Biosynthetic infochemical communication.

There is an ever-increasing demand for data to be embedded in our environment at ever-decreasing temporal and spatial scales. Whilst current communication and storage technologies generally exploit the electromagnetic properties of media, chemistry offers us a new alternative for nanoscale signaling using molecules as messengers with high information content. Biological systems effectively overcome the challenges of chemical communication using highly specific biosynthetic pathways for signal generation together with specialized protein receptors and nervous systems. Here we consider a new approach for information transmission based upon nature's quintessential example of infochemical communication, the moth pheromone system. To approach the sensitivity, specificity and versatility of infochemical communication seen in nature, we describe an array of biologically-inspired technologies for the production, transmission, detection, and processing of molecular signals. We show how it is possible to implement each step of the moth pheromone pathway for biosynthesis, transmission, receptor protein binding/transduction, and antennal lobe processing of monomolecular and multimolecular signals. For each implemented step, we discuss the value, current limitations, and challenges for the future development and integration of infochemical communication technologies. Together, these building blocks provide a starting point for future technologies that can utilize programmable emission and detection of multimolecular information for a new and robust means of communicating chemical information.

Transition from marine to terrestrial ecologies: changes in olfactory and tritocerebral neuropils in land-living isopods.

In addition to the ancestors of insects, representatives of five lineages of crustaceans have colonized land. Whereas insects have evolved sensilla that are specialized to allow the detection of airborne odors and have evolved olfactory sensory neurons that recognize specific airborne ligands, there is so far little evidence for aerial olfaction in terrestrial crustaceans. Here we ask the question whether terrestrial Isopoda have evolved the neuronal substrate for the problem of detecting far-field airborne chemicals. We show that conquest of land of Isopoda has been accompanied by a radical diminution of their first antennae and a concomitant loss of their deutocerebral olfactory lobes and olfactory computational networks. In terrestrial isopods, but not their marine cousins, tritocerebral neuropils serving the second antenna have evolved radical modifications. These include a complete loss of the malacostracan pattern of somatotopic representation, the evolution in some species of amorphous lobes and in others lobes equipped with microglomeruli, and yet in others the evolution of partitioned neuropils that suggest modality-specific segregation of second antenna inputs. Evidence suggests that Isopoda have evolved, and are in the process of evolving, several novel solutions to chemical perception on land and in air.

Function of the heater: the dead horse arum revisited.

The dead horse arum, Helicodiceros muscivorus, is a conspicuous, foul smelling and thermogenic plant of the Araceae family. This Mediterranean arum lily copies several aspects of a carcass in order to attract carrion blowflies, which are subsequently exploited as unrewarded pollinators. We have previously shown that this plant exhibits a highly accurate olfactory carrion mimicry, which serves to attract the blowflies. In this study, we have investigated the role of thermogeny in the arum. We show that the thermogeny has a direct effect on the pollinators, altering their behaviour. By manipulating heat and odour release of the plant, we can show that the heat, produced along the appendix, is important to lure the flies to this structure, which is vital as the flies from the appendix are more prone to enter the trap chamber that houses the female and male florets. This study provides rare evidence for a direct functional role of thermogeny.

Pre-exposure modulates attraction to sex pheromone in a moth.

In behavioural experiments we investigated the influence of previous short exposure to sex pheromone on subsequent response of male Spodoptera littoralis moths to sex pheromone. We found that pre-exposed males showed increased sensitivity to female sex pheromone after a single exposure to a pheromone plume compared to that found in naïve males. The increased responsiveness lasted for at least 27 h after the exposure, showing that it was not just a short-term sensitization of the males. Exposure to the odour source without upwind movement towards the source was enough to increase the responsiveness. Physical activation without exposure to odour did not affect responsiveness. The increase in responsiveness after exposure was higher when the males were pre-exposed to natural female pheromone gland extract than when they were exposed to a higher dose of the main component, even though both odour sources elicited similar upwind attraction in naïve males. Thus, the quality of the pheromone mixture to which males were exposed influenced the subsequent response.

Olfactory activation patterns in the antennal lobe of the sphinx moth, Manduca sexta.

The sphinx moth Manduca sexta is a well-studied insect with regard to central olfactory functions. Until now, the innervation patterns of olfactory receptor neurons into the array of olfactory glomeruli in the antennal lobe have, however, been unclear. Using optical imaging to visualize calcium dynamics within the antennal lobe we demonstrate specific patterns elicited by sex pheromone components and plant-derived odours. These patterns mainly reflect receptor neuron activity. Within the male-specific macroglomerular complex the two major pheromone components evoke stereotyped activity in either of two macroglomerular complex glomeruli. Based on previous knowledge of output neuron specificity, our results suggest a matching of information between input and output in the macroglomerular complex. Plant odours evoked activity in the sexually isomorphic glomeruli. Two major results were obtained: (1). terpenes and aromatic compounds activate different clusters of glomeruli with only minor overlapping, and (2). the position of certain key glomeruli is fixed in both males and females, which suggests that host-plant related odorants are processed in a similar way in both sexes.

Detection of fruit- and flower-emitted volatiles by olfactory receptor neurons in the polyphagous fruit chafer Pachnoda marginata (Coleoptera: Cetoniinae).

Olfactory receptor neurons on the antennae of the African fruit chafer species Pachnoda marginata (Coleoptera: Scarabaeidae) were examined through extensive use of gas chromatography linked with electrophysiological recordings from single olfactory receptor neurons. Contacted neurons were stimulated with a large number of extracted volatiles from 22 different fruits and with 64 synthetic plant compounds. Extracted fruit volatiles were identified using linked gas chromatography-mass spectrometry. In total, 48 different odor compounds were found to elicit responses. Analysis of the response spectra of the contacted neurons (n = 232) revealed the presence of 28 classes of receptor neurons. The neurons exhibited strong selectivity as well as high sensitivity. Eleven of the identified classes were selectively activated by single compounds, while the remaining were activated by 2-6 compounds. Several receptor neurons that were activated by more than one compound responded to compounds sharing basic structural similarities. The results support the growing hypothesis that a significant proportion of plant-odor receptor neurons in insects are highly sensitive and selective for single odors.

Detection of sex pheromone components in Manduca sexta (L.).

The ability of olfactory receptor neurons to detect female-produced sex pheromone components and a limited sample of potential host plant odours was studied by single-sensillum recordings from olfactory sensilla present on male and female antennae in Manduca sexta. The majority of pheromone-sensitive receptor neurons examined in males was specialized for detection of the two major pheromone components, E10,Z12-hexadecadienal and E10,E12,Z14-hexadecatrienal or E10,E12,E14-hexadecatrienal. New olfactory receptor neurons tuned to the minor components E10,E12-hexadecadienal and Z11-hexadecenal were found. In females, olfactory receptor neurons specific to Z11-hexadecanal were discovered. Pheromone components and host volatiles were detected by separate sets of receptor neurons.

Olfactory receptor neurons detecting plant odours and male volatiles in Anomala cuprea beetles (Coleoptera: Scarabaeidae).

We have identified several types of olfactory receptor neurons in male and female Anomala cuprea beetles. The receptor neurons were sensitive to female sex pheromone components, flower volatiles, green leaf volatiles and unknown volatiles from males. Olfactory sensilla were located on three lamellae forming the antennal club. There was a clear spatial separation between some types of sensilla on each lamella. Receptor neurons for the two sex pheromone components were situated in sensilla placodea covering a specific area on each lamella in both males and females. All sex pheromone receptor neurons were found in these sensilla. Most other receptor neurons were located in a longitudinal, heterogeneous streak formed by various types of sensilla. Receptor neurons for plant-derived compounds appeared to be specialists with a high sensitivity to their respective key compound. The most remarkable among these are the green leaf volatile-specific receptor neurons, which were both sensitive and selective, with the key compound being at least 1000 times as effective as any other compound. These green leaf volatile detectors are apparently homologous to detectors recently found in the scarab Phyllopertha diversa. Our results emphasize the role of single-sensillum recordings as a tool in the identification of biologically active odours.

The antennal lobe of orthoptera - anatomy and evolution.

The first odor-processing neuropils of insects comprise glomeruli, islets of neuropil, that are supplied by olfactory receptor neurons and give rise to efferent axons to higher brain centers. Glomeruli size and organization varies in a taxon-specific manner across the Insecta, suggesting possible correlates between their organization and chemosensory behaviors in different insect groups. Comparative studies of antennal lobe glomeruli within the Orthoptera have been used to infer how the various taxon-specific arrangements of odorant-processing structures (glomeruli) might have evolved. The cellular arrangements in glomeruli have been surveyed using anterograde filling and Golgi impregnation of antennal receptor neurons projecting to the antennal lobe in Stenopelmatidae, Tettigoniidae, Gryllidae, Tetrigidae and Acrididae. These taxa, which represent the two sub-orders of Orthoptera, reveal a high correlation between the neural architecture of the glomeruli and structures within the glomeruli. Using a recent molecular phylogeny of the Orthoptera we have mapped the occurrence of glomerular characteristics to infer the evolution of antennal lobe structures in orthopterans. The functional implications of these results are discussed.

Olfactory discrimination conditioning in the moth Spodoptera littoralis.

We used a proboscis extension reflex (PER) to study the olfactory discrimination capability in the moth Spodoptera littoralis. Already after a single experience, moths were capable to discriminate a rewarded from an unrewarded odor. In the first experiment, when rewarded and unrewarded odors were substituted for each other, moths were able to undergo reversal conditioning already after two experiences. Both shorter and longer inter-trial intervals (ITIs) supported high degrees of learning. In a second experiment, moths could solve both feature-positive and -negative discrimination tasks. Two hypotheses for the way in which these associations exert their discrimination performance are considered. The moth's olfactory physiology has been extensively studied. This animal thus provides a powerful system in which to study the neurobiology of olfactory discrimination and odor recognition.

Olfactory protocerebral pathways processing sex pheromone and plant odor information in the male moth Agrotis segetum.

We investigated protocerebral processing of behaviorally relevant signals in the turnip moth, Agrotis segetum. Single neurons were studied both physiologically and morphologically using intracellular recording techniques. In moth pheromone communication systems, the presence of the complete, female-produced pheromone blend is necessary for male attraction. We predicted that more protocerebral neurons, compared with AL, would display blend interactions. However, only a few protocerebral neurons responded differently to the blend than could be deduced from the response to single components. The majority of the pheromone-sensitive protocerebral neurons identified in this study responded to the major pheromone component. In coding time, most AL neurons can follow a 5-Hz odor stimulus, whereas most protocerebral neurons failed at higher frequencies than 1 Hz. The majority of neurons that responded to the odorants tested innervated one or both of the protocerebral lateral accessory lobes. If only one of these was innervated, then the innervation always displayed a varicose appearance, suggesting a presynaptic function. Thus, information seems to be transferred from other protocerebral areas to the lateral accessory lobes. Into these, descending neurons sent smooth, postsynaptic branches. A majority of the neurons innervating the superior medial protocerebrum were found to display single-component specificity. Few additional correlations between odor specificity and structural characteristics were apparent.

Sex pheromone mimicry in the early spider orchid (ophrys sphegodes): patterns of hydrocarbons as the key mechanism for pollination by sexual deception.

We investigated the female-produced sex pheromone of the solitary bee Andrena nigroaenea and compared it with floral scent of the sexually deceptive orchid Ophrys sphegodes which is pollinated by Andrena nigroaenea males. We identified physiologically and behaviorally active compounds by gas chromatography with electroantennographic detection, gas chromatography-mass spectrometry, and behavioral tests in the field. Dummies scented with cuticle extracts of virgin females or of O. sphegodes labellum extracts elicited significantly more male reactions than odorless dummies. Therefore, copulation behavior eliciting semiochemicals are located on the surface of the females' cuticle and the surface of the flowers. Within bee and orchid samples, n-alkanes and n-alkenes, aldehydes, esters, all-trans-farnesol and all-trans-farnesyl hexanoate triggered electroantennographic responses in male antennae. Most of the alkanes and alkenes occurred in similar patterns both in the bees and orchids. O. sphegodes leaf extracts contained mostly the same compounds but in different proportions. In behavioral tests with synthetic compounds, blends of alkenes triggered significantly more approaches and pounces of the males whereas alkanes were not more attractive than odorless dummies. Since alkanes and alkenes together were most attractive, we conclude they constitute the bees' sex pheromone as well as the pseudocopulation-behavior releasing orchid-odor bouquet.

Function and morphology of the antennal lobe: new developments.

The antennal lobe of insects has emerged as an excellent model for olfactory processing in the CNS. In the present review we compile data from areas where substantial progress has been made during recent years: structure-function relationships within the glomerular array, integration and blend specificity, time coding and the effects of neuroactive substances and hormones on antennal lobe processing.

The maxillary palp sensory pathway of Orthoptera.

Primary sensory projections and arborisations of higher-order neurons associated with the maxillary palps were examined in Tettigoniidae, Gryllidae, Tetrigidae and Acrididae representing the two sub-orders of Orthoptera, Ensifera and Caelifera. Anterograde filling and Golgi impregnation of maxillary receptor neurons revealed two patterns of innervation, the ensiferous and the caeliferous type. In both ensiferans and caeliferans, receptor neurons arborised within the tritocerebrum, the antennal motor- and mechano-sensory centre and the lobus glomerulatus. In ensiferans, additional areas of innervation were found in the lobus glomerulatus and in a previously undescribed neuropil, here referred to as the accessory lobus glomerulatus. In relation to the anatomical data a putative functional segregation of the neuropil into gustatory-, olfactory- and mechano-sensory centres is implied.

Appetitive learning of odours with different behavioural meaning in moths.

Moth behaviour is to a great extent guided by olfactory stimuli with different relevance. We investigated whether olfactory learning of odours is influenced by the behavioural significance of the odorant. In proboscis extension conditioning experiments species-specific sex pheromones, which normally elicit an innate behaviour in males, and a flower odour were used as olfactory stimuli. After 10 conditioning trials, both sexes showed similar response levels to individual pheromone components and to the flower odour geraniol. However, when the female gland extract was used as conditioning stimulus, the response level was significantly lower than that for geraniol in both sexes. Significant learning nevertheless occurred in females, but not in males. Experiments with different numbers of training trials revealed that, in females, fewer learning trials with individual pheromone components were necessary for significant memory formation than in males.

Arthropod sensilla: morphology and phylogenetic considerations.

The structures of different types of arthropod sensilla are compared and theories regarding the evolution of these sensory organs are presented. Arthropod sensilla are built according to a common plan, and are probably homologous to scolopidia. Certain similarities in the structure of sensilla in different arthropod groups can be the result of adaptations to specific environments. The structure of sensilla in insect groups, which are regarded to be ancestral, do not appear to be less sophisticated than in groups considered to be more advanced. The different types of pore systems, as well as the structural differentiations of insect olfactory sensillar types remain unexplained. Olfactory sensilla display a large degree of similarity among terrestrial arthropods, whereas crustacean sensilla diverge in structure. In holometabolous insects larval sensilla appear to be structurally quite advanced, and more complex than in the adult. During the ontogeny of both sensilla and scolopidia, these are differentiated in an epithelial layer, resulting in the formation of both sensory and enveloping cells. The developmental patterns of sensilla in the studied insect groups are similar. During the development of sensilla apoptotic process are usually active.

Central processing of pulsed pheromone signals by antennal lobe neurons in the male moth Agrotis segetum.

Male moths use female-produced pheromones as orientation cues during the mate-finding process. In addition to the needs of evaluating the quality and quantity of the pheromone signal, the male moth also needs to resolve the filamentous structure of the pheromone plume to proceed toward the releasing point successfully. To understand how a discontinuous olfactory signal is processed at the central level, we used intracellular recording methods to characterize the response patterns of antennal lobe (AL) neurons to pulsatile stimulation with the full female-produced pheromone blend and its single components in male turnip moths, Agrotis segetum. Air puffs delivered at frequencies of 1, 3, 5, 7, or 10 Hz were used to carry the stimulus. Two types of AL neurons were characterized according to their capabilities to resolve stimulus pulses. The most common type could resolve at least 1-Hz pulses, thus termed fast neurons; another type could not resolve any pulses, thus termed slow neurons. When fast neurons were excited by stimuli, they always displayed biphasic response patterns, a depolarization phase followed by a hyperpolarization phase. This pattern could be evoked by stimulation with both the single pheromone components and the blend. The pulse-resolving capability of the fast neurons correlated significantly with the size of the hyperpolarization phase. When the amplitude was higher and the fall time of the hyperpolarization faster, the neuron could follow more pulses per second. Moreover, interactions between different pheromone components eliciting different response patterns did not improve the pulse-resolving capability of fast neurons.

Integration of behaviourally relevant odours at the central nervous level in solitary and gregarious third instar locusts, Schistocerca gregaria.

Integration of behaviourally relevant odours at the central nervous level of 3rd instar nymphal desert locusts, Schistocerca gregaria, showed phase- and (developmental) stage-dependent characteristics which correlated with differences in the number of olfactory sensilla. Antennal lobe (AL) neurons of gregarious locusts generally responded more frequently and showed a higher sensitivity to the tested stimuli. However, AL neurons of solitary locusts responded significantly more frequently to phenylacetonitril, the major component of the adult aggregation pheromone. Pheromone-specific, plant-specific and pheromone-plant generalist neurons were found in both phases. The response spectra of pheromone-specific neurons correlated with the potential behavioural significance of attractant chemical cues in the environment. Neurons of both phases responded specifically to stage-specific aggregation-pheromone components and to other chemical cues that may be involved in the location of suitable roosting and foraging sites.