Venom alkaloid and cuticular hydrocarbon profiles are associated with social organization, queen fertility status, and queen genotype in the fire ant Solenopsis invicta

A supergene in seaweed flies modulates male traits and female perception

Supergenes, tightly linked sets of alleles, offer some of the most spectacular examples of polymorphism persisting under long-term balancing selection. However, we still do not understand their evolution and persistence, especially in the face of accumulation of deleterious elements. Here, we show that an overdominant supergene in seaweed flies, Coelopa frigida, modulates male traits, potentially facilitating disassortative mating and promoting intraspecific polymorphism. Across two continents, the Cf-Inv(1) supergene strongly affected the composition of male cuticular hydrocarbons (CHCs) but only weakly affected CHC composition in females. Using gas chromatography-electroantennographic detection, we show that females can sense male CHCs and that there may be differential perception between genotypes. Combining our phenotypic results with RNA-seq data, we show that candidate genes for CHC biosynthesis primarily show differential expression for Cf-Inv(1) in males but not females. Conversely, candidate genes for odorant detection were differentially expressed in both sexes but showed high levels of divergence between supergene haplotypes. We suggest that the reduced recombination between supergene haplotypes may have led to rapid divergence in mate preferences as well as increasing linkage between male traits, and overdominant loci. Together this probably helped to maintain the polymorphism despite deleterious effects in homozygotes.

Chemistry and Functions of Imported Fire Ant Venom

In the United States, imported fire ants are often referred to as red imported fire ants, Solenopsis invicta Buren, black imported fire ants, S. richteri Forel, and their hybrid (S. invicta × S. richteri). Due to their aggressive stings and toxic venom, imported fire ants pose a significant threat to public health, agriculture, and ecosystem health. However, venom plays a vital role in the survival of fire ants by serving various crucial functions in defense, foraging, and colony health maintenance. Numerous reviews and book chapters have been published on fire ant venom. Due to its medical importance and the expanding global distribution of these ants, fire ant venom research remains an active and highly productive area, leading to the discovery of new components and functions. This review summarizes the recent advances in our understanding of fire ant venom chemistry and its functions within fire ant colonies.

Synthesis and Insecticidal Activity of Fire Ant Venom Alkaloid-Based 2-Methyl-6-alkyl-Δ1,6-piperideines

2,6-dialkylpiperideines found in the venom of Solenopsis (Hymenoptera, Formicidae) fire ants are a range of compounds possessing various biological activities. A series of racemic 2-methyl-6-alkyl-Δ^1,6-piperideines were synthesized for chemical confirmation of the natural products found in fire ant venom, and the evaluation of their biological activity. Synthetic Δ^1,6-piperideines and the natural compounds in the cis-alkaloid fraction of Solenopsis invicta had identical mass spectra and retention times. Their insecticidal activities against the third-instar larvae of cotton bollworm (Helicoverpa armigera) were evaluated by using injection and topical application methods. All three compounds exhibited no lethal effect at concentrations of 0.05–0.4 mol/L by topical treatment, but moderate lethal effect at 0.4 mol/L through injection treatment. Compound 6a showed significantly higher activity than the natural insecticide nicotine. The differences in activity among compounds 6b, 6c and nicotine were not significant. The elongation of the carbon chain at the 6-position of the piperideine ring appears to decrease insecticidal activity.

On the Biological Diversity of Ant Alkaloids

Ants have outstanding capacity to mediate inter- and intraspecific interactions by producing structurally diverse metabolites from numerous secretory glands. Since Murray Blum's pioneering studies dating from the 1950s, there has been a growing interest in arthropod toxins as natural products. Over a dozen different alkaloid classes have been reported from approximately 40 ant genera in five subfamilies, with peak diversity within the Myrmicinae tribe Solenopsidini. Most ant alkaloids function as venom, but some derive from other glands with alternative functions. They are used in defense (e.g., alarm, repellants) or offense (e.g., toxins) but also serve as antimicrobials and pheromones. We provide an overview of ant alkaloid diversity and function with an evolutionary perspective. We conclude that more directed integrative research is needed. We suggest that comparative phylogenetics will illuminate compound diversification, while molecular approaches will elucidate genetic origins. Biological context, informed by natural history, remains critical not only for research about focal species, but also to guide applied research.

Characterization of Queen Supergene Pheromone in the Red Imported Fire Ant Using Worker Discrimination Assays

Ants use chemical signals to communicate for various purposes related to colony function. Social organization in the red imported fire ant, Solenopsis invicta, is determined by the Sb supergene, with colonies of the monogyne (single-queen) form lacking the element and colonies of the polygyne (multiple-queen) form possessing it. Polygyne workers accept new reproductive queens in their nest, but only those carrying Sb; young winged queens lacking this genetic element are executed as they mature sexually in their natal nest or as they attempt to enter a foreign nest to initiate reproduction after mating and shedding their wings. It has been suggested that queen supergene genotype status is signaled to workers by unsaturated cuticular hydrocarbons, while queen reproductive status is signaled by piperidines (venom alkaloids). We used high-throughput behavioral assays to study worker acceptance of paper dummies dosed with fractions of extracts of polygyne queens, or blends of synthetic counterparts of queen cuticular compounds. We show that the queen supergene pheromone comprises a blend of monoene and diene unsaturated hydrocarbons. Our assays also reveal that unsaturated hydrocarbons elicit discrimination by polygyne workers only when associated with additional compounds that signal queen fertility. This synergistic effect was obtained with a polar fraction of queen extracts, but not by the piperidine alkaloids, suggesting that the chemical(s) indicating queen reproductive status are compounds more polar than cuticular hydrocarbons but are not the piperidine alkaloids. Our results advance understanding of the role of chemical signaling that is central to the regulation of social organization in an important invasive pest and model ant species.

Non-kin Cooperation in Ants

Eusociality represents an extreme form of social behavior characterized by a reproductive division of labor. Eusociality necessarily evolved through kin selection, which requires interactions among related individuals. However, many eusocial taxa also show cooperation between non-kin groups, challenging the idea that cooperative actions should only occur among relatives. This review explores the causes and consequences of non-kin cooperation in ants. Ants display a diversity of behaviors that lead to non-kin cooperation within and between species. These interactions occur among both reproductive and non-reproductive individuals. The proximate and ultimate mechanisms leading to non-kin cooperative interactions differ substantially depending on the biotic and abiotic environment. We end this review with directions for future research and suggest that the investigation of non-kin cooperative actions provides insight into processes leading to social evolution.

Preference and effect of gustatory sense on sugar-feeding of fire ants

BACKGROUND

The red imported fire ant is one of the notorious species of ants all over the world. Sugar is one of the most important components of food and necessary for the survival of ants. Because more than 70% food of fire ants is honeydew produced by Homopteran insects such as aphids and scales.

METHODOLOGY

It is well known that beetles, flies, and honey bees can recognize the sugar taste through their legs and antennae, but in the case of fire ants, no records regarding gustatory sense were published. In the current study, considering the importance of sugar bait, we investigated the gustatory sense of the fire ant workers to sucrose via behavioral sequence and gustatory behavior. First, the feeding sequence (ethogram) of the fire ant workers on most preferred sugar (sucrose) solution was observed and categorized. Secondly, the gustatory behavior of treated fire ant workers (without flagellum and foreleg tarsi treated with HCL solution) was observed on the sucrose solution. In addition, using scanning electron microscopy (SEM) techniques, we identified the possible porous sensilla types on antenna flagellum and foreleg tarsi of fire ant workers.

RESULTS

Based on the results of feeding sequence, foreleg tarsi of workers were the main body appendages in the detection of the sucrose droplet as compared to antennae flagellum and palps. Feeding time of treated workers with HCL solution was significantly decreased on sucrose solution as compared to those workers having no flagellum. While both types of treated workers have less feeding time in comparison to normal workers. Based on the results of feeding sequence analysis and feeding time, it is indicating that the foreleg tarsi of workers play a more important role in the detection of sucrose solution as compared to antennae flagellum. Based on the SEM results, sensilla chaetic, trichoid II, and basiconic I and II have a clear pore at their tip. This study provides a substantial basis for elucidating the gustatory function of antennal and tarsal sensilla on appendages of fire ant workers to sugars and further baits improvement for the management of fire ants.

Simple inheritance, complex regulation: Supergene-mediated fire ant queen polymorphism

The fire ant Solenopsis invicta exists in two alternate social forms: monogyne nests contain a single reproductive queen and polygyne nests contain multiple reproductive queens. This colony-level social polymorphism corresponds with individual differences in queen physiology, queen dispersal patterns and worker discrimination behaviours, all evidently regulated by an inversion-based supergene that spans more than 13 Mb of a "social chromosome," contains over 400 protein-coding genes and rarely undergoes recombination. The specific mechanisms by which this supergene influences expression of the many distinctive features that characterize the alternate forms remain almost wholly unknown. To advance our understanding of these mechanisms, we explore the effects of social chromosome genotype and natal colony social form on gene expression in queens sampled as they embarked on nuptial flights, using RNA-sequencing of brains and ovaries. We observe a large effect of natal social form, that is, of the social/developmental environment, on gene expression profiles, with similarly substantial effects of genotype, including: (a) supergene-associated gene upregulation, (b) allele-specific expression and (c) pronounced extra-supergene trans-regulatory effects. These findings, along with observed spatial variation in differential and allele-specific expression within the supergene region, highlight the complex gene regulatory landscape that emerged following divergence of the inversion-mediated Sb haplotype from its homologue, which presumably largely retained the ancestral gene order. The distinctive supergene-associated gene expression trajectories we document at the onset of a queen's reproductive life expand the known record of relevant molecular correlates of a complex social polymorphism and point to putative genetic factors underpinning the alternate social syndromes.

Different lethal treatments induce changes in piperidine (1,1'-(1,2-ethanediyl)bis-) in the epidermal compounds of red imported fire ants and affect corpse-removal behavior

Corpse-removal behavior of the red imported fire ant (RIFA) and the effects of lethal substances on RIFA signal communication were investigated in this study. The RIFA corpses, obtained through freezing, ether, 0.25 mg/L thiamethoxam, and starvation to death treatments, and naturally dead red fire ants were subjected to gas chromatography-mass spectrometry to identify the cuticular hydrocarbon profiles that had an effect on the corpse-removal behavior. The results showed that lethal toxic substances altered the epidermal compounds of RIFA and affected their corpse-removal behavior. Lethal toxic substances increased the number of worker touches with corpses and identification time of corpses. In addition, the content of piperidine (1,1'-(1,2-ethanediyl)bis-) on the surface of the corpse was different following the various treatments. Contamination with toxic substances resulted in the increased secretion of piperidine and led to increased identification time of corpses, number of touch with corpses, and total time for removal of corpses. Piperidine content was higher under conditions of natural death (4.67 ± 0.55%) and with thiamethoxam (10.43 ± 0.78%), freezing (0.83 ± 0.25%), and ether treatment (12.50 ± 0.70%) than under starvation treatment (0). The higher content of piperidine led to a longer number of touches with corpses and identification time. Piperidine compounds may be an element in warning information, which could affect the occurrence of different corpse-removal behaviors.

Near-Complete Genome Sequences of New Strains of Nylanderia Fulva Virus 1 from Solenopsis invicta

Nylanderia fulva virus 1 (NfV-1) is a single-stranded positive-sense RNA virus that infects the tawny crazy ant. Three near-complete genomes of NfV-1SI (92% to 94% nucleotide identity to reference strain NfV-1) found infecting the red imported fire ant were determined. The genomes have 10,904 to 10,918 nucleotides and include most of the coding region for the polyprotein.

Nylanderia fulva virus 1 (NfV-1) is a single-stranded positive-sense RNA virus that infects the tawny crazy ant. Three near-complete genomes of NfV-1SI (92% to 94% nucleotide identity to reference strain NfV-1) found infecting the red imported fire ant were determined. The genomes have 10,904 to 10,918 nucleotides and include most of the coding region for the polyprotein.

Development of an LC-MS multivariate nontargeted methodology for differential analysis of the peptide profile of Asian hornet venom (Vespa velutina nigrithorax): application to the investigation of the impact of collection period variation

Insect venom is a highly complex mixture of bioactive compounds, containing proteins, peptides, and small molecules. Environmental factors can alter the venom composition and lead to intraspecific variation in its bioactivity properties. The investigation of discriminating compounds caused by variation impacts can be a key to manage sampling and explore the bioactive compounds. The present study reports the development of a peptidomic methodology based on UHPLC-ESI-QTOF-HRMS analysis followed by a nontargeted multivariate analysis to reveal the profile variance of Vespa velutina venom collected in different conditions. The reliability of the approach was enhanced by optimizing certain XCMS data processing parameters and determining the sample peak threshold to eliminate the interfering features. This approach demonstrated a good repeatability and a criterion coefficient of variation (CV) > 30% was set for deleting nonrepeatable features from the matrix. The methodology was then applied to investigate the impact of collection period variation. PCA and PLS-DA models were used and validated by cross-validation and permutation tests. A slight discrimination was found between winter and summer hornet venom in two successive years with 10 common discriminating compounds. Graphical abstract.

Efficacy of the InvictDetectTM Immunostrip® to Taxonomically Identify the Red Imported Fire Ant, Solenopsis invicta, Using a Single Worker Ant

The early detection and identification of the red imported fire ant Solenopsis invicta are crucial to intercepting and preventing it from becoming established in new areas. Unfortunately, the visual identification of fire ants to species is difficult and ant samples must often be couriered to an expert for positive identification, which can delay control interventions. A lateral flow immunoassay that provides a rapid and portable method for the identification of S. invicta ants was developed and commercialized, and it is available from Agdia, Inc. under the trade name InvictDetect^TM. While the test was 100% accurate when using the recommended minimum sample of three ant workers, InvictDetect^TM was field tested for the first time while using homogenates prepared from single S. invicta workers to determine the effectiveness of the method under these non-recommended conditions. Disregarding social form, the false negative rate was 25.5% for an initial single worker ant test and 10% after a repeat test was performed. The InvictDetect^TM false negative response was independent of worker weight. Though InvictDetect^TM requires a minimum of three worker ants, the test improves upon current identification methods because it can be conducted in the field, be completed in 10–30 min, and requires no special training or expertise.

The fire ant social supergene is characterized by extensive gene and transposable element copy number variation

In the fire ant Solenopsis invicta, a supergene composed of ~600 genes and having two variants, SB and Sb, regulates colony social form. In single queen colonies, all individuals carry only the SB allele, while in multiple queen colonies, some individuals carry the Sb allele. In this study, we characterized genes with copy number variation between SB and Sb-carrying individuals. We showed extensive acquisition of gene duplicates in the Sb genome, with some likely involved in polygyne-related phenotypes. We found 260 genes with copy number differences between SB and Sb, of which 239 have greater copy number in Sb. We observed transposable element (TE) accumulation on Sb, likely due to the accumulation of repetitive elements on the nonrecombining chromosome. We found a weak correlation between TE copy number and differential expression, suggesting some TEs may still be proliferating in Sb while many of the duplicated TEs have presumably been silenced. Among the 115 non-TE genes with higher copy in Sb, enzymes responsible for cuticular hydrocarbon synthesis were highly represented. These include a desaturase and an elongase, both potentially responsible for differential queen odour and likely beneficial for polygyne ants. These genes seem to have translocated into the supergene from other chromosomes and proliferated by multiple duplication events. While the presence of TEs in supergenes is well documented, little is known about duplication of non-TE genes and their possible adaptive role. Overall, our results suggest that gene duplications may be an important factor leading to monogyne and polygyne ant societies.

Has gene expression neofunctionalization in the fire ant antennae contributed to queen discrimination behavior?

Queen discrimination behavior in the fire ant Solenopsis invicta maintains its two types of societies: colonies with one (monogyne) or many (polygyne) queens, yet the underlying genetic mechanism is poorly understood. This behavior is controlled by two supergene alleles, SB and Sb, with ~600 genes. Polygyne workers, having either the SB/SB or SB/Sb genotype, accept additional SB/Sb queens into their colonies but kill SB/SB queens. In contrast, monogyne workers, all SB/SB, reject all additional queens regardless of genotype. Because the SB and Sb alleles have suppressed recombination, determining which genes within the supergene mediate this differential worker behavior is difficult. We hypothesized that the alternate worker genotypes sense queens differently because of the evolution of differential expression of key genes in their main sensory organ, the antennae. To identify such genes, we sequenced RNA from four replicates of pooled antennae from three classes of workers: monogyne SB/SB, polygyne SB/SB, and polygyne SB/Sb. We identified 81 differentially expressed protein-coding genes with 13 encoding potential chemical metabolism or perception proteins. We focused on the two odorant perception genes: an odorant receptor SiOR463 and an odorant-binding protein SiOBP12. We found that SiOR463 has been lost in the Sb genome. In contrast, SiOBP12 has an Sb-specific duplication, SiOBP12b', which is expressed in the SB/Sb worker antennae, while both paralogs are expressed in the body. Comparisons with another fire ant species revealed that SiOBP12b' antennal expression is specific to S. invicta and suggests that queen discrimination may have evolved, in part, through expression neofunctionalization.

Pyridine Alkaloids in the Venom of Imported Fire Ants

Venomous imported fire ants cause significant medical problems. Alkaloids are an important component of imported fire ant venom. Piperidine and piperideine alkaloids have been identified in fire ant venom. In this study, we studied the venom alkaloids of the red imported fire ant, Solenopsis invicta Buren, the black imported fire ant, Solenopsis richteri Forel, and the hybrid, S. invicta × S. richteri. Pyridine alkaloids were detected the first time in fire ants using solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (SPME-GC-MS). The thermal desorption process was manipulated to facilitate the isolation and identification of pyridine alkaloids that were coeluted with piperidine or piperideine alkaloids in GC. After SPME extraction of ant venom, we conducted a series of consecutive GC-MS injections, each with a partial desorption. Hidden pyridine alkaloid peaks were revealed after the overlapping piperidine or piperidiene alkaloid peaks had been desorbed. Using this approach, 10 2-methyl-6-alkyl (or alkenyl)pyridines (1-10) were found the first time in the venom of imported fire ants. Structures of three pyridine alkaloids were confirmed by synthesis, including 2-methyl-6-undecylpyridine (1), 2-methyl-6-tridecylpyridine (7), and 2-methyl-6-pentadecylpyridine (10). We also developed a silica gel column chromatography method to separate the pyridine alkaloids from other alkaloids. Using column chromatography and GC-MS with single ion monitoring at 107 m/z, five pyridine alkaloids were quantified for both workers and female alates of S. invicta and S. richteri.

Joint Evolution of Asexuality and Queen Number in an Ant

Ants exhibit a striking diversity of reproductive systems, varying in traits such as the number of reproductives per colony [1], the mode of daughter production (sexual or asexual) [2], and the mode of caste determination (genetic or environmental) [3]. Species employing mixed reproductive systems present a unique opportunity to explore the causes and consequences of alternative breeding strategies. Mixed reproductive systems in ants include social polymorphism in colony queen number, whereby single-queen (monogyne) and multiple-queen (polygyne) colonies co-occur within species [4-7], and facultative asexuality, in which female offspring may be produced sexually or asexually within colonies [8-13]. Here, we document a remarkable confluence of multiple mixed reproductive systems in the tropical fire ant, Solenopsis geminata, in a population with three important features: (1) polygyne colonies produce workers sexually but queens asexually, whereas monogyne colonies produce both castes sexually; (2) polygyne queens mate with monogyne males to produce workers, but monogyne queens do not mate with polygyne males; and (3) different asexual/polygyne lineages evidently were founded separately by genetically distinct founder queens, which appear to have originated from the same neighboring monogyne population. Multiple asexual/polygyne genomes are transmitted undiluted in this system, but sterile workers produced with sperm from a sexually-reproducing/monogyne population are necessary for the persistence of these lineages. The intersection of social polymorphism, facultative asexuality, and genetic caste determination marks this population of S. geminata as an embodiment of the diversity of ant reproductive systems and suggests previously unknown connections between these phenomena.

Differential Gene Expression in Red Imported Fire Ant (Solenopsis invicta) (Hymenoptera: Formicidae) Larval and Pupal Stages

Solenopsis invicta Buren is an invasive ant species that has been introduced to multiple continents. One such area, the southern United States, has a history of multiple control projects using chemical pesticides over varying ranges, often resulting in non-target effects across trophic levels. With the advent of next generation sequencing and RNAi technology, novel investigations and new control methods are possible. A robust genome-guided transcriptome assembly was used to investigate gene expression differences between S. invicta larvae and pupae. These life stages differ in many physiological processes; of special importance is the vital role of S. invicta larvae as the colonies' "communal gut". Differentially expressed transcripts were identified related to many important physiological processes, including digestion, development, cell regulation and hormone signaling. This dataset provides essential developmental knowledge that reveals the dramatic changes in gene expression associated with social insect life stage roles, and can be leveraged using RNAi to develop effective control methods.

Evolution of Olfactory Functions on the Fire Ant Social Chromosome

Understanding the molecular evolutionary basis of social behavior is a major challenge in evolutionary biology. Social insects evolved a complex language of chemical signals to coordinate thousands of individuals. In the fire ant Solenopsis invicta, chemical signals are involved in the determination of a polymorphic social organization. Single-queen (monogyne) or multiqueen (polygyne) social structure is determined by the "social chromosome," a nonrecombining region containing ∼504 genes with two distinct haplotypes, SB and Sb. Monogyne queens are always SBB, while polygyne queens are always SBb. Workers discriminate monogyne from polygyne queens based on olfactory cues. Here, we took an evolutionary genomics approach to search for candidate genes in the social chromosome that could be responsible for this discrimination. We compared the SB and Sb haplotypes and analyzed the evolutionary rates since their divergence. Notably, we identified a cluster of 23 odorant receptors in the nonrecombining region of the social chromosome that stands out in terms of nonsynonymous changes in both haplotypes. The cluster includes twelve genes formed by recent Solenopsis-specific duplications. We found evidence for positive selection on several tree branches and significant differences between the SB and Sb haplotypes of these genes. The most dramatic difference is the complete deletion of two of these genes in Sb. These results suggest that the evolution of polygyne social organization involved adaptations in olfactory genes and opens the way for functional studies of the molecular mechanisms underlying social behavior.

Cuticular hydrocarbon chemistry, an important factor shaping the current distribution pattern of the imported fire ants in the USA

Two sibling species, Solenopsis richteri and S. invicta, were both introduced into the southern USA from South America in the early 20th century. Today, S. richteri occupies higher latitudes and colder areas, while S. invicta occupies lower latitudes. Between the distributions of the two species, there is a large area of viable hybrid (S. richteri × S. invicta) populations. This study aimed to characterize the forces driving this distribution pattern and the underlying mechanisms. Cuticular hydrocarbons (CHCs) of freshly killed workers of S. invicta, hybrids, and S. richteri were removed using hexane. Both intact and CHCs-extracted workers were subjected to a constant rate of increasing temperature from 10 to 60 °C to obtain relative water loss and the water loss transition temperature (T_c-ant). Mass loss and T_c-ant were both significantly increased with CHCs removal. We then examined the CHC composition of three species. CHC profiles of S. richteri are characterized by significant amounts of short-chain (C₂₃-C₂₇) saturated and unsaturated hydrocarbons. In contrast, profiles of S. invicta consist primarily of long-chain (C₂₇-C₂₉) saturated hydrocarbons; unsaturated alkenes are completely lacking. Hybrid fire ants show intermediate profiles of the two parent species. We measured the melting point (T_m) and water-loss transition temperature of CHC blends (T_c-CHC) of different ant species colonies using differential scanning calorimetry (DSC) and an artificial membrane system, respectively. There were 3-5 T_ms of each CHCs sample of different ant colonies due to their complex chemistry. The highest T_ms (T_m-maxs) of CHCs samples from S. invicta and the hybrid were significantly higher than that from S. richteri. The correlation between T_c-CHC and T_m-max obtained from the same CHCs sample was highly significant. These results reveal that species having higher T_c and T_m-max retain more water under relatively higher temperature, and consequently are able to occupy warmer environments. We conclude that CHC chemistry plays a role in shaping current distribution patterns of S. richteri, S. invicta and their hybrid in the United States.

Positive selection on sociobiological traits in invasive fire ants

The fire ant Solenopsis invicta and its close relatives are highly invasive. Enhanced social cooperation may facilitate invasiveness in these and other invasive ant species. We investigated whether invasiveness in Solenopsis fire ants was accompanied by positive selection on sociobiological traits by applying a phylogenomics approach to infer ancient selection, and a population genomics approach to infer recent and ongoing selection in both native and introduced S. invicta populations. A combination of whole-genome sequencing of 40 haploid males and reduced-representation genomic sequencing of 112 diploid workers identified 1,758,116 and 169,682 polymorphic markers, respectively. The resulting high-resolution maps of genomic polymorphism provide high inference power to test for positive selection. Our analyses provide evidence of positive selection on putative ion channel genes, which are implicated in neurological functions, and on vitellogenin, which is a key regulator of development and caste determination. Furthermore, molecular functions implicated in pheromonal signalling have experienced recent positive selection. Genes with signatures of positive selection were significantly more often those overexpressed in workers compared with queens and males, suggesting that worker traits are under stronger selection than queen and male traits. These results provide insights into selection pressures and ongoing adaptation in an invasive social insect and support the hypothesis that sociobiological traits are under more positive selection than nonsocial traits in such invasive species.

Queen Control or Queen Signal in Ants: What Remains of the Controversy 25 Years After Keller and Nonacs' Seminal Paper?

Ant queen pheromones (QPs) have long been known to affect colony functioning. In many species, QPs affect important reproductive functions such as diploid larvae sexualization and egg-laying by workers, unmated queens (gynes), or other queens. Until the 1990s, these effects were generally viewed to be the result of queen manipulation through the use of coercive or dishonest signals. However, in their seminal 1993 paper, Keller and Nonacs challenged this idea, suggesting that QPs had evolved as honest signals that informed workers and other colony members of the queen's presence and reproductive state. This paper has greatly influenced the study of ant QPs and inspired numerous attempts to identify fertility-related compounds and test their physiological and behavioral effects. In the present article, we review the literature on ant QPs in various contexts and pay special attention to the role of cuticular hydrocarbons (CHCs). Although the controversy generated by Keller and Nonacs' (Anim Behav 45:787-794, 1993) paper is currently less intensively debated, there is still no clear evidence which allows the rejection of the queen control hypothesis in favor of the queen signal hypothesis. We argue that important questions remain regarding the mode of action of QPs, and their targets which may help understanding their evolution.

The evolution of queen control over worker reproduction in the social Hymenoptera

A trademark of eusocial insect species is reproductive division of labor, in which workers forego their own reproduction while the queen produces almost all offspring. The presence of the queen is key for maintaining social harmony, but the specific role of the queen in the evolution of eusociality remains unclear. A long‐discussed scenario is that a queen either behaviorally or chemically sterilizes her workers. However, the demographic and ecological conditions that enable such manipulation are still debated. We study a simple model of evolutionary dynamics based on haplodiploid genetics. Our model is set in the commonly observed case where workers have lost the ability to lay female (diploid) eggs by mating, but retain the ability to lay male (haploid) eggs. We consider a mutation that acts in a queen, causing her to control the reproductive behavior of her workers. Our mathematical analysis yields precise conditions for the evolutionary emergence and stability of queen‐induced worker sterility. These conditions do not depend on the queen's mating frequency. We find that queen control is always established if it increases colony reproductive efficiency, but can evolve even if it decreases colony efficiency. We further derive the conditions under which queen control is evolutionarily stable against invasion by mutant workers who have recovered the ability to lay male eggs.

Fire ant social chromosomes: Differences in number, sequence and expression of odorant binding proteins

Variation in social behavior is common yet our knowledge of the mechanisms underpinning its evolution is limited. The fire ant Solenopsis invicta provides a textbook example of a Mendelian element controlling social organization: alternate alleles of a genetic element first identified as encoding an odorant binding protein (OBP) named Gp-9 determine whether a colony accepts one or multiple queens. The potential roles of such a protein in perceiving olfactory cues and evidence of positive selection on its amino acid sequence made it an appealing candidate gene. However, we recently showed that recombination is suppressed between Gp-9 and hundreds of other genes as part of a >19 Mb supergene-like region carried by a pair of social chromosomes. This finding raises the need to reassess the potential role of Gp-9. We identify 23 OBPs in the fire ant genome assembly, including nine located in the region of suppressed recombination with Gp-9. For six of these, the alleles carried by the two variants of the supergene-like region differ in protein-coding sequence and thus likely in function, with Gp-9 showing the strongest evidence of positive selection. We identify an additional OBP specific to the Sb variant of the region. Finally, we find that 14 OBPs are differentially expressed between single- and multiple-queen colonies. These results are consistent with multiple OBPs playing a role in determining social structure.

Insects, arachnids and centipedes venom: A powerful weapon against bacteria. A literature review

Currently, new antimicrobial molecules extracted or obtained by natural sources, could be a valide alternative to traditional antibiotics. Most of these molecules are represented by antimicrobial peptides (AMPs), which are essential compounds of insect, arachnids and centipedes venom. AMPs, due to their strong effectiveness, low resistance rates and peculiar mode of action, seem to have all the suitable features to be a powerful weapon against several bacteria, especially considering the increasing antibiotic-resistance phenomena. The present literature review focuses on the antibacterial activity of bee, wasp, ant, scorpion, spider and scolopendra crude venom and of their main biological active compounds. After a brief overview of each animal and venom use in folkloristic medicine, this review reports, in a comprehensive table, the results obtained by the most relevant and recent researches carried out on the antibacterial activity of different venom and their AMPs. For each considered study, the table summarizes data concerning minimal inhibitory concentration values, minimal bactericidal concentration values, the methods employed, scientific name and common names and provenience of animal species from which the crude venom and its respective compounds were obtained.

Update on the defensive chemicals of the little black ant, Monomorium minimum (Hymenoptera: Formicidae)

Alkaloids, including 2,5-dialkylpyrrolidines and 2,5-dialkylpyrrolines, have been reported to be components in the venom of little black ants, Monomorium minimum (Buckley). Two fatty amines were recently reported as minor compounds. By analyzing the discharge collected from the stinger apparatus (milking), this study revealed the presence of an additional seven compounds in the defensive secretion of this ant species. Compounds identified were 9-decenyl-1-amine, N-methylenedecan-1-amine, N-methylenedodecan-1-amine, 2-(1-non-8-enyl)-5-(1-hex-5-enyl)-1-pyrroline, N-methyl-2-(hex-5-enyl)-5-nonanyl-1-pyrrolidine, β-springene ((E,E)-7,11,15-trimethyl-3-methylene-1,6,10,14-hexadecatetraene) and neocembrene ((E,E,E)-1-isopropenyl-4,8,12-trimethylcyclotetradeca-3,7,11-triene). β-springene and neocembrene were found only in the defensive secretion of queens. Analyses of the contents of isolated poison and Dufour's glands of the queen indicated that all amines and alkaloids were from the poison gland and that β-springene and neocembrene were from the Dufour's gland. This demonstrated that the defensive secretion in M. minimum queens consists of components from both glands. This is also the first report on the natural occurrence of 9-decenyl-1-amine, N-methylenedecan-1-amine, and N-methyllenedodecan-1-amine.

Chemical communication of queen supergene status in an ant

Traits of interest to evolutionary biologists often have complex genetic architectures, the nature of which can confound traditional experimental study at single levels of analysis. In the fire ant Solenopsis invicta, the presence of a Mendelian 'supergene' is both necessary and sufficient to induce a shift in a fundamental property of social organization, from single-queen (monogyne) to multiple-queen (polygyne) colonies. This selfish genetic element, termed the Social b (Sb) supergene, contains > 600 genes that collectively promote its fitness by inducing the characteristic polygyne syndrome, in part by causing polygyne workers to accept only queens bearing the Sb element (a behaviour termed 'worker Sb discrimination'). Here, we employ a newly developed behavioural assay to reveal that polygyne workers, many of which bear the Sb element, employ chemical cues on the cuticle of queens to achieve worker Sb discrimination, but we found no evidence for such pheromonally mediated worker Sb discrimination in monogyne workers, which universally lack the Sb element. This polygyne worker Sb discrimination was then verified through a 'green beard' effect previously described in this system. We thus have demonstrated that the Sb element is required both for production of relevant chemical cues of queens and for expression of the behaviours of workers that collectively result in worker Sb discrimination. This information fills a critical gap in the map between genotype and complex phenotype in S. invicta by restricting the search for candidate genes and molecules involved in producing this complex social trait to factors associated with the Sb element itself.

Honeybee venom proteome profile of queens and winter bees as determined by a mass spectrometric approach

Venoms of invertebrates contain an enormous diversity of proteins, peptides, and other classes of substances. Insect venoms are characterized by a large interspecific variation resulting in extended lists of venom compounds. The venom composition of several hymenopterans also shows different intraspecific variation. For instance, venom from different honeybee castes, more specifically queens and workers, shows quantitative and qualitative variation, while the environment, like seasonal changes, also proves to be an important factor. The present study aimed at an in-depth analysis of the intraspecific variation in the honeybee venom proteome. In summer workers, the recent list of venom proteins resulted from merging combinatorial peptide ligand library sample pretreatment and targeted tandem mass spectrometry realized with a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS/MS). Now, the same technique was used to determine the venom proteome of queens and winter bees, enabling us to compare it with that of summer bees. In total, 34 putative venom toxins were found, of which two were never described in honeybee venoms before. Venom from winter workers did not contain toxins that were not present in queens or summer workers, while winter worker venom lacked the allergen Api m 12, also known as vitellogenin. Venom from queen bees, on the other hand, was lacking six of the 34 venom toxins compared to worker bees, while it contained two new venom toxins, in particularly serine proteinase stubble and antithrombin-III. Although people are hardly stung by honeybees during winter or by queen bees, these newly identified toxins should be taken into account in the characterization of a putative allergic response against Apis mellifera stings.

Differential necrophoric behaviour of the ant Solenopsis invicta towards fungal-infected corpses of workers and pupae

Necrophoric behaviour is critical sanitation behaviour in social insects. However, little is known about the necrophoric responses of workers towards different developmental stages in a colony as well as its underlying mechanism. Here, we show that Solenopsis invicta workers display distinct necrophoric responses to corpses of workers and pupae. Corpses of workers killed by freezing (dead for <1 h) were carried to a refuse pile, but pupal corpses would take at least 1 day to elicit workers' necrophoric response. Metarhizium anisopliae-infected pupal corpses accelerated the necrophoric behaviour of resident workers, with 47.5% of unaffected corpses and 73.8% infected corpses discarded by 1 day post-treatment). We found that fungus-infected pupal corpses had a higher concentration of fatty acids (palmitic acid, oleic acid and linoleic acid) on their surface. We experimentally confirmed that linoleic and oleic acids would elicit a necrophoric response in workers. The appearance of linoleic and oleic acids appeared to be chemical signals involved in recognition of pupal corpses, and M. anisopliae infection could promote the accumulation of fatty acids on surface of pupal corpses resulting in accelerated necrophoric responses of workers.

Did the fire ant supergene evolve selfishly or socially?

The genetic basis for animal social organization is poorly understood. Fire ants provide one of the rare cases where variation in social organization has been demonstrated to be under genetic control, which amazingly, segregates as a single Mendelian locus. A recent genetic, genomic, and cytological analysis revealed that this locus actually consists of over 600 genes locked together in a supergene that possesses many characteristics of sex chromosomes. The fire ant social supergene also behaves selfishly, and an interesting evolutionary question is whether the genes incorporated first into the social supergene were those for social adaptation, selfish genetic drive, or something else. In depth, functional molecular genetic analysis in fire ants and comparative genomics in other closely related socially polymorphic species will be required to resolve this question.