The scent of senescence: age-dependent changes in the composition of the cephalic gland secretion of the male European beewolf, Philanthus triangulum

Rapid Loss of Resistance to Satyrization in Invasive Mosquitoes and the Effects of Age on Interspecific Mating Frequency

In several areas where Aedes aegypti (L.) (Diptera: Culicidae) and Aedes albopictus (Skuse) (Diptera: Culicidae) have come in contact following successful invasions, Ae. aegypti have been rapidly displaced by Ae. albopictus. Recent work has confirmed that mating interference, in the form of satyrization, is likely a driving factor in these competitive displacements. However, in sites of sympatry, Ae. aegypti females evolve resistance to satyrization, and in the laboratory, satyrization-susceptible Ae. aegypti can evolve resistance within a few generations of cage exposure to Ae. albopictus. The resistance trait may be costly to maintain, as satyrization-resistant females have been shown to invest more time in mate selection. Here, we show that increased satyrization-resistance does not persist in the absence of satyrization pressure, confirming a cost to maintaining this trait. Ae. aegypti lines that had been previously selected for increased satyrization-resistance demonstrated rapid decreases in this trait over eight generations. Support for the hypothesis that condition-dependent mate preferences lead to variation in mating choice among individuals of differing quality within a population has been accumulating. Here we examine how age-related changes in reproductive effort or reproductive value may result in age-dependent mate choice. We hypothesize that older, lower quality individuals may be more likely to accept a subpar mating, in this case an interspecific mating. Our results demonstrate that mosquito age significantly affects interspecific mating rates with older mosquitoes (male and female) engaging in interspecific mating more frequently than younger counterparts (26.32 vs 9.41%). We discuss the possibility of age-related signal variation causing the breakdown of mating barriers with age.

Effect of antibiotic treatment and gamma-irradiation on cuticular hydrocarbon profiles and mate choice in tsetse flies (Glossina m. morsitans)

BACKGROUND

Symbiotic microbes represent a driving force of evolutionary innovation by conferring novel ecological traits to their hosts. Many insects are associated with microbial symbionts that contribute to their host's nutrition, digestion, detoxification, reproduction, immune homeostasis, and defense. In addition, recent studies suggest a microbial involvement in chemical communication and mating behavior, which can ultimately impact reproductive isolation and, hence, speciation. Here we investigated whether a disruption of the microbiota through antibiotic treatment or irradiation affects cuticular hydrocarbon profiles, and possibly mate choice behavior in the tsetse fly, Glossina morsitans morsitans. Four independent experiments that differentially knock down the multiple bacterial symbionts of tsetse flies were conducted by subjecting tsetse flies to ampicillin, tetracycline, or gamma-irradiation and analyzing their cuticular hydrocarbon profiles in comparison to untreated controls by gas chromatography - mass spectrometry. In two of the antibiotic experiments, flies were mass-reared, while individual rearing was done for the third experiment to avoid possible chemical cross-contamination between individual flies.

RESULTS

All three antibiotic experiments yielded significant effects of antibiotic treatment (particularly tetracycline) on cuticular hydrocarbon profiles in both female and male G. m. morsitans, while irradiation itself had no effect on the CHC profiles. Importantly, tetracycline treatment reduced relative amounts of 15,19,23-trimethyl-heptatriacontane, a known compound of the female contact sex pheromone, in two of the three experiments, suggesting a possible implication of microbiota disturbance on mate choice decisions. Concordantly, both female and male flies preferred non-treated over tetracycline-treated flies in direct choice assays.

CONCLUSIONS

While we cannot exclude the possibility that antibiotic treatment had a directly detrimental effect on fly vigor as we are unable to recolonize antibiotic treated flies with individual symbiont taxa, our results are consistent with an effect of the microbiota, particularly the obligate nutritional endosymbiont Wigglesworthia, on CHC profiles and mate choice behavior. These findings highlight the importance of considering host-microbiota interactions when studying chemical communication and mate choice in insects.

Larval rearing temperature influences amount and composition of the marking pheromone of the male beewolf, Philanthus triangulum

Pheromones play an important role for courtship and mating in many insect species, and they are shaped by a complex interaction of genetic and environmental factors. Developmental temperature is known to have a strong influence on adult life history, morphology, and physiology, but little is known about its effect on pheromone characteristics. In the present study, the influence of temperature during larval development on the amount and composition of the complex marking pheromone from the cephalic glands of the adult male beewolf, Philanthus triangulum F. (Hymenoptera: Crabronidae), was investigated. Additionally, the effects of temperature on several life-history traits were examined. European beewolf larvae were reared at three constant temperatures (20, 25, and 30 degrees C). Males reared at 20 degrees C showed longer development times and higher mortality, suggesting that low temperatures constitute stressful conditions for developing larvae. After eclosion, the amount and composition of the scent marking secretion of the adult males was analyzed by coupled gas chromatography-mass spectrometry. Males that had been reared at 20 degrees C had significantly less secretion than individuals reared under warmer conditions (25 degrees C and 30 degrees C). Furthermore, larval rearing temperature had a significant effect on the composition of the adult males' pheromone gland content, with warmer rearing conditions leading to higher relative amounts of compounds with high molecular weight. The results show that the temperature during larval development significantly affected the amount and composition of the content of the male pheromone glands, probably due to physiological constraints and competing processes for limited energetic resources. Thus, the pheromone gland content may contain information on developmental conditions of males, which may have consequences for female mate choice decisions and male reproductive success.

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.

The odor of origin: kinship and geographical distance are reflected in the marking pheromone of male beewolves (Philanthus triangulum F., Hymenoptera, Crabronidae)

BACKGROUND

Pheromones play an important role for mate finding and courtship in many insects. In species where males are the signaling sex, females are expected to choose among potential mates with regard to the emitter's quality and/or genetic compatibility. One important aspect is the balance between negative and positive effects of in- vs. outbreeding. In the present study, we aimed to assess the potential of the territory marking pheromone of European beewolves as an indicator for genetic compatibility in the context of female choice.

RESULTS

We analyzed the sex pheromone composition of male European beewolves (Philanthus triangulum F., Hymenoptera, Crabronidae) from eight different locations across Central Europe (six in Germany, one in England, and one in Italy). The pheromone constitutes a complex blend of various long-chain hydrocarbons (alkanes, alkenes, alcohols, ketones, and a carbon acid). We demonstrate that pheromone composition differs significantly among distant populations (regional scale), among subpopulations (local scale) and between families within subpopulations. The differences in the pheromone blend are positively correlated with geographical distances as might be expected according to an isolation-by-distance model. On a local scale, family membership has a larger effect on pheromone composition than subpopulation affiliation, while the reverse is true for the regional scale.

CONCLUSION

Our results show that male pheromones can contain information on both kinship and geographical origin that may be used by females to choose adaptively among potential mates on the basis of their genetic distance.

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.