Carla Masala

Gustatory Sensitivity and Food Acceptance in Two Phylogenetically Closely Related Papilionid Species: Papilio hospiton and Papilio machaon

In herbivorous insects, food selection depends on sensitivity to specific chemical stimuli from host-plants as well as to secondary metabolites (bitter) and to sugars (phagostimulatory). Bitter compounds are noxious, unpalatable or both and evoke an aversive feeding response. Instead, sugars and sugar alcohols play a critical role in determining and enhancing the palatability of foods. We assumed that peripheral taste sensitivity may be related to the width of the host selection. Our model consists of two closely phylogenetically related Papilionid species exhibiting a difference in host plant choice: Papilio hospiton and Papilio machaon. The spike activity of the lateral and medial maxillary styloconic taste sensilla was recorded following stimulation with several carbohydrates, nicotine and NaCl, with the aim of characterizing their gustatory receptor neurons and of comparing their response patterns in the light of their different acceptability in feeding behaviour. The results show that: a) each sensillum houses phagostimulant and phagodeterrent cells; b) the spike activity of the gustatory neurons in response to different taste stimuli is higher in P. hospiton than in P. machaon; c) sugar solutions inhibit the spike activity of the deterrent and salt cells, and the suppression is higher in P. machaon than in P. hospiton. In conclusion, we propose that the different balance between the phagostimulant and phagodeterrent inputs from GRNs of maxillary sensilla may contribute in determining the difference in food choice and host range.

Effects of Avermectins on Olfactory Responses of Culicoides imicola (Diptera: Ceratopogonidae)

Abstract The aim of this study was to examine the role of the olfactory system of the midge Culicoides imicola Kieffer as the major system mediating repellency to antihelminthic avermectins. Incidental observations indicate that treatment with Dectomax or Ivomec (commercial formula of the avermectins doramectin and ivermectin, respectively) protects sheep from infection by bluetongue (BT) viruses. Our electrophysiological data from midge antennae showed that the stimulating effectiveness of l-(+)-lactic acid, butanone, and sheep fleece odor decreased after addition of avermectins. The results show that these antihelminthics affect the olfactory sensitivity of the insect toward the animal host by reducing the response to those compounds that attract the insect, consequently reducing the possibility of biting the sheep and thereby transferring the BT virus.

Morpho-functional identification of abdominal olfactory receptors in the midge Culicoides imicola

The aim of the present study was to examine the presence and the possible role of abdominal olfactory sensilla in Culicoides imicola mediating the search for potential hosts and oviposition sites, by means of a morphological, electrophysiological and behavioural approach. The results reported here show that in the midge C. imicola the whole abdomen, comprising the ovipositor, are endowed with three morphotypes of multiporous sensilla that display olfactory sensitivity towards kairomones related to the host-animal skin such as l-(+)-lactic acid and 1-octen-3-ol, to the host-animal urine such as 3-ethylphenol and 4-propylphenol, and to the potent attractant sesame seed oil. Electrophysiological and behavioural data for the first time suggest in the midge the involvement of abdominal olfactory structures in the choice of the oviposition sites and allow in discussing their possible role in the host-animal localisation. Field experiments showed that light traps baited with the aforementioned compounds elicited a stronger degree of attractiveness on midges with respect to the unbaited traps (control), although to a different extent. Our results, while implying a number of considerations concerning the role of molecules tested as kairomones, also suggest their use in the control of the midge C. imicola population.

Transduction mechanism(s) of Na-saccharin in the blowfly Protophormia terraenovae: evidence for potassium and calcium conductance involvement

The study on transduction mechanisms underlying bitter stimuli is a particularly intriguing challenge for taste researchers. The present study investigates, in the labellar chemosensilla of the blowfly Protophormia terraenovae, the transduction mechanism by which saccharin evokes the response of the “deterrent” cell, with particular attention to the contribution of K+ and Ca2+ current and the role of cyclic nucleotides, since second messengers modulate Ca2+, Cl− and K+ currents to different extents. As assessed by extracellular single-sensillum recordings, our results show that the addition of a Ca2+ chelator such as EGTA or the Ca2+ current blockers SK&F-96365, Mibefradil, Nifedipine and W-7 decrease the response of the “deterrent” cell to saccharin. A similar decreasing effect was also obtained following the addition of 4-aminopyridine, a K+ current blocker. On the contrary, the membrane-permeable cyclic nucleotide 8-bromoguanosine 3′,5′-cyclic monophosphate (8Br-cGMP) activates this cell and shows an additive effect when presented mixed with saccharin. Our results are consistent with the hypothesis that in the labellar chemosensilla of the blowfly both Ca2+ and K+ ions are involved in the transduction mechanism of the “deterrent” cell in response to saccharin. Our results also suggest a possible pathway common to saccharin and 8Br-cGMP.

Antennular Morphology and Contribution of Aesthetascs in the Detection of Food-related Compounds in the Shrimp Palaemon adspersus Rathke, 1837 (Decapoda: Palaemonidae)

Shrimp are an essential ecological component of marine ecosystems, and have commercial importance for human consumption and aquaculture. Like other decapod crustaceans, shrimp rely on chemical senses to detect and localize food resources by means of chemosensilla that are located mainly on the cephalothoracic appendages. Using the shrimp Palaemon adspersus, a model organism with omnivorous feeding behavior, we aimed to provide comparative information on the role of aesthetascs, antennular sensilla, and flicking behavior in food detection. To this end, we examined i) the morphology of antennular sensilla by field emission scanning electron microscopy, ii) the shrimp’s sensitivity to a number of food-related compounds (amino acids and sugars) by means of whole-animal bioassays, and iii) the contribution of the aesthetasc sensilla to food detection. Our results showed that, aside from the aesthetascs, only three other main morphotypes of setae with chemoreceptive features were present in the antennules, thus accounting for relatively simple sensillar equipment. Nevertheless, we found broad-spectrum sensitivity of the shrimp to a number of amino acids (i.e., isoleucine, leucine, methionine, phenylalanine, glycine, tryptophan, cysteine, and tyrosine) and carbohydrates (trehalose, maltose, cellobiose, and fructose) that was consistent with the omnivorous or scavenging habits of the animal. Although aesthetasc ablation attenuated flicking behavior in a chemical stimulus-independent manner, success in detection and short-range localization of food did not rely on the presence of aesthetasc sensilla. This finding confirms the existence of a non-aesthetasc alternative pathway for feeding, with functional redundancy in simple generalist feeder models such as shrimp.

Release mechanism of sex pheromone in the female gypsy moth Lymantria dispar: a morpho-functional approach

A morpho-functional investigation of the sex pheromone-producing area was correlated with the pheromone release mechanism in the female gypsy moth Lymantria dispar. As assessed by male electroantennograms (EAG) and morphological observations, the pheromone gland consists of a single-layered epithelium both in the dorsal and ventral halves of the intersegmental membrane between the 8th and 9th abdominal segments. By using the male EAG as a biosensor of real-time release of sex pheromone from whole calling females, we found this process time coupled with extension movements of the ovipositor. Nevertheless, in females in which normal calling behavior was prevented, pheromone release was detected neither in absence nor in presence of electrical stimulation of the ventral nerve cord/terminal abdominal ganglion (TAG) complex. Tetramethylrhodamine-conjugated dextran amine stainings also confirm the lack of any innervation of the gland from nerves IV to VI emerging from the TAG. These findings indicate that the release of sex pheromone from the glands in female gypsy moths is independent of any neural control exerted by the TAG on the glands, at least by way of its three most caudally located pairs of nerves, and appears as a consequence of a squeezing mechanism in the pheromone-producing area.

Morphological and electrophysiological analysis of tarsal sensilla in the medfly Ceratitis capitata (Wiedemann, 1824) (Diptera: Tephritidae)

Abstract The Mediterranean fruit fly, Ceratitis capitata (Wiedemann, 1824) (Diptera: Tephritidae), is a polyphagous pest in horticulture, mainly targeting pomaceous and citrus fruits. To gain better knowledge about its chemosensory system related to taste, essential for behavioural strategies and localisation of host plants, we examined the tarsal external morphology by high-resolution scanning electron microscopy, focusing on sensilla. Numerous trichoid and chaetica sensilla, related to taste and mechanoreceptor systems, and other types of sensilla are present in tarsal segments. The responses of specific trichoid chemosensilla were also studied through electrophysiological experiments by stimulation with sodium chloride, fructose and four bitter stimuli. Electrophysiological data allowed us to distinguish three types of different neurons: high (“H”), middle (“M”) and small (“S”). Increasing concentrations of sodium chloride enhanced the activity of H cells, while fructose boosted M cells activity. On the contrary, S cells showed no increase in spike activity after stimulation with these substances. Bitter stimuli induced no significant response on these cells. Behavioural assays showed no statistical difference among the responses to sugars, bitter substances and citric acid. Our results on taste stimuli will be useful in increasing knowledge of Ceratitis chemoreception, fundamental in new integrated pest management.

Sensing with the legs: contribution of pereiopods in the detection of food-related compounds in the red swamp crayfish Procambarus clarkii

The red swamp crayfish Procambarus clarkii (Girard, 1852) is one of the most hazardous invasive alien species of freshwater habitats. Like other decapod crustaceans, crayfish rely on chemical senses to detect and localize food resources. By way of extracellular nerve recordings coupled with behavioural bioassays, we investigated the sensitivity spectra of the walking leg chemoreceptors of P. clarkii in response to different food-related compounds. Recordings from isolated legs confirmed a marked sensitivity of the legs to trehalose, cellobiose, sucrose, maltose, glycine, and leucine. Some sensitivity to glucose, fructose, asparagine, and taurocholic acid was also found. Under confined experimental conditions, the behavioural bioassays excluded involvement of antennules in the detection of food-related compounds, thus emphasizing the role of the legs as the main short-distance, broad-spectrum sensors involved in feeding. Such information could be valuable for the identification of key chemicals aimed at the future development of strategies for crayfish population control programs.

A pheromone analogue affects the evaporation rate of (+)-disparlure in Lymantria dispar.

BACKGROUND The gypsy moth Lymantria dispar L. is a widespread pest that causes economic damage to cork oak forests. Females produce the sex pheromone (+)-(7R,8S)-epoxy-2-methyloctadecane, known as (+)-disparlure [(+)D], for long-distance attraction of conspecific males. A (+)D analogue, 2-decyl-1-oxaspiro[2.2]pentane (OXP-01), neither stimulating nor attractive by itself, causes short-time inhibition of male response in a 1:1 blend with (+)D. The authors investigated whether and how the biological activity of the natural pheromone is affected by OXP-01 on a long-time basis (up to 16 days), also by looking at possible physicochemical reciprocal interactions. RESULTS Blending of (+)D with OXP-01 decreased, under low evaporation rate, the pheromone effectiveness, as assessed by electroantennogram recordings. In male trappings, within the first 24 h, OXP-01 decreased and later enhanced the blend attractiveness, but only under high evaporation rate. Gas chromatography-mass spectroscopy indicates that quantitative retrieval of (+)D from blend cartridges is higher than for pure pheromone, and nuclear magnetic resonance measurements show that OXP-01 produces, possibly by Van der Waals interactions, a bimolecular entity with pheromone causing retention and lengthening of its attractiveness over time. CONCLUSION The biological and physicochemical interactions between (+)D and OXP-01 may provide valuable information for the optimisation of pheromone-based control strategies for gypsy moths.

Taste response profiles of the labellar chemosensilla of the medfly Ceratitis capitata (Diptera: Tephritidae)

Abstract The medfly (Ceratitis capitata Wiedemann, 1824) is a widespread pest for horticulture, mainly targeting fruits of pomaceous and citrus cultivars. Scanty data is available about its chemosensory responses to contact salty, sweet, sour and bitter stimuli. High Resolution Scanning Electron Microscopy (HRSEM) and Transmission Electron Microscopy (TEM) observations indicate the presence of six pairs of Long type and about 40 pairs of Intermediate type labellar chemosensilla, each containing four different neurons. Spike activity was electrophysiologically recorded from the labellar sensilla in response to sodium chloride (NaCl), fructose, acids (citric and malic) and bitter compounds (quinine, quercetin and 6-n-propyl-thiouracil (PROP)). By analysis of spike waveforms in the responses, four neurons were identified responding to different stimuli in both sensillum types of C. capitata. The response specificity of three of these cells (named M1, M2 and S) was determined on the basis of their dose-response profiles. “M1” was specific to NaCl, “M2” to fructose. Both acids appeared to excite the “S” cell and to inhibit the “M1” cell. Citric acid also partly inhibited the response of the “M2” cell. No cell clearly responded to any of the bitter stimuli.