Female volatiles as sex attractants in the invasive population of Vespa velutina nigrithorax

Due to its huge invasion potential and specialization in honeybee predation, the invasive hornet Vespa velutina nigrithorax represents a high-concern species under both an ecological and economical perspective. In light of the development of specific odorant attractants to be used in sustainable control strategies, we carried out both behavioural assays and chemical analyses to investigate the possibility that, in the invasive population of V. velutina nigrithorax, reproductive females emit volatile pheromones to attract males, as demonstrated in a Chinese non-invasive population. We focused on the secretions produced by sternal and venom glands; because of the volatility and complexity of their composition, both of them could potentially allow an attraction and a species-specific response, decreasing therefore non-target species by-catches. Results of chemical analyses and behavioural assays showed that venom volatiles, although population-specific, are unlikely candidates as male attractants since they do not differ in composition or in quantity between reproductive females and workers and do not attract males. Conversely, sternal gland secretion differs between female castes for the presence of some ketoacids exclusive of gynes already reported as sex pheromones for the non-invasive subspecies V. velutina auraria. Despite such a difference, males are attracted by the sternal gland secretion of both workers and gynes. These results provide a first step to understand the reproductive biology of V. velutina nigrithorax in its invasive range and to develop effective and sustainable management strategies for the species.

Wide-scale analysis of protein expression in head and thorax of Aedes albopictus females

The recently available genome of Aedes albopictus - the most worldwide-spread human arbovirus vector - has revealed a large genome repertory and a great plasticity which are believed to have contributed to the species success as an invasive species and opened the way to genomic, transcriptomic and proteomic studies. We carried out the first wide-scale quantitative proteomic analysis of Ae. albopictus female head and thorax by means of a 'shotgun' approach based on nano liquid chromatography-high resolution mass spectrometry associated to protein Label Free Quantification (LFQ) which allows to assess differences in protein expression between tissues and different physiological stages. We identified 886 and 721 proteins in heads and thoraxes respectively, 5 of which were exclusively expressed in thoraxes and 170 in heads, consistently with the more complex head physiology. Head-protein expression was found to be highly divergent between virgin and mated females and limited before and after blood-feeding and oviposition. The large repertoire of proteins identified represents an instrumental source of data for genome annotation and gene-expression studies, and may contribute to studies aimed at investigating the molecular bases of physiological processes of this successful invasive species.

Candidate biomarkers for mosquito age-grading identified by label-free quantitative analysis of protein expression in Aedes albopictus females

We applied a "shotgun" approach based on nanoliquid chromatography-high resolution mass spectrometry associated to label free quantification (LFQ) to identify proteins varying with age, independently from the physiological state, in Aedes albopictus, a mosquito species which in the last decades invaded temperate regions in North America and Europe, creating concerns for associated high nuisance and risk of arbovirus transmission. The combined "shotgun" and LFQ approach was shown to be highly suitable to simultaneously compare several biological samples, as needed in a study aimed to analyze different age-groups and physiological states of adult mosquito females. The results obtained represent the first wide-scale analysis of protein expression in Ae. albopictus females: >1000 and 665 proteins were identified from few micrograms of crude protein extracts of mosquito heads and thoraxes, respectively. Six of these proteins were shown to significantly vary from 2- to 16-day-old females, independently from their physiological state (i.e. virgin, mated, host-seeking, blood-fed, and gravid).

BIOLOGICAL SIGNIFICANCE

Mosquito-borne diseases, such as malaria, dengue and other arboviroses, are a persistent cause of global mortality and morbidity, affecting hundreds of thousands of people. Billions of people living in tropical areas are at risk of being bitten every day by an infective mosquito female and the spread of tropical species such as Aedes albopictus to temperate areas is creating alarm in the northern hemisphere. Mosquito longevity is a critical factor affecting mosquito-borne pathogen transmission cycles and the mosquito capacity to transmit pathogens. However, large scale analyses of the age structure of mosquito field populations is hampered by the lack of optimal age-grading approaches. Our findings open new perspectives for the development of reliable, simple and cheap protein-based assays to age-grade Ae. albopictus females and, most likely, other mosquito species of higher medical relevance, such as the main dengue vector, Aedes aegypti, and the major Afrotropical malaria vectors. These assays would greatly contribute to epidemiological studies aimed at defining the actual vectorial capacity of a given mosquito species. Moreover, they would be very valuable in assessing the effectiveness of mosquito control interventions based on the relative ratio between young and old individuals before and after the intervention.

Ant repellent effect of the sternal gland secretion ofPolistes dominulus (Christ) andP. sulcifer (Zimmermann). (Hymenoptera: Vespidae)

The long-chain carboxylic acids identified in the sternal gland secretion ofPolistes dominulus andP. sulcifer females were tested individually on three species of ants,Crematogaster scutellaris, Formica cunicularia, andLasius sp., in order to verify if they have a repellent effect. The unsaturated acids (palmitoleic, linoleic, and oleic) act as repellents of all three ant species, while the saturated acids (lauric, myristic, palmitic, and stearic) have no effect. The mixture reproducing the secretion of the sternal glands ofP. dominulus maintained its repellency for at least four days.

Comparative analysis of epicuticular lipid profiles of sympatric and allopatric field populations of Anopheles gambiae s.s. molecular forms and An. arabiensis from Burkina Faso (West Africa)

We analysed by gas chromatography-mass spectrometry (GC-MS) and Gas Chromatography-Flame Ionization Detector (GC-FID) the epicuticular lipid profiles of field females of the major Afro-tropical malaria vector, Anopheles gambiae. The samples were collected in three villages in Burkina Faso (West Africa), where An. gambiae M and S molecular forms and An. arabiensis live sympatrically. The aim was to compare the cuticular hydrocarbon (CHC) composition of individual field specimens of these three taxa, to highlight possible differences among them. All the samples analysed by GC-MS (55 individuals and eight pools) were characterized by the same 48 CHCs and 10 oxygenated compounds. The 19 most abundant CHCs were quantified in 174 specimens by GC-FID: quantitative intra-taxon differences were found between allopatric populations of both An. arabiensis and S-form. Inter-taxa quantitative differences in the relative abundances of some hydrocarbons between pairs of sympatric taxa were also found, which appear to be mainly linked to local situations, with the possible exception of diMeC(35) between An. arabiensis and S-form. Moreover, MeC(29) shows some degree of differentiation between S- and M-form in all three villages. Possible causes of these differences are discussed.

Expression of odorant-binding proteins and chemosensory proteins in some Hymenoptera

The expression of chemosensory proteins (CSPs) and odorant-binding proteins (OBPs) in individuals of different castes and ages have been monitored in three species of social hymenopterans, Polistes dominulus (Hymenoptera, Vespidae), Vespa crabro (Hymenoptera, Vespidae) and Apis mellifera (Hymenoptera, Apidae), using PCR with specific primers and polyclonal antibodies. In the paper wasp P. dominulus, OBP is equally expressed in antennae, wings and legs of all castes and ages, while CSP is often specifically present in antennae and in some cases also in legs. In the vespine species V. crabro CSP is antennal specific, while OBP is also expressed in legs and wings. The three CSPs and the five OBPs of A. mellifera show a complex pattern of expression, where both classes of proteins include members specifically expressed in antennae and others present in other parts of the body. These data indicate that at least in some hymenopteran species CSPs are specifically expressed in antennae and could perform roles in chemosensory perception so far assigned only to OBPs.

Soluble proteins of chemical communication in the social wasp Polistes dominulus

Members of the odorant-binding protein (OBP) and chemosensory protein (CSP) families were identified and characterised in the sensory tissues of the social wasp Polistes dominulus (Hymenoptera: Vespidae). Unlike most insects so far investigated, OBPs were detected in antennae, legs and wings, while CSPs appeared to be preferentially expressed in the antennae. The OBP is very different from the homologous proteins of other Hymenopteran species, with around 20% of identical residues, while the CSP appears to be much better conserved. Both OBP and CSP, not showing other post-translational modifications apart from disulphide bridges, were expressed with high yields in a bacterial system. Cysteine pairing in the recombinant and native proteins follows the classical arrangements described for other members of these classes of proteins. OBPs isolated from the wings were found to be associated with a number of long-chain aliphatic amides and other small organic molecules. Binding of these ligands and other related compounds was measured for both recombinant OBP and CSP.

Recognition of social parasites as nest-mates: adoption of colony-specific host cuticular odours by the paper wasp parasite Polistes sulcifer

Colonies of the polistine wasp Polistes dominulus are parasitized by the permanent worker-less social parasite Polistes sulcifer. After usurpation of the host colony, parasite females are characterized by a change in the relative proportions of their cuticular hydrocarbons to match those of the host species. In this paper we present evidence from field data and laboratory experiments that P. sulcifer females adopt a colony-specific host odour that facilitates their acceptance by host females of the usurped colony. Presentation experiments demonstrate that parasite females are recognized as foreign individuals by workers of other parasitized nests. We show that the modification of parasite cuticular compounds is sufficient for this recognition. This provides evidence that, after invasion, P. sulcifer queens do not require appeasement or propaganda substances for their acceptance by host colonies. Furthermore, multivariate discriminant analysis of the cuticular hydrocarbon proportions of the parasites after usurpation assigns the parasites together with P. dominulus females of their own host colony. To the authors' knowledge, this is the first confirmation that social parasites adopt colony-specific host odours.

(Z)-3-hexenyl (R)-3-hydroxybutanoate: a male specific compound in three North American decorator wasps Eucerceris rubripes, E. conata and E. tricolor

The males of the decorator wasps Eucerceris have been observed to display abdomen-dragging behavior on plants surrounding their nest. It is thought that this applies a territorial-marking sex pheromone that serves to alert females to the males' territory for courtship and mating. The extracts of three species E. rubripes, E. conata, and E. tricolor have been analyzed by GC-MS. The gas chromatograms revealed the presence, in large quantities, of one common volatile compound in the male head extracts. By analogy with the closely related bee wolves Philanthus, we believe the pheromone is produced in the mandibular glands. Utilizing mass spectrometry, microreactions, FT-IR, and 1H and 13C NMR spectroscopy the structure of the pheromone was proposed to be (Z)-3-hexenyl 3-hydroxybutanoate, which was confirmed by synthesis. The absolute configuration of the chiral center was determined to be R for the three species by preparing the Mosher esters of the insect samples and comparing their GC retention times with a synthetic sample of known absolute configuration. In addition 2- and 3-hexenoic acid and some aromatic compounds were also found in varying quantities in both males and females along with hydrocarbons and fatty acids, although no species-specific profiles emerged.

Social hackers: integration in the host chemical recognition system by a paper wasp social parasite

Obligate social parasites in the social insects have lost the worker caste and the ability to establish nests. As a result, parasites must usurp a host nest, overcome the host recognition system, and depend on the host workers to rear their offspring. We analysed cuticular hydrocarbon profiles of live parasite females of the paper wasp social parasite Polistes sulcifer before and after usurpation of host nests, using the non-destructive technique of solid-phase micro-extraction. Our results reveal that hydrocarbon profiles of parasites change after usurpation of host nests to match the cuticular profile of the host species. Chemical evidence further shows that the parasite queen changes the odour of the nest by the addition of a parasite-specific hydrocarbon. We discuss the possible role of this in the recognition and acceptance of the parasite and its offspring in the host colony.