A 'social' gland in a solitary wasp? The postpharyngeal gland of female European beewolves (Hymenoptera, Crabronidae)

Sexual selection and the evolution of male pheromone glands in philanthine wasps (Hymenoptera, Crabronidae)

Background

Sexual selection is thought to promote evolutionary changes and diversification. However, the impact of sexual selection in relation to other selective forces is difficult to evaluate. Male digger wasps of the tribe Philanthini (Hymenoptera, Philanthinae) scent mark territories to attract receptive females. Consequently, the organs for production and storage of the marking secretion, the mandibular gland (MG) and the postpharyngeal gland (PPG), are subject to sexual selection. In female Philanthini, these glands are most likely solely subject to natural selection and show very little morphological diversity. According to the hypothesis that sexual selection drives interspecific diversity, we predicted that the MG and PPG show higher interspecific variation in males than in females. Using histological methods, 3D-reconstructions, and multivariate statistical analysis of morphological characters, we conducted a comparative analysis of the MG and the PPG in males of 30 species of Philanthini and three species of the Cercerini and Aphilanthopsini, two related tribes within the Philanthinae.

Results

We found substantial interspecific diversity in gland morphology with regard to gland incidence, size, shape and the type of associated secretory cells. Overall there was a phylogenetic trend: Ensuing from the large MGs and small PPGs of male Cercerini and Aphilanthopsini, the size and complexity of the MG was reduced in male Philanthini, while their PPG became considerably enlarged, substantially more complex, and associated with an apparently novel type of secretory cells. In some clades of the Philanthini the MG was even lost and entirely replaced by the PPG. However, several species showed reversals of and exceptions from this trend. Head gland morphology was significantly more diverse among male than among female Philanthinae.

Conclusion

Our results show considerable variation in male head glands including the loss of an entire gland system and the evolution of a novel kind of secretory cells, confirming the prediction that interspecific diversity in head gland morphology is higher in male than in female Philanthini. We discuss possible causes for the remarkable evolutionary changes in males and we conclude that this high diversity has been caused by sexual selection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0963-6) contains supplementary material, which is available to authorized users.

Comparative morphology of the postpharyngeal gland in the Philanthinae (Hymenoptera, Crabronidae) and the evolution of an antimicrobial brood protection mechanism

BACKGROUND

Hymenoptera that mass-provision their offspring have evolved elaborate antimicrobial strategies to ward off fungal infestation of the highly nutritive larval food. Females of the Afro-European Philanthus triangulum and the South American Trachypus elongatus (Crabronidae, Philanthinae) embalm their prey, paralyzed bees, with a secretion from a complex postpharyngeal gland (PPG). This coating consists of mainly unsaturated hydrocarbons and reduces water accumulation on the prey's surface, thus rendering it unfavorable for fungal growth. Here we (1) investigated whether a North American Philanthus species also employs prey embalming and (2) assessed the occurrence and morphology of a PPG among females of the subfamily Philanthinae in order to elucidate the evolution of prey embalming as an antimicrobial strategy.

RESULTS

We provide clear evidence that females of the North American Philanthus gibbosus possess large PPGs and embalm their prey. The comparative analyses of 26 species from six genera of the Philanthinae, using histological methods and 3D-reconstructions, revealed pronounced differences in gland morphology within the subfamily. A formal statistical analysis based on defined characters of the glands confirmed that while all members of the derived tribe Philanthini have large and complex PPGs, species of the two more basal tribes, Cercerini and Aphilanthopsini, possess simple and comparatively small glands. According to an ancestral state reconstruction, the complex PPG most likely evolved in the last common ancestor of the Philanthini, thus representing an autapomorphy of this tribe.

CONCLUSION

Prey embalming, as described for P. triangulum and T. elongatus, and now also for P. gibbosus, most probably requires a complex PPG. Hence, the morphology and size of the PPG may allow for inferences about the origin and distribution of the prey embalming behavior within the Philanthinae. Based on our results, we suggest that prey embalming has evolved as an antimicrobial strategy in and is restricted to the tribe Philanthini, which seems to face exceptional threats with regard to fungal infestations of their larval provisions.

Morphology, chemistry and function of the postpharyngeal gland in the South American digger wasps Trachypus boharti and Trachypus elongatus

Microbes pose severe threats to animals as competitors or pathogens and strongly affect the evolution of life history traits like parental care. Females of the European beewolf Philanthus triangulum, a solitary digger wasp, provision their offspring with paralyzed honeybees and embalm them with the secretion from large postpharyngeal glands (PPG) that contain mainly unsaturated hydrocarbons. This coating changes the physico-chemical properties of the prey surface, causes a reduction of water condensation and retards growth of mold fungi. Here we examined the closely related South American genus Trachypus, which shows a life-history similar to Philanthus. We investigated whether Trachypus spp. also possess PPGs and embalm larval provisions. Using histological methods and 3D reconstructions we show that Trachypus boharti and T. elongatus possess PPGs that are similar to P. triangulum but somewhat smaller. The ultrastructure of the gland epithelium suggests that the gland content is at least partly sequestered from the hemolymph. Chemical analyses using gas chromatography / mass spectrometry revealed that both the cuticle and PPGs of Trachypus contain mainly unsaturated long-chain hydrocarbons. The gland of T. boharti additionally contains long-chain ketones. The hydrocarbons from the PPG of T. elongatus occurred on prey bees excavated from nests in the field but not on conspecific control bees. While the embalming only slightly elevated the amount of hydrocarbons on prey bees, the proportion of unsaturated hydrocarbons, which is crucial for the antifungal effect, was significantly increased. The Trachypus species under study possess PPGs that are very similar to the PPG of P. triangulum with regard to morphology, ultrastructure and chemistry. Moreover, we provide clear evidence that T. elongatus females embalm their prey, presumably as a means of prey preservation. The observed differences among Trachypus and Philanthus in gland size and prey embalming may have evolved in response to divergent ecological conditions.

Structure, chemical composition and putative function of the postpharyngeal gland of the emerald cockroach wasp, Ampulex compressa (Hymenoptera, Ampulicidae)

The postpharyngeal gland (PPG) plays a major role in the social integration of ant colonies. It had been thought to be restricted to ants but was recently also described for a solitary wasp, the European beewolf (Philanthus triangulum). This finding posed the question whether the gland has evolved independently in the two taxa or has been inherited from a common ancestor and is hence homologous. The latter alternative would be supported if a PPG was found in more basal taxa. Therefore, we examined a species at the base of the Apoidea, the solitary ampulicid wasp Ampulex compressa, for the existence of a PPG. Both sexes of this species possess a cephalic gland that branches off the posterior part of the pharynx, is lined by a cuticular intima and surrounded by a monolayered epithelium with the epithelial cells bearing long hairs. Most of these morphological characteristics conform to those of the PPG of ants and beewolves. Chemical analysis of the gland content revealed that it contains mainly hydrocarbons and that there is a congruence of the pattern of hydrocarbons in the gland, on the cuticle, and in the hemolymph, as has also been reported for both ants and beewolves. Based on these morphological and chemical results we propose that the newly described cephalic gland is a PPG and discuss its possible function in A. compressa. The present study supports the view of a homologous origin of the PPG in the aculeate Hymenoptera.

Is the postpharyngeal gland of a solitary digger wasp homologous to ants? Evidence from chemistry and physiology

The postpharyngeal gland (PPG) was thought to be restricted to ants where it serves a crucial function in the generation of the colony odour. Recently, head glands that closely resemble the PPG of ants were discovered in females of a solitary digger wasp, the European beewolf. The function of this gland necessarily differs from ants: beewolf females apply the secretion of their PPG onto the bodies of paralysed honeybees that serve as larval provisions in order to delay fungus growth. Since ants and digger wasps are not closely related, the occurrence of this gland in these two taxa might either be due to convergent evolution or it is a homologous organ inherited from a common ancestor. Here we test the hypothesis that the PPGs of both taxa are homologous by comparing characteristics of chemical composition and physiology of the PPG of beewolves and ants. Based on reported characteristics of the PPG content of ants, we tested three predictions that were all met. First, the PPG of beewolves contained mainly long-chain hydrocarbons and very few compounds with functional groups. Second, the composition of hydrocarbons in the beewolf PPG was similar to that of the hemolymph. Taking the structure of the gland epithelium and the huge requirements of beewolf females for gland secretion into account this result suggests that the content of the PPG is also sequestered from the hemolymph in beewolves. Third, the chemical composition of the PPG and the cuticle was similar in beewolves since cuticular hydrocarbons derive either from the hemolymph or the PPG. Taking the considerable morphological similarities into account, our results support the hypothesis of a homologous origin of the PPG in beewolves and ants.

Mandibular glands of male European beewolves, Philanthus triangulum (Hymenoptera, Crabronidae)

Males of a solitary digger wasp, the European beewolf, Philanthus triangulum, possess large mandibular glands that have been reported to produce a scent marking pheromone. We analysed the morphology and ultrastructure of these glands using light microscopy as well as scanning and transmission electron microscopy. The paired glands are located laterally in the head and each side consists of a larger and a smaller part. Both parts possess a collecting duct each with distinct openings at the mandible base. However, the collecting duct of the larger part is additionally connected to the pharynx through a lateral extension. The collecting ducts are bordered by a monolayered epithelium lined with cuticle that exhibits conspicuous ramified protuberances. About 1400 acini consisting of class 3 gland cells surround the ducts and are connected to them through conducting canals. The main components in the cytoplasm of these gland cells are mitochondria, well-developed smooth endoplasmatic reticulum, and electron lucent vesicles suggesting a high secretory activity. The connection between the large gland parts and the pharynx suggests that the secretion of the mandibular glands might not only be delivered directly onto the mandibles but might also be transported to and stored in the postpharyngeal gland.

The chemistry of the postpharyngeal gland of female European beewolves

Females of the European beewolf, Philanthus triangulum, possess a large glove-shaped gland in the head, the postpharyngeal gland (PPG). They apply the content of the PPG to their prey, paralyzed honeybees, where it delays fungal infestation. Here, we describe the chemical composition of the gland by using combined GC-MS, GC-FTIR, and derivatization. The PPG of beewolves contains mainly long-chain unsaturated hydrocarbons (C23-C33), lower amounts of saturated hydrocarbons (C14-C33), and minor amounts of methyl-branched hydrocarbons (C17-C31). Additionally, the hexane-soluble gland content is comprised of small amounts of an unsaturated C25 alcohol, an unknown sesquiterpene, an octadecenylmethylester, and several long-chain saturated (C25, C27) and unsaturated (C23-C27) ketones, some of which have not yet been reported as natural products. Surprisingly, we found a dimorphism with regard to the major component of the PPG with some females having (Z)-9-pentacosene, whereas others have (Z)-9-heptacosene as their predominant component. The biological relevance of the compounds for the prevention of fungal growth on the prey and the significance of the chemical dimorphism are discussed.

A cuckoo in wolves' clothing? Chemical mimicry in a specialized cuckoo wasp of the European beewolf (Hymenoptera, Chrysididae and Crabronidae)

Background

Host-parasite interactions are among the most important biotic relationships. Host species should evolve mechanisms to detect their enemies and employ appropriate counterstrategies. Parasites, in turn, should evolve mechanisms to evade detection and thus maximize their success. Females of the European beewolf (Philanthus triangulum, Hymenoptera, Crabronidae) hunt exclusively honeybee workers as food for their progeny. The brood cells containing the paralyzed bees are severely threatened by a highly specialized cuckoo wasp (Hedychrum rutilans, Hymenoptera, Chrysididae). Female cuckoo wasps enter beewolf nests to oviposit on paralyzed bees that are temporarily couched in the nest burrow. The cuckoo wasp larva kills the beewolf larva and feeds on it and the bees. Here, we investigated whether H. rutilans evades detection by its host. Since chemical senses are most important in the dark nest, we hypothesized that the cuckoo wasp might employ chemical camouflage.

Results

Field observations suggest that cuckoo wasps are attacked by beewolves in front of their nest, most probably after being recognized visually. In contrast, beewolves seem not to detect signs of the presence of these parasitoids neither when these had visited the nest nor when directly encountered in the dark nest burrow.

In a recognition bioassay in observation cages, beewolf females responded significantly less frequently to filter paper discs treated with a cuticular extract from H. rutilans females, than to filter paper discs treated with an extract from another cuckoo wasp species (Chrysis viridula). The behavior to paper discs treated with a cuticular extract from H. rutilans females did not differ significantly from the behavior towards filter paper discs treated with the solvent only.

We hypothesized that cuckoo wasps either mimic the chemistry of their beewolf host or their host's prey. We tested this hypothesis using GC-MS analyses of the cuticles of male and female beewolves, cuckoo wasps, and honeybee workers. Cuticle extracts of Hedychrum nobile (Hymenoptera: Chrysididae) and Cerceris arenaria (Hymenoptera: Crabronidae) were used as outgroups. There was little congruence with regard to cuticular compounds between H. rutilans females and honeybees as well as females of C. arenaria and H. nobile. However, there was a considerable similarity between beewolf females and H. rutilans females. Beewolf females show a striking dimorphism regarding their cuticular hydrocarbons with one morph having (Z)-9-C25:1 and the other morph having (Z)-9-C27:1 as the major component. H. rutilans females were more similar to the morph having (Z)-9-C27:1 as the main component.

Conclusion

We conclude that H. rutilans females closely mimic the composition of cuticular compounds of their host species P. triangulum. The occurrence of isomeric forms of certain compounds on the cuticles of the cuckoo wasps but their absence on beewolf females suggests that cuckoo wasps synthesize the cuticular compounds rather than sequester them from their host. Thus, the behavioral data and the chemical analysis provide evidence that a specialized cuckoo wasp exhibits chemical mimicry of the odor of its host. This probably allows the cuckoo wasp to enter the nest with a reduced risk of being detected by olfaction and without leaving traitorous chemical traces.

Males of a solitary wasp possess a postpharyngeal gland

The postpharyngeal gland has long been thought to occur only in ants. Here we characterize, by use of light and electron microscopy as well as 3D reconstruction based on nuclear magnetic resonance (NMR) imaging data, a large cephalic gland reservoir of males of a solitary digger wasp, the European beewolf, Philanthus triangulum. Several lines of evidence suggest that this reservoir is a postpharyngeal gland. The gland reservoir originates from the posterior part of the pharynx and consists of two pairs of unbranched tubular structures that occupy a large portion of the head capsule. Its wall is composed of a unicellular epithelium that is lined by a cuticle. The gland contains a blend of hydrocarbons and compounds with functional groups, and we show that the hydrocarbon fraction of the pheromone is congruent with the hydrocarbons on the cuticle. We discuss the implications of our findings for the evolution of the postpharyngeal gland in ants.

A selfish function of a "social" gland? A postpharyngeal gland functions as a sex pheromone reservoir in males of the solitary wasp Philanthus triangulum

The postpharyngeal gland (PPG) has long been assumed to be restricted to ants, where it mainly functions in the maintenance of social integrity. Recently, a PPG has been described in both sexes of a solitary digger wasp, the European beewolf, Philanthus triangulum (Hymenoptera, Crabronidae). Female beewolves use the contents of their PPG to embalm their honeybee prey to delay microbial growth. Here we show that in male beewolves, the PPG serves as a reservoir of the pheromone used to scent-mark their territories. Gas chromatography-mass spectrometry (GC-MS) analysis of PPG contents identified 55 substances including long-chain aliphatic hydrocarbons, and 13 substances with functional groups. The composition was consistent with the composition of the marking pheromone of male European beewolves described earlier. Comparisons of the PPG contents, and total-head extracts showed a strong congruency, suggesting that total-head extracts can be used for the analysis of marking secretion in beewolves. Furthermore, we found a dimorphism in the composition of the PPG contents, based on significant differences in the proportions of seven compounds between the two morphs.

Food wrapping with the postpharyngeal gland secretion by females of the European beewolf Philanthus triangulum

Ground-nesting animals share their habitat with countless microorganisms that can play important roles as pathogens or competitors for food resources. Thus, species that store food in the soil, either for themselves or for their progeny, must protect these resources against microbial degradation. Females of the European beewolf, Philanthus triangulum, hunt honeybees as provisions for their brood and store the paralyzed prey in their subterranean nests. A previous study had shown that females lick the surface of prey before oviposition and that this licking treatment delays mold growth. Here, we showed that females apply large amounts of a secretion from their postpharyngeal glands onto the surface of their prey during the licking behavior. Inhibition-zone assays showed that comparatively large amounts of the gland secretion had no direct antimycotic effect. We discuss our findings with regard to other possible mechanisms of the postpharyngeal gland secretion against fungal growth.