Medial Styloconic Sensilla Research Articles

Electrophysiological and Morphological Characterization of Contact Chemosensilla in Adults and Larvae of the Butterfly, Atrophaneura alcinous

Simple SummaryIn butterflies, oviposition and feeding are induced by the recognition of host plant compounds with contact sensory organs called chemosensilla. Oviposition behavior of the butterfly Atrophaneura alcinous involves detection of two plant compounds—an alkaloid and a monosaccharide—using tarsal contact chemosensilla of the foreleg. Feeding behavior of the adults and larvae involves detection of sucrose, sodium chloride, and other chemicals. We examined the distribution and electrophysiological response of contact chemosensilla in A. alcinous adults and larvae. In adults, tarsal contact chemosensilla were classified into long- and short-type sensilla. Long-type sensilla were distributed widely much more in females than in males, whereas short-type sensilla were found at the edge of the tarsi in a similar manner in both sexes. Long- and short-type sensilla showed different responses to methanol extracts of host plant compounds. Our findings suggest that the adult butterfly discriminates between host plant compounds using long-type sensilla during oviposition and recognizes the taste using short-type sensilla. In the larval mouthparts, contact chemosensilla were present on the maxillary galea and epipharynx. Electrophysiological responses of chemosensilla suggest that larvae can discriminate between host plant compounds.Distribution and electrophysiological responses of contact chemosensilla were examined in the Aristolochiaceae-feeding butterfly Atrophaneura alcinous. In adult butterflies, tarsal contact chemosensilla of the foreleg were classified into two groups based on length: long- and short-type sensilla. Long-type sensilla were distributed much more widely in females than in males, whereas short-type sensilla were found at the edge of the tarsi in a similar manner in both sexes. Taste responses of the long- and short-type sensilla to methanol extracts of Aristolochia debilis and Citrus spp. were recorded. Aristolochia debilis extracts evoked spikes with different amplitudes, whereas Citrus spp. extracts evoked spikes with a single amplitude in the long-type sensilla. Short-type sensilla did not respond to either extract. Moreover, we recorded responses to different concentrations of sucrose and NaCl. Results suggest that adult butterflies can discriminate the taste of host plant components from other chemicals using long-type sensilla during oviposition and may recognize diets containing sugar and salts during feeding using short-type sensilla. In the larval mouthparts, there were lateral and medial styloconic sensilla on the maxillary galea and epipharyngeal sensillum on the epipharynx. Electrophysiological responses of these sensilla suggest that larvae can discriminate between host plant compounds.

Chemosensory Receptors in the Larval Maxilla of Papilio hospiton

Among the butterflies of the genus Papilio (Lepidoptera: Papilionidae), Papilio hospiton (Géné) has a geographical distribution limited to the Mediterranean islands of Sardinia (Italy) and Corsica (France). This is mainly due to the host range that includes only a few plant species of Apiaceae and Rutaceae growing on these islands. In a previous electrophysiological investigation conducted on the maxillary gustatory system of larvae of P. hospiton and its closely phylogenetically related species Papilio machaon, a significantly higher spike activity was shown for the gustatory neurons of lateral and medial styloconic sensilla in P. hospiton when bitter compounds were tested. This effect was possibly correlated to the limited host choice range for P. hospiton. To shed light on the molecular aspects of this phenomenon, we investigated the expression pattern of sensory-related sequences by conducting a transcriptomic analysis from total RNA isolates of P. hospiton larval maxillae. We identified several transcripts that may be involved in taste (one gustatory receptor, one divergent ionotropic receptor, and several transient receptor potential channels, TRPs) as well as transcripts supporting an olfactory function for this appendage, including odorant receptors (ORs), antennal ionotropic receptors (A-IRs), sensory neuron membrane proteins (SNMPs), and odorant-binding proteins (OBPs). We used Human Embryonic Kidney (HEK293A) cells to heterologously express two of the identified receptors, PhospOR1 and PhospPain, together with their orthologs from P. machaon, for functional characterization. While our data suggest no activation of these two receptors by the ligands known so far to activate the electrophysiological response in larval maxillary neurons of Papilio species, nor temperature activation of both Papilio TRPA-channel Painless, they represent the first attempt in connecting neuronal activity with their molecular bases to unravel diet specialization between closely related Papilio species.

Taste discriminating capability to different bitter compounds by the larval styloconic sensilla in the insect herbivore Papilio hospiton (Géné)

Herbivorous animals may benefit from the capability to discriminate the taste of bitter compounds since plants produce noxious compounds, some of which toxic, while others are only unpalatable. Our goal was to investigate the contribution of the peripheral taste system in the discrimination of different bitter compounds by an herbivorous insect using the larvae of Papilio hospiton Géné as the experimental model, showing a narrow choice range of host plants. The spike activity from the lateral and medial styloconic sensilla, housing two and one bitter-sensitive gustatory receptor neurons (GRNs), respectively, was recorded following stimulation with nicotine, caffeine, salicin and quercitrin and the time course of the discharges was analyzed. Nicotine and caffeine activated all three bitter-sensitive GRNs, while salicin and quercitrin affected only two of them. In feeding behavior bioassays, intact larvae ate glass-fiber disks moistened with salicin and quercitrin, but rejected those with nicotine and caffeine, while lateral sensillum-ablated insects also ate the disks with the two latter compounds. The capability to discriminate bitter taste stimuli and the neural codes involved are discussed.

Neurophysiological and behavioral responses of gypsy moth larvae to insect repellents: DEET, IR3535, and picaridin.

The interactions between insect repellents and the olfactory system have been widely studied, however relatively little is known about the effects of repellents on the gustatory system of insects. In this study, we show that the gustatory receptor neuron (GRN) located in the medial styloconic sensilla on the maxillary palps of gypsy moth larvae, and known to be sensitive to feeding deterrents, also responds to the insect repellents DEET, IR3535, and picaridin. These repellents did not elicit responses in the lateral styloconic sensilla. Moreover, behavioral studies demonstrated that each repellent deterred feeding. This is the first study to show perception of insect repellents by the gustatory system of a lepidopteran larva and suggests that detection of a range of bitter or aversive compounds may be a broadly conserved feature among insects.

Gustatory Receptor Neurons in Manduca sexta Contain a TrpA1-Dependent Signaling Pathway that Integrates Taste and Temperature

Temperature modulates the peripheral taste response of many animals, in part by activating transient receptor potential (Trp) cation channels. We hypothesized that temperature would also modulate peripheral taste responses in larval Manduca sexta. We recorded excitatory responses of the lateral and medial styloconic sensilla to chemical stimuli at 14, 22, and 30 °C. The excitatory responses to 5 chemical stimuli-a salt (KCl), 3 sugars (sucrose, glucose, and inositol) and an alkaloid (caffeine)-were unaffected by temperature. In contrast, the excitatory response to the aversive compound, aristolochic acid (AA), increased robustly with temperature. Next, we asked whether TrpA1 mediates the thermally dependent taste response to AA. To this end, we 1) identified a TrpA1 gene in M. sexta; 2) demonstrated expression of TrpA1 in the lateral and medial styloconic sensilla; 3) determined that 2 TrpA1 antagonists (HC-030031 and mecamylamine) inhibit the taste response to AA, but not caffeine; and then 4) established that the thermal dependence of the taste response to AA is blocked by HC-030031. Taken together, our results indicate that TrpA1 serves as a molecular integrator of taste and temperature in M. sexta.

Fine Structure of the Galeal Styloconic Sensilla of Larval Lymantria dispar (Lepidoptera: Lymantriidae)

Lepidopteran larvae possess two pairs of styloconic sensilla located on the maxillary galea. These sensilla, namely the lateral and medial styloconic sensilla, are each comprised of a smaller cone, which is inserted into a style. They are thought to play an important role in host-plant selection and are the main organs involved in feeding. Ultrastructural examination of these sensilla of fifth instar Lymantria dispar (L.) larvae reveal that they are each approximately 70 um in length and 30 um in width. Each sensillum consists of a single sensory peg inserted into the socket of a large style. Each peg bears a slightly subapical terminal pore averaging 317 nm in lateral and 179 nm in medial sensilla. Each sensillum houses five bipolar neurons. The proximal dendritic segment of each neuron gives rise to an unbranched distal dendritic segment. Four of these dendrites terminate near the tip of the sensillum below the pore and bear ultrastructural features consistent with contact chemosensilla. The fifth distal dendrite terminates near the base of the peg and bears ultrastructural features consistent with mechanosensilla. Thus, these sensilla each bear a bimodal chemo-mechanosensory function. The distal dendrites lie within the dendritic channel and are enclosed by a dendritic sheath. The intermediate and outer sheath cells enclose a large sensillar sinus, whereas the smaller ciliary sinus is enclosed by the inner cell. The neurons are ensheathed successively by the inner, intermediate, and outer sheath cells.

Determining the Sensitivity of Gustatory Neurons in the Maxillary Styloconic Sensilla of Gypsy Moth Larvae

Gypsy moth larvae, Lymantria dispar (L.), are highly polyphagous and display a wide host preference, feeding on the foliage of many species, but favoring leaves of deciduous hardwood trees, such as oak, maple, and sweet gum. Gypsy moth larvae are major pest defoliators in the United States and destroy millions of acres of trees annually. These lepidopteran insects possess gustatory sensory organs located on the maxillae, namely the medial and lateral galeal styloloconic sensilla, which play an important role in host-plant selection. Using a single cell electrophysiological recording method, this study characterized the sensitivity of the receptor cells housed within each sensillum of gypsy moth larvae when exposed to a panel of selected phytochemicals by performing dose response experiments. Electrophysiological tip recordings from these sensilla revealed that medial styloconic sensilla responded to the alkaloids, strychnine and atropine, while lateral styloconic sensilla responded to aristolochic acid and atropine. In general, these different taste cells exhibited characteristic temporal firing patterns. Thus, this study provides correlative insights into the feeding behavior and taste physiology of this larval insect. It also provides a gateway to use other alkaloids in temporal and dose-response experiments as a possible means of biocontrol.

Deficiency of gustatory sensitivity to some deterrent compounds in "polyphagous" mutant strains of the silkworm, Bombyx mori.

Sawa-J, CSJ02 and N601 x C601 are selected mutant strains of Bombyx mori, which grow on various artificial diets or temporarily ingest various plant leaves. To examine the mechanisms mediating diet breadth of caterpillars, gustatory spike responses of the silkworms, called 'polyphagous" strains, were compared with normal strains, N137 x C146 and C02. There were notable differences in their feeding habits and in their sensitivity to salicin in deterrent cells in the maxillary medial styloconic sensilla and the epipharyngeal sensilla. By contrast, the deterrent cells of all strains responded similarly to strychnine nitrate in a dose-dependent manner. In additional comparisons of Sawa-J and N137 x C146, Sawa-J maxillary deterrent cells were significantly less sensitive to phloridzin, amygdalin and arbutin, but responded to some alkaloids and 20-hydroxyecdysone with similar or even higher firing rate. These results suggest that the deficiency of the sensitivity to some deterrent compounds on the deterrent cell of the polyphagous strains may be caused by mutant genes and affects the diet breadth of caterpillars.

Taste cell responses of stem-borer larvae Chilo partellus (Swinhoe), Eldana saccharina Wlk. and Maruca testulalis (Geyer) to plant substances

Eleetrophysiological recordings from taste sensilla of larvae of Chilo partellus (Swinhoe), Eldana saccharina Wlk. and Maruca testulalis (Geyer) during stimulation with various plant substances revealed that in the medial and lateral maxillary sensilla styloconica of each of these three species a NaCl, water and sucrose sensitive cell is present. In the medial styloconic sensilla of C. partellus, an additional chlorogenic acid sensitive cell occurs. Chilo partellus and E. saccharina appeared to be more sensitive to sucrose than M. testulalis. In all three species, the lateral sensilla styloconica are more responsive to sucrose and less responsive to NaCl than the medial ones.

Contact chemoreception of host plant specific chemicals in larvae of variousYponomeuta species (Lepidoptera)

Gustatory sensitivity of larvae belonging to nine differentYponomeuta species was studied. A conspicuous behavioural difference between these closely related species is represented in their host plant preferences. Electrophysiological and morphological examination of the anteriorly-located sensilla revealed that a restricted number of contact chemoreceptors are present. A tip recording technique was used to record gustatory responses of neurones in the lateral and medial styloconic sensilla. The following chemicals were applied as stimuli: sucrose, sedoheptulose, sorbitol, dulcitol, coumarin, phloridzin, salicin, prunasin, rutin, (+)-catechin, the trisodium salt of isocitric acid, ZnCl2, NaCl, and KCl. Most of these stimuli were selected on the basis of chemical composition of the array of host plants of the larvae studied (Table 1). Different spike amplitudes displayed in the recordings indicated that most constituents tested are perceived by single cells. Ionized substances sometimes excited more than one cell. In several recordings, a delayed response appeared. This phenomenon seems related to variation in diameter of the distal pore by which inward diffusion of the stimulus takes place. Analysis of the time course of the recordings indicated that the effect of diffusion of stimulus molecules on the initital neural response may be considerable for the non-ionized substances tested. The functional diversity displayed in the interspecific sensitivity patterns can be partly explained by composition of the host plants on which the larvae feed (Table 2). Compounds of one type act as phagostimulants, and some of these compounds are sufficiently host-specific to act as host-recognition factors. A second type of chemical signal inhibits “wrong” food intake; these substances are classified as deterrents. A few gustatory responses appeared non-adaptive, this type of sensitivity is discussed from an evolutionary point of view.