permanent caste differentiation and extreme sex-differentiation induced major changes in the neural circuitry of ants.

Using single-cell transcriptomics, the authors generate a brain cell atlas for the pharaoh ant including individuals of different sexes and castes and show changes in cell composition underlying division of labour and reproductive specialization.

Influence of caste and subcaste characteristics in ant locomotion (Camponotus fellah)

Worker polymorphism in ants has evolved repeatedly, with considerable differences in the morphometry of worker subcastes. Such body size differences and especially caste- and subcaste-specific characteristics might significantly influence locomotion. Therefore, we performed a comprehensive locomotion analysis along gradients in both body size and walking speed of Camponotus fellah worker subcastes, and of males, which have rarely been studied to date due to short life spans associated with mating flights. We provide a detailed description of the morphometry and size differences of C. fellah castes and subcastes and analyse locomotion in the different polymorphic groups in terms of absolute and relative walking speeds (mesosoma lengths per second). Our results reveal that body size and shape affect locomotion behaviour to different extents in the worker subcastes (minor workers, medias, major workers) and in males. Nevertheless, C. fellah ants use the same overall locomotion strategy, with males and major workers reaching considerably lower walking speeds than minors and medias. Body size thus mainly affects walking speed. Minor workers reach the highest relative velocities by high relative stride lengths in combination with large vertical and lateral COM oscillations and clearly higher stride frequencies of up to 25 Hz. Locomotion of males was characterised by clearly lower walking speeds, wider footprint positions, significant phase shifts and a notable dragging of the shorter hind legs. However, general walking parameters of males differed less from those of the female workers than expected due to division of labour in the colony.

D, Barrios H. Male ant reproductive investment in a seasonal wet tropical forest: Consequences of future climate change

Tropical forests sustain many ant species whose mating events often involve conspicuous flying swarms of winged gynes and males. The success of these reproductive flights depends on environmental variables and determines the maintenance of local ant diversity. However, we lack a strong understanding of the role of environmental variables in shaping the phenology of these flights. Using a combination of community-level analyses and a time-series model on male abundance, we studied male ant phenology in a seasonally wet lowland rainforest in the Panama Canal. The male flights of 161 ant species, sampled with 10 Malaise traps during 58 consecutive weeks (from August 2014 to September 2015), varied widely in number (mean = 9.8 weeks, median = 4, range = 1 to 58). Those species abundant enough for analysis (n = 97) flew mainly towards the end of the dry season and at the start of the rainy season. While litterfall, rain, temperature, and air humidity explained community composition, the time-series model estimators elucidated more complex patterns of reproductive investment across the entire year. For example, male abundance increased in weeks when maximum daily temperature increased and in wet weeks during the dry season. On the contrary, male abundance decreased in periods when rain receded (e.g., at the start of the dry season), in periods when rain fell daily (e.g., right after the beginning of the wet season), or when there was an increase in the short-term rate of litterfall (e.g., at the end of the dry season). Together, these results suggest that the BCI ant community is adapted to the dry/wet transition as the best timing of reproductive investment. We hypothesize that current climate change scenarios for tropical regions with higher average temperature, but lower rainfall, may generate phenological mismatches between reproductive flights and the adequate conditions needed for a successful start of the colony.

Queens remate despite traumatic mating in stingless bees

Males can control female reproduction using genital plugs to impede access by rivals. In social bees, ants, and wasps, plugging may involve traumatic mating, with females being harmed. In stingless bees, chances are that plugs may promote ovarian activan, and are thought to ensure single mating-a general tendency among the social Hymenoptera. However, understanding on relationships between mating plugs, traumatic mating, and mating systems in stingless bees remains limited. To address this, we (1) compared mated queens of 7 Neotropical species to understand the patterns of copulatory marks in females and (2) compared pre- and post-mating genitalia of males and females in Melipona fasciculata to depict plug functional morphology. Data revealed an unprecedented consequence of mating in stingless bees: the characteristic marks left by mating plugs on female abdomens and the inferences that can be made from them. To our surprise, in 1 species M. fasciculata we found that queens retain the plug long after mating, and may carry it for the rest of their lives. All the other 6 species retained the plug for only a short period. Remated queens were only found in M. seminigra, whose multiple copulatory marks match previous findings of polyandry in this species. Our study shows that queens can remate, and suggests that male genital morphology may determine in part the time persistence of plugs. We conclude that traumatic mating plugs do not fully prevent remating in stingless bees and that mating systems are not uniform in this group. Nonetheless, exceptional cases of facultative polyandry in social insects-for example, when mating plugs fail-may confirm a general tendency for single mating in close link with efficient mating plugs.

Thiamethoxam as an inadvertent anti-aphrodisiac in male bees

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There is consensus that neonicotinoids can impact non-target animal fertility.

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Thiamethoxam reduced both mating success and sperm physiology in bumblebees.

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Queens mated by exposed males had 50% less total living sperm in their spermatheca.

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Thiamethoxam may act as anti-aphrodisiac, thereby limiting conservation efforts.

Sexual reproduction is common to almost all multi-cellular organisms and can be compromised by environmental pollution, thereby affecting entire populations. Even though there is consensus that neonicotinoid insecticides can impact non-target animal fertility, their possible impact on male mating success is currently unknown in bees. Here, we show that sublethal exposure to a neonicotinoid significantly reduces both mating success and sperm traits of male bumblebees. Sexually mature male Bombus terrestris exposed to a field-realistic concentration of thiamethoxam (20 ng g⁻¹) or not (controls) were mated with virgin gynes in the laboratory. The results confirm sublethal negative effects of thiamethoxam on sperm quantity and viability. While the latency to mate was reduced, mating success was significantly impaired in thiamethoxam-exposed males by 32% probably due to female choice. Gynes mated by exposed males revealed impaired sperm traits compared to their respective controls, which may lead to severe constraints for colony fitness. Our laboratory findings demonstrate for the first time that neonicotinoid insecticides can negatively affect male mating success in bees. Given that holds true for the field, this provides a plausible mechanism contributing to declines of wild bee populations globally. The widespread prophylactic use of neonicotinoids may therefore have previously overlooked inadvertent anti-aphrodisiac effects on non-target animals, thereby limiting conservation efforts.

Sperm Storage Costs Determine Survival and Immunocompetence in Newly Mated Queens of the Leaf-Cutting Ant Atta colombica

In the leaf-cutting ant Atta colombica, queens receive ejaculates from multiple males during one single mating event early in their lives. A queen’s fertility and fitness therefore depend on maximizing the number of sperm cells she can store and maintain inside her spermatheca. Previous studies implied significant physiological mating costs, either originating from energetic investments maximizing sperm survival, or from resolving sexual conflicts to terminate male-driven incapacitation of rival sperm via serine proteases found in seminal fluid. Here we conducted an artificial insemination experiment, which allowed us to distinguish between the effects of sperm and seminal fluid within the queen’s sexual tract on her survival and immunocompetence. We found significantly higher mortality in queens that we had inseminated with sperm, independently of whether seminal fluid was present or not. Additionally, after receiving sperm, heavier queens had a higher probability of survival compared to lightweight queens, and immunocompetence decreased disproportionally for queens that had lost weight during the experiment. These findings indicate that queens pay significant physiological costs for maintaining and storing sperm shortly after mating. On the other hand, the presence of seminal fluid within the queens’ sexual tract neither affected their survival nor their immunocompetence. This suggests that the energetic costs that queens incur shortly after mating are primarily due to investments in sperm maintenance and not costs of terminating conflicts between competing ejaculates. This outcome is consistent with the idea that sexually selected traits in social insects with permanent castes can evolve only when they do not affect survival or life-time fitness of queens in any significant way.

OUP accepted manuscript

Many animal species are haplodiploid: their fertilized eggs develop into diploid females and their unfertilized eggs develop into haploid males. The unique genetic features of haplodiploidy raise the prospect that these systems can be used to disentangle the population genetic consequences of haploid and diploid selection. To this end, sex-specific reproductive genes are of particular interest because, while they are shared within the same genome, they consistently experience selection in different ploidal environments. However, other features of these genes, including sex-specific expression and putative involvement in postcopulatory sexual selection, are potentially confounding factors because they may also impact the efficacy of selection asymmetrically between the sexes. Thus, to properly interpret evolutionary genomic patterns, it is necessary to generate a null expectation for the relative amount of polymorphism and divergence we expect to observe among sex-specific genes in haplodiploid species, given differences in ploidal environment, sex-limited expression, and their potential role in sexual selection. Here, we derive the theoretical expectation for the rate of evolution of sex-specific genes in haplodiploid species, under the assumption that they experience the same selective environment as genes expressed in both sexes. We find that the null expectation is that reproductive genes evolve more rapidly than constitutively expressed genes in haplodiploid genomes. However, despite the aforementioned differences, the null expectation does not differ between male- and female-specific reproductive genes, when assuming additivity. Our theoretical results provide an important baseline expectation that should be used in molecular evolution studies comparing rates of evolution among classes of genes in haplodiploid species.

Evolution of anticipatory effects mediated by epigenetic changes

Anticipatory effects mediated by epigenetic changes occur when parents modify the phenotype of their offspring by making epigenetic changes in their gametes, guided by information from an environmental cue. To investigate when do anticipatory effects mediated by epigenetic changes evolve in a fluctuating environment, I use an individual-based simulation model with explicit genetic architecture. The model allows for the population to respond to environmental changes by evolving plasticity, bet hedging, or by tracking the environment with genetic adaptation, in addition to the evolution of anticipatory effects. The results show that anticipatory effects evolve when the environmental cue provides reliable information about the environment and the environment changes at intermediate rates, provided that fitness costs of anticipatory effects are rather low. Moreover, evolution of anticipatory effects is quite robust to different genetic architectures when reliability of the environmental cue is high. Anticipatory effects always give smaller fitness benefits than within-generation plasticity, suggesting a possible reason for generally small observed anticipatory effects in empirical studies.

Equivalent learning, but unequal participation: Male bumble bees learn comparably to females, but participate in cognitive assessments at lower rates

Sex-specific cognitive abilities are well documented. These can occur when sexes engage in different ecological contexts. Less known is whether different ecological contexts can also drive sex-specific participation rates in behavioral tests. Here, we explore this question in bumble bees, a group of eusocial insects where worker females and males exhibit stark socioecological differences. Among myriad colony maintenance tasks, workers forage for themselves and developing brood, while males forage only for themselves while mate-searching. Following upon previous studies suggesting no sex differences in bumble bee learning, we test the hypothesis that despite having equivalent associative learning abilities, males participate in cognitive assessments offering nutritional rewards at lower rates. Testing > 500 bees from nine colonies in a differential conditioning protocol, we find support for our hypothesis. An equivalent proportion of workers and males successfully completed our cognitive assessment, while a significantly lower proportion of males participated in the entire protocol. Unequal participation is a perennial issue in the behavioral sciences, limiting sample size and potentially biasing results. Our results suggest that to understand the true range of variation in cognition, sex-differences in participation must be accounted for.

Anatomy and evolution of the head of Dorylus helvolus (Formicidae: Dorylinae): Patterns of sex- and caste-limited traits in the sausagefly and the driver ant

Ants are highly polyphenic Hymenoptera, with at least three distinct adult forms in the vast majority of species. Their sexual dimorphism, however, is overlooked to the point of being a nearly forgotten phenomenon. Using a multimodal approach, we interrogate the near total head microanatomy of the male of Dorylus helvolus, the "sausagefly," and compare it with the conspecific or near-conspecific female castes, the "driver ants." We found that no specific features were shared uniquely between the workers and males to the exclusion of the queens, indicating independence of male and worker development; males and queens, however, uniquely shared several features. Certain previous generalizations about ant sexual dimorphism are confirmed, while we also discover discrete muscular presences and absences, for which reason we provide a coarse characterization of functional morphology. Based on the unexpected retention of a medial carinate line on the structurally simplified mandible of the male, we postulate a series of developmental processes to explain the patterning of ant mandibles. We invoke functional and anatomical principles to classify sensilla. Critically, we observe an inversion of the expected pattern of male-queen mandible development: male Dorylus mandibles are extremely large while queen mandibles are poorly developed. To explain this, we posit that the reproductive-limited mandible phenotype is canalized in Dorylus, thus partially decoupling the queen and worker castes. We discuss alternative hypotheses and provide further comparisons to understand mandibular evolution in army ants. Furthermore, we hypothesize that the expression of the falcate phenotype in the queen is coincidental, that is, a "spandrel," and that the form of male mandibles is also generally coincidental across the ants. We conclude that the theory of ant development and evolution is incomplete without consideration of the male system, and we call for focused study of male anatomy and morphogenesis, and of trait limitation across all castes.

Age-related mushroom body expansion in male sweat bees and bumble bees

A well-documented phenomenon among social insects is that brain changes occur prior to or at the onset of certain experiences, potentially serving to prime the brain for specific tasks. This insight comes almost exclusively from studies considering developmental maturation in females. As a result, it is unclear whether age-related brain plasticity is consistent across sexes, and to what extent developmental patterns differ. Using confocal microscopy and volumetric analyses, we investigated age-related brain changes coinciding with sexual maturation in the males of the facultatively eusocial sweat bee, Megalopta genalis, and the obligately eusocial bumble bee, Bombus impatiens. We compared volumetric measurements between newly eclosed and reproductively mature males kept isolated in the lab. We found expansion of the mushroom bodies-brain regions associated with learning and memory-with maturation, which were consistent across both species. This age-related plasticity may, therefore, play a functionally-relevant role in preparing male bees for mating, and suggests that developmentally-driven neural restructuring can occur in males, even in species where it is absent in females.

Prospects in Connecting Genetic Variation to Variation in Fertility in Male Bees

Bees are economically and ecologically important pollinating species. Managed and native bee species face increasing pressures from human-created stressors such as habitat loss, pesticide use, and introduced pathogens. There has been increasing attention towards how each of these factors impacts fertility, especially sperm production and maintenance in males. Here, we turn our attention towards another important factor impacting phenotypic variation: genetics. Using honey bees as a model, we explore the current understanding of how genetic variation within and between populations contributes to variation in sperm production, sperm maintenance, and insemination success among males. We conclude with perspectives and future directions in the study of male fertility in honey bees and non-Apis pollinators more broadly, which still remain largely understudied.

Courtship with two spoons-Anatomy and presumed function of the bizarre antennae of Cardiocondyla zoserka ant males

Mating in ants often occurs on the wing during nuptial flights or on the ground when scattered female sexuals attract males by pheromones. In both scenarios, there is little opportunity for males to engage in prolonged aggressive competition or elaborate courtship displays. Male morphology is therefore adapted to locating female sexuals and mating, and it lacks specific weapons or other traits associated with courtship. In contrast, sexuals of the ant genus Cardiocondyla typically mate in their natal nests. As a consequence, in many species winged males have been replaced by wingless fighter or territorial males, which kill or expel rival males with their strong mandibles and show complex mating behavior. However, no wingless males are known from Cardiocondyla zoserka from West Africa, and instead, winged males have evolved a bizarre secondary sexual trait: uniquely shaped antennae with spoon-like tips that show heavily sculptured ventral surfaces with numerous invaginations. We here report on the courtship behavior of C. zoserka males and describe antennal glands with class 3 gland cells, which presumably secrete a close range sex pheromone. Antennal glands have not yet been found in males of other ant species, including a close relative of C. zoserka, suggesting that in ants with intranidal mating sexual selection can rapidly lead to highly divergent adaptations and the evolution of novel structures.

The plasticity of lifespan in social insects

One of the central questions of ageing research is why lifespans of organisms differ so tremendously among related taxa and, even more surprising, among members of the same species. Social insects provide a particularly pronounced example for this. Here, we review previously published information on lifespan plasticity in social insects and provide new data on worker lifespan in the ant Cardiocondyla obscurior, which because of its relatively short lifespan is a convenient model to study ageing. We show that individual lifespan may vary within species with several reproductive and social traits, such as egg-laying rate, queen number, task, colony size and colony composition. For example, in Cardiocondyla, highly fecund queens live longer than reproductively less active queens, and workers tend to live longer when transferred into a novel social environment or, as we show with new data, into small colonies. We hypothesize that this plasticity of lifespan serves to maximize the reproductive output of the colony as a whole and thus the inclusive fitness of all individuals. The underlying mechanisms that link the social environment or reproductive status with lifespan are currently unresolved. Several studies in honeybees and ants indicate an involvement of nutrient-sensing pathways, but the details appear to differ among species. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Emigrating Together but Not Establishing Together: A Cockroach Rides Ants and Leaves

AbstractSymbionts of ant colonies can hitchhike on winged ant reproductives (alates) during colony nuptial flights. Attaphila fungicola Wheeler, a miniature cockroach that lives in the nests of Texas leaf-cutter ants (Atta texana Buckley), hitchhikes on female alates (winged queens). Hitchhiking roaches are presumably vertically transmitted from leaf-cutter parent colonies to daughter colonies, remaining with female alates as they transition into foundresses (workerless queens); however, foundresses have limited resources and high mortality rates. Rather than remaining with foundresses likely to die (vertical transmission), roaches might abandon them during dispersal to infect higher-quality later stages of colony development (female alate-vectored transmission). In field experiments, I find evidence for female alate-vectored transmission and discover that roaches use a second hitchhiking step (riding foraged plant material) to infect established colonies. This work reveals a novel relationship between host dispersal and symbiont transmission and shows that colony development can be an important selection pressure on transmission.

Metabolite Support of Long-Term Storage of Sperm in the Spermatheca of Honeybee (Apis mellifera) Queens

The polyandrous mating system of honeybees (Apis mellifera L.) has garnered widespread attention. Long-lived honeybee queens only mate early in maturation, and the sperm obtained from the aerial mating is stored in the spermatheca. The maintenance of sperm viability in the spermatheca is an intriguing and complex process. However, the key physiological and biochemical adaptations underlying the long-term storage of sperm remain unclear. Analysis of the metabolite profile could help better understand the biology of the spermatheca and offer insights into the breeding and conservation of honeybees and even pest control strategies. Here, the changes in metabolites in the spermatheca were quantified between virgin queens and new-laying queens (with stored sperm) via liquid chromatography-mass spectrometry. Compared with virgin queens, changes occurred in lipids and lipid-like molecules, including fatty acyls and glycerophospholipids (GPL), prenol lipids, and sterol lipids, during storage of sperm in new-laying honeybee queens. Furthermore, the metabolic pathways that were enriched with the differentially expressed metabolites were identified and included GPL metabolism, biosynthesis of amino acids, and the mTOR signaling pathway. The likely roles of the pathways in the maintenance and protection of sperm are discussed. The study identifies key metabolites and pathways in the complex interplay of substances that contribute to the long-term storage of sperm and ultimately reproductive success of honeybee queens.

Morphological evolution in the ant reproductive caste

The evolution of eusociality has led to considerable changes in the general hymenopteran body plan. In particular, the evolution of reproductive division of labour caused the worker caste to be largely freed from the demands involved in reproduction. As a consequence, workers were able to evolve highly specialized morphologies for foraging and colony maintenance, whereas the reproductive caste became specialized for reproduction. Despite these important changes, little is known about the general patterns of morphological evolution within the ant reproductive caste. Our goals here were to characterize morphological variation in the ant reproductive caste and to test whether different sexes display variation in their evolutionary rates. We obtained measurements of 897 specimens from a total of 678 ant species. The shapes of the size distributions were similar between sexes, with queens being larger than males in all traits except for eye length. Contrary to the expectation based on Rensch’s rule, although queens were larger, the degree of dimorphism increased with body size. Finally, there was strong evidence for an accelerated tempo of morphological evolution in queens in relation to males. These results represent the first comprehensive treatment of morphological variation in the ant reproductive caste and provide important new insights into their evolution.

Histopathological Findings in Testes from Apparently Healthy Drones of Apis mellifera ligustica

It is well known that factors acting on the decrease of population of honeybees, can act on the male and female reproductive system, compromising the fertility of queens and drones. While there are many studies on female fertility, only a few studies have focused on male fertility and the possible alterations of the reproductive system. The testes of 25 samples of adult drones of Apis mellifera ligustica were analyzed by histopathology using an innovative histological processing technique and the alterations that were found are here described. Most of the samples showed unaltered testes but, in some cases, samples showed degenerated seminiferous tubules, while others appeared immature. Although a limited number of samples were analyzed, the results obtained displayed that histopathological alterations of the testes exist also in honeybees and that more interest should be put to the matter, as honeybees could be considered as bioindicators for endocrine disruptors. Future studies on a larger number of samples are necessary to analyze how different environmental factors can act and induce alterations in the honeybee reproductive system.

Antennal Proteome of the Solenopsis invicta (Hymenoptera: Formicidae): Caste Differences in Olfactory Receptors and Chemosensory Support Proteins

Little is known about the expression pattern of odorant and pheromone transporters, receptors, and deactivation enzymes in the antennae of ants carrying out different tasks. In order to begin filling in this information gap, we compared the proteomes of the antennae of workers and males of the red fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae). Male ants do not perform any colony work, and their only activity is to leave the nest on a mating flight. Previous studies showed that male ants express fewer types of odorant receptors than workers. Thus, we expected to find large differences between male and worker antennae for expression of receptors, transporters, and deactivators of signaling chemicals. We found that the abundance of receptors was consistent with the expected caste-specific signaling complexity, but the numbers of different antenna-specific transporters and deactivating enzymes in males and workers were similar. It is possible that some of these proteins have antenna-specific functions that are unrelated to chemosensory reception. Alternatively, the similar complexity could be a vestige of ant progenitors that had more behaviorally active males. As the reduced behavior of male ants evolved, the selection process may have favored a complex repertoire of transporters and deactivating enzymes alongside a limited repertoire of odorant receptors.

Superorganismal anisogamy: queen-male dimorphism in eusocial insects

Colonies of insects such as ants and honeybees are commonly viewed as 'superorganisms', with division of labour between reproductive 'germline-like' queens and males and 'somatic' workers. On this view, properties of the superorganismal colony are comparable with those of solitary organisms to such an extent that the colony itself can be viewed as a unit analogous to an organism. Thus, the concept of a superorganism can be useful as a guide to thinking about life history and allocation traits of colonies as a whole. A pattern that seems to reoccur in insects with superorganismal societies is size dimorphism between queens and males, where queens tend to be larger than males. It has been proposed that this is analogous to the phenomenon of anisogamy at the level of gametes in organisms with separate sexes; more specifically, it is suggested that this caste dimorphism may have evolved via similar selection pressures as gamete dimorphism arises in the 'gamete competition' theory for the evolution of anisogamy. In this analogy, queens are analogous to female gametes, males are analogous to male gametes, and colony survival is analogous to zygote survival in gamete competition theory. Here, we explore if this question can be taken beyond an analogy, and whether a mathematical model at the superorganism level, analogous to gamete competition at the organism level, may explain the caste dimorphism seen in superorganismal insects. We find that the central theoretical idea holds, but that there are also significant differences between the way this generalized 'propagule competition' theory operates at the levels of solitary organisms and superorganisms. In particular, we find that the theory can explain superorganismal caste dimorphism, but compared with anisogamy evolution, a central coevolutionary link is broken, making the requirements for the theory to work less stringent than those found for the evolution of anisogamy.

Sexually dimorphic traits and male fertility in a paper wasp

Females of many social Hymenoptera commit their reproductive potential to a single male; they mate once and never re-mate. Sexual selection theory predicts that under this regimen female precopulatory mate choice is crucial. The targets of female choice, male secondary sexual traits, should convey information about the functional fertility of the male. Here, we evaluated semen quality and its correlates in the paper wasp Polistes dominula, a monandric species with a lek-based mating system. The main goal of the study was to test whether sperm viability and the total number of sperm covaried with sexually dimorphic abdominal spots, body size and parasitic infection by Xenos vesparum. Surprisingly, superior male fertility was not predicted by attractive sexual traits, such as little round spots and large body size. We also found no effect of the parasite on male fertility, sexual ornamentation or body size. We found only that sperm viability was positively associated with the number of sperm. Moreover, there was evidence of a counterintuitive relationship between (sexually unattractive) large irregular spots and large body size of dominant successful males. Overall, our results suggest a condition-dependent trade-off between body size and costly sexual signals.

Heat stress during development affects immunocompetence in workers, queens and drones of Africanized honey bees (Apis mellifera L.) (Hymenoptera: Apidae)

Though social insects generally seem to have a reduced individual immunoresponse compared to solitary species, the impact of heat stress on that response has not been studied. In the honey bee, the effect of heat stress on reproductives (queens and males/drones) may also vary compared to workers, but this is currently unknown. Here, we quantified the activity of an enzyme linked to the immune response in insects and known to be affected by heat stress in solitary species: phenoloxidase (PO), in workers, queens and drones of Africanized honey bees (AHBs) experimentally subjected to elevated temperatures during the pupal stage. Additionally, we evaluated this marker in individuals experimentally infected with the entomopathogenic fungus Metarhizium anisopliae. Differences in PO activity were found between sexes and castes, with PO activity generally higher in workers and lower in reproductives. Such differences are associated with the likelihood of exposure to infection and the role of different individuals in the colony. Contrary to our expectation, heat stress did not cause an increase in PO activity equally in all classes of individual. Heat stress during the pupal stage significantly decreased the PO activity of AHB queens, but not that of workers or drones, which more frequently engage in extranidal activity. Experimental infection with Metarhizium anisopliae reduced PO activity in queens and workers, but increased it in drones. Notably, heat stressed workers lived significantly shorter after infection despite exhibiting greater PO activity than queens or drones. We suggest that this discrepancy may be related to trade-offs among immune response cascades in honey bees such as between heat shock proteins and defensin peptides used in microbial defence. Our results provide evidence for complex relationships among humoral immune responses in AHBs and suggest that heat stress could result in a reduced life expectancy of individuals.

Body size and sperm quality in queen- And worker-produced ant males

Workers of many species of social Hymenoptera have functional ovaries and are capable of laying haploid, unfertilized eggs, at least in the absence of a queen. Except for honeybees, it remains largely unknown whether worker-produced males have the same quality as queen-produced males and whether workers benefit in direct fitness by producing their sons. Previous studies in the monogynous ant Temnothorax crassispinus revealed that a high proportion of males in natural and laboratory colonies are worker offspring. Here, we compare longevity, body size, sperm length and sperm viability between queen- and worker-produced males. We either split queenright colonies into queenright and queenless halves or removed the queen from a fraction of the queenright colonies and then examined the newly produced males. Male quality traits varied considerably among colonies but differed only slightly between queen- and worker-produced males. Worker-produced males outnumbered queen-produced males and also had a longer lifespan, but under certain rearing conditions sperm from queen-produced males had a higher viability.

Structural variation in Drosophila melanogaster spermathecal ducts and its association with sperm competition dynamics

The ability of female insects to retain and use sperm for days, months, or even years after mating requires specialized storage organs in the reproductive tract. In most orders, these organs include a pair of sclerotized capsules known as spermathecae. Here, we report that some Drosophila melanogaster females exhibit previously uncharacterized structures within the distal portion of the muscular duct that links a spermatheca to the uterus. We find that these 'spermathecal duct presences' (SDPs) may form in either or both ducts and can extend from the duct into the sperm-storing capsule itself. We further find that the incidence of SDPs varies significantly between genotypes, but does not change significantly with the age or mating status of females, the latter indicating that SDPs are not composed of or stimulated by sperm or male seminal proteins. We show that SDPs affect neither the number of first male sperm held in a spermatheca nor the number of offspring produced after a single mating. However, we find evidence that SDPs are associated with a lack of second male sperm in the spermathecae after females remate. This raises the possibility that SDPs provide a mechanism for variation in sperm competition outcome among females.

Development and comparative morphology of the reproductive system in different aged males of the drywood termite Cryptotermes brevis (Blattaria, Isoptera, Kalotermitidae)

Termites are eusocial cockroaches, which have received great attention due to their diversity of reproductive strategies. Although these novelties allow new interpretations concerning the mating biology of these insects, studies highlighting the structure of the reproductive system are limited to some termite lineages. Here we provide the first comparative analysis of the reproductive system of a drywood termite, using different aged males of Cryptotermes brevis as models. This species represents an important structural pest in tropical regions, and most aspects of its reproductive biology remain unknown, especially on males. The reproductive apparatus of C. brevis is equipped with paired testes, composed of seven testicular lobes, in which developing spermatozoa are located. The basal portion of the lobes connects to the vasa deferentia and transport spermatozoa to a pair of enlarged chambers, the seminal vesicles. These structures join in a median ejaculatory duct, which opens to the external region through a retractile penis. Spermatozoa were observed in all C. brevis males, exhibiting elongated morphology and measuring about 10 μm in length/4 μm in width. Compared with last-instar nymphs and alates, functional kings showed enlarged testes and seminal vesicles, as well as an intense secretory activity towards the lumen of the latter structures. Histochemical tests evidenced strongly PAS and xylidine Ponceau positive reactions of the secretion only in functional kings, indicating the occurrence of glycoproteins. Thus, we suggest that morphophysiological changes establish during the maturation of the reproductive system in C. brevis.

Evolutionary relationship of fat body endoreduplication and queen fecundity in termites

Endoreduplication or nuclear genome replication without cell division is widely observed in the metabolically active tissues of plants and animals. The fat body cells of adult female insects produce abundant yolk proteins and become polyploid, which is assumed to accelerate egg production. Recently, it was reported that in termites, endopolyploidy in the fat body occurs only in queens but not in the other females; however, the relationship between the fecundity and ploidy level in the fat body remains unclear. Termite queens exhibit a huge variation in their egg-producing capacity among different species; queens in the species with a foraging lifestyle, in which workers leave the nest to forage outside, are much more fecund than those in the species living in a single piece of wood. In this study, we conducted ploidy analyses on three foragings and three wood-dwelling termites via flow cytometry. In all the species, the fat body of queens contained significantly more polyploid cells than that of other nonreproductive females, considering their body size effect. However, the male fat body, which is not involved in yolk production, did not show consistency in polyploid cell numbers among the species studied. Moreover, highly fecund queens in foraging termites exhibit higher levels of endopolyploidy in their fat body than those with less fecundity in wood-dwelling termites. These results suggest that endopolyploidy in the fat body of termite queens can boost their egg production, and the level of endopolyploidy in their fat body is linked to their fecundity. Our study provides a novel insight into the evolutionary relationship between endoreduplication and caste specialization in social insects.

Seminal fluid compromises visual perception in honeybee queens reducing their survival during additional mating flights

Queens of social insects make all mate-choice decisions on a single day, except in honeybees whose queens can conduct mating flights for several days even when already inseminated by a number of drones. Honeybees therefore appear to have a unique, evolutionarily derived form of sexual conflict: a queen’s decision to pursue risky additional mating flights is driven by later-life fitness gains from genetically more diverse worker-offspring but reduces paternity shares of the drones she already mated with. We used artificial insemination, RNA-sequencing and electroretinography to show that seminal fluid induces a decline in queen vision by perturbing the phototransduction pathway within 24–48 hr. Follow up field trials revealed that queens receiving seminal fluid flew two days earlier than sister queens inseminated with saline, and failed more often to return. These findings are consistent with seminal fluid components manipulating queen eyesight to reduce queen promiscuity across mating flights.

For social insects like honeybees it is beneficial if their queens mate with many males, because genetic diversity can protect the hive against parasites. Early in life, a honeybee queen has a short period of time in which she can fly out to mate with males before returning to the hive with all the sperm needed to last for a lifetime. Queens that have mated on their first flight may embark on additional mating flights over a few consecutive days to further increase genetic variability in their offspring. This is problematic for a male that has already mated because the more males that inseminate the queen the fewer offspring will carry on his specific genes. This results in sexual conflict between males and queens over the number of mating flights.

In many animals, males manipulate females using molecules in seminal fluid to reduce the chances of the female mating again and honeybee males may use a similar strategy. Previous studies revealed that insemination alters the activity of genes related to vision in a honeybee queen’s brain. This could be one way for the males to prevent queens from embarking on additional mating flights.

Now, Liberti et al. find support for this idea by showing that seminal fluid can indeed trigger changes in the activity of vision-related genes in the brains of honeybee queens, which in turn reduce a queen’s opportunity to complete additional mating flights. Queens inseminated with seminal fluid were less responsive to light compared to queens that were exposed to saline instead. Electronic tracking devices affixed to queens showed that the seminal fluid-exposed queens left for mating flights sooner but were more likely to get lost and to not return to their hives compared to the saline-exposed queens.

The experiments support the idea of a sexual arms race in honeybees. Males use seminal fluid to cause rapid deteriorating vision in queens, thus reducing their likelihood of leaving the hive to mate again and to find males when they do fly again. The queens try to counteract these effects by leaving for mating flights sooner, thereby increasing offspring genetic diversity and the success of their colonies. Further studies will be needed to find out how the honeybee sexual arms race varies across seasons, bee races, and geographic ranges. Such information will be useful for honeybee breeding programs, which rely on queen mating success and hive genetic diversity to ensure hive health.

Population and colony structure of an ant with territorial males, Cardiocondyla venustula

Background

Many species of social insects have large-scale mating and dispersal flights and their populations are therefore often relatively homogenous. In contrast, dispersal on the wing appears to be uncommon in most species of the ant genus Cardiocondyla, because its males are wingless and the winged queens mate in their natal nests before dispersing on foot. Here we examine the population structure of C. venustula from South Africa. This species is of particular interest for the analysis of life history evolution in Cardiocondyla, as it occupies a phylogenetic position between tropical species with multi-queen (polygynous) colonies and fighting males and a Palearctic clade with single-queen colonies and mutually peaceful males. Males of C. venustula exhibit an intermediate strategy between lethal fighting and complete tolerance – they mostly engage in non-lethal fights and defend small territories inside their natal nests. We investigated how this reproductive behavior influences colony and population structure by analyzing samples on two geographic scales in South Africa: a small 40 × 40m² plot and a larger area with distances up to 5 km between sampling sites in Rietvlei Nature Reserve near Pretoria.

Results

Colonies were found to be facultatively polygynous and queens appear to mate only with a single male. The extraordinarily high inbreeding coefficient suggests regular sib-mating. Budding by workers and young queens is the predominant mode of colony-founding and leads to high population viscosity. In addition, some queens appear to found colonies independently or through adoption into foreign nests.

Conclusion

While C. venustula resembles tropical Cardiocondyla in queen number and mating frequency, it differs by the absence of winged disperser males. Dispersal by solitary, mated queens on foot or by short flights and their adoption by alien colonies might promote gene flow between colonies and counteract prolonged inbreeding. The abundance of suitable habitat and the high density of nests facilitate the spread of this species by budding and together with the apparent resistance against inbreeding make it a highly successful pioneer species and invader of degraded and man-made habitats.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1448-6) contains supplementary material, which is available to authorized users.

Ants as Object of Gerontological Research

Social insects with identical genotype that form castes with radically different lifespans are a promising model system for studying the mechanisms underlying longevity. The main direction of progressive evolution of social insects, in particular, ants, is the development of the social way of life inextricably linked with the increase in the colony size. Only in a large colony, it is possible to have a developed polyethism, create large food reserves, and actively regulate the nest microclimate. The lifespan of ants hugely varies among genetically similar queens, workers (unproductive females), and males. The main advantage of studies on insects is the determinism of ontogenetic processes, with a single genome leading to completely different lifespans in different castes. This high degree of determinacy is precisely the reason why some researchers (incorrectly) call a colony of ants the "superorganism", emphasizing the fact that during the development, depending on the community needs, ants can switch their ontogenetic programs, which influences their social roles, ability to learn (i.e., the brain [mushroom-like body] plasticity), and, respectively, the spectrum of tasks performed by a given individual. It has been shown that in many types of food behavior, older ants surpass young ones in both performing the tasks and transferring the experience. The balance between the need to reduce the "cost" of non-breeding individuals (short lifespan and small size of workers) and the benefit from experienced long-lived workers possessing useful skills (large size and "non-aging") apparently determines the differences in the lifespan and aging rate of workers in different species of ants. A large spectrum of rigidly determined ontogenetic trajectories in different castes with identical genomes and the possibility of comparison between "evolutionarily advanced" and "primitive" subfamilies (e.g., Formicinae and Ponerinae) make ants an attractive object in the studies of both normal aging and effects of anti-aging drugs.

Protein-Level Interactions as Mediators of Sexual Conflict in Ants

All social insects with obligate reproductive division of labor evolved from strictly monogamous ancestors, but multiple queen-mating (polyandry) arose de novo, in several evolutionarily derived lineages. Polyandrous ant queens are inseminated soon after hatching and store sperm mixtures for a potential reproductive life of decades. However, they cannot re-mate later in life and are thus expected to control the loss of viable sperm because their lifetime reproductive success is ultimately sperm limited. In the leaf-cutting ant Atta colombica,, the survival of newly inseminated sperm is known to be compromised by seminal fluid of rival males and to be protected by secretions of the queen sperm storage organ (spermatheca). Here we investigate the main protein-level interactions that appear to mediate sperm competition dynamics and sperm preservation. We conducted an artificial insemination experiment and DIGE-based proteomics to identify proteomic changes when seminal fluid is exposed to spermathecal fluid followed by a mass spectrometry analysis of both secretions that allowed us to identify the sex-specific origins of the proteins that had changed in abundance. We found that spermathecal fluid targets only seven (2%) of the identified seminal fluid proteins for degradation, including two proteolytic serine proteases, a SERPIN inhibitor, and a semen-liquefying acid phosphatase. In vitro, and in vivo, experiments provided further confirmation that these proteins are key molecules mediating sexual conflict over sperm competition and viability preservation during sperm storage. In vitro, exposure to spermathecal fluid reduced the capacity of seminal fluid to compromise survival of rival sperm in a matter of hours and biochemical inhibition of these seminal fluid proteins largely eliminated that adverse effect. Our findings indicate that A. colombica, queens are in control of sperm competition and sperm storage, a capacity that has not been documented in other animals but is predicted to have independently evolved in other polyandrous social insects.

Patterns of sexual size dimorphism in stingless bees: Testing Rensch's rule and potential causes in highly eusocial bees (Hymenoptera: Apidae, Meliponini)

Eusocial insects offer a unique opportunity to analyze the evolution of body size differences between sexes in relation to social environment. The workers, being sterile females, are not subject to selection for reproductive function providing a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other kinds of natural selection. Patterns of sexual size dimorphism (SSD) and testing of Rensch's rule controlling for phylogenetic effects were analyzed in the Meliponini or stingless bees. Theory predicts that queens may exhibit higher selection for fecundity in eusocial taxa, but contrary to this, we found mixed patterns of SSD in Meliponini. Non-Melipona species generally have a female-biased SSD, while all analyzed species of Melipona showed a male-biased SSD, indicating that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. The phylogenetic regressions revealed that the rate of divergence has not differed between the two castes of females and the males, that is, stingless bees do not seem to follow Rensch's rule (a slope >1), adding this highly eusocial taxon to the various solitary insect taxa not conforming with it. Noteworthy, when Melipona was removed from the analysis, the phylogenetic regressions for the thorax width of males on queens had a slope significantly smaller than 1, suggesting that the evolutionary divergence has been larger in queens than males, and could be explained by stronger selection on female fecundity only in non-Melipona species. Our results in the stingless bees question the classical explanation of female-biased SSD via fecundity and provide a first evidence of a more complex determination of SSD in highly eusocial species. We suggest that in highly eusocial taxa, additional selection mechanisms, possibly related to individual and colonial interests, could influence the evolution of environmentally determined traits such as body size.

Strong, but incomplete, mate choice discrimination between two closely related species of paper wasp

Paper wasps (genus Polistes) are one of the most species-rich genera of social insect. Prior studies have found that male coloration, male colour pattern, territory choice and female caste are potential drivers of intraspecific mate choice in paper wasps. However, there has been no formal assessment of interspecific mate choice in this group; therefore, the mechanism driving diversification in paper wasps remains an open question. In this study, we measured interspecific and intraspecific mating behaviour between two closely related species of paper wasps, Polistes fuscatus and Polistes metricus. These two species have ample opportunity to interbreed because P. fuscatus and P. metricus forage, nest and mate in the same habitats. We tested the strength of reproductive isolation between these species using no-choice and choice mating trials. Our results show strong, symmetric, prezygotic isolation between P. fuscatus and P. metricus. Males discriminated between conspecifics and heterospecifics but attempted to mate with females of the other species in ~10% of heterospecific mating trials. Female wasps were more discriminating than males and probably evaluated species identity and male quality through visual or olfactory cues. We additionally report sexual dimorphism in P. metricus body size.

Exposure to pesticides during development negatively affects honey bee (Apis mellifera) drone sperm viability

Honey bee (Apis mellifera) colonies invest a substantial amount of colony resources in the production of drones during the reproductive season to enable mating with virgin queens from nearby colonies. Recent studies have shown significant differences in the production of sperm cells that are viable (i.e., sperm viability) and can fertilize an ovule among sexually mature drones that are exposed to different environmental conditions during development or as adults. In particular, sperm viability may be negatively affected during drone development from exposure to pesticides in contaminated beeswax. To assess whether sperm viability is negatively affected during drone development from exposure to beeswax contaminated with in-hive pesticides, we compared the viability of sperm collected from drones reared in pesticide-free beeswax with that of drones reared in beeswax contaminated with field-relevant concentrations of the pesticides most commonly found in wax from commercial beekeeping operations in the United States. These pesticides include the miticides fluvalinate, coumaphos and amitraz, and the agro-chemicals chlorothalonil and chlorpyrifos. Sperm from drones collected at 10 and 18 days post emergence were classified as viable or non-viable to calculate sperm viability. For all pesticide treatment groups, drones that were reared in pesticide-laden beeswax had lower sperm viability compared to those reared in pesticide-free beeswax. This difference was especially pronounced among drones reared in miticide-laden wax. Our results reinforce the notion that pesticide contamination of beeswax negatively affects the reproductive quality of drones, which can affect the queens they mate with, ultimately compromising colony health.

Methuselah's daughters: Paternal age has little effect on offspring number and quality in Cardiocondyla ants

Male age may directly or indirectly affect the fitness of their female mating partners and their joint progeny. While in some taxa of insects, old males make better mates and fathers, young males excel in others. Males of most social Hymenoptera are relatively short lived and because of testis degeneration have only a limited sperm supply. In contrast, the wingless fighter males of the ant Cardiocondyla obscurior live for several weeks and produce sperm throughout their lives. Wingless males engage in lethal combat with rival males and the winner of such fights can monopolize mating with all female sexuals that emerge in their nests over a prolonged timespan. Here, we investigate if male age has an influence on sperm quality, the queen's lifespan and productivity, and the size and weight of their offspring. Queens mated to one-week or six-week-old males did not differ in life expectancy and offspring production, but the daughters of young males were slightly heavier than those of old males. Our data suggest negligible reproductive senescence of C. obscurior males even at an age, which only few of them reach. This matches the reproductive strategy of Cardiocondyla ants, in which freshly emerging female sexuals rarely have the option to mate with males other than the one present in their natal nest.

High expression of the breast cancer susceptibility gene BRCA1 in long-lived termite kings

Aging is associated with the accumulation of DNA damage. High expression of DNA repair genes has been suggested to contribute to prolonged lifespan in several organisms. However, the crucial DNA repair genes contributing to longevity remain unknown. Termite kings have an extraordinary long lifespan compared with that of non-reproductive individuals such as workers despite being derived from the same genome, thus providing a singular model for identifying longevity-related genes. In this study, we demonstrated that termite kings express higher levels of the breast cancer susceptibility gene BRCA1 than other castes. Using RNA sequencing, we identified 21 king-specific genes among 127 newly annotated DNA repair genes in the termite Reticulitermes speratus. Using quantitative PCR, we revealed that some of the highly expressed king-specific genes were significantly upregulated in reproductive tissue (testis) compared to their expression in somatic tissue (fat body). Notably, BRCA1 gene expression in the fat body was more than 4-fold higher in kings than in workers. These results suggest that BRCA1 partly contributes to DNA repair in somatic and reproductive tissues in termite kings. These findings provide important insights into the linkage between BRCA1 gene expression and the extraordinary lifespan of termite kings.

Delayed sexual maturity in males of Vespa velutina

Vespa velutina var nigrithorax (Lepelletier, 1835) is an invasive predator of bees accidentally introduced in France in 2004, and it is having a serious impact on apiculture and ecosystems. Studying the reproduction of an invasive species is key to assess its population dynamic. This study explores the sexual maturation of V. velutina males and the evolution of their fertility. The main studied parameters were physiologic (spermiogenesis, spermatogenesis) and anatomic (testes size and structure, head width). Two populations of males were described based on their emergence period: early males in early summer or classic males in autumn. Each testis has an average of 108 testicular follicles. Spermatogenesis is synchronous, with only 1 sperm production wave, and completed, on average, at 10.3 d after emergence with the degeneration of the testes. The sperm counts in seminal vesicles of mature males are 3 × 10⁶ in October/November and 0.8 × 10⁶ in June. In comparison, females store 0.1 × 10⁶ sperm in their spermathecae. The early males emerged from colonies made by fertilized queens. The reproductive potential of these early males seemed limited, and their function in the colony is discussed. The sperm stock evolution in autumn males suggests the occurrence of a reproductive pattern of male competition for the access to females and a single copulation per male. The synchronicity of male and foundress emergences and sexual maturation is of primary importance for the mating success and the future colony development.