Recognition of Plant Odor Information in Moths

Abstract

Herbivorous moth species are important models in studies of olfaction, starting with the pioneering work on the silk moth. Their survival directly depends on innate and learned olfactory cues, both insect produced pheromones and plant odors, for feeding and reproduction (mating and oviposition). Therefore, gaining knowledge about the crucial olfactory system in moth, also contribute to strategies in species-specific pest management. Extensive studies over the last decades have given good insight into the pheromone system. The plant odor sensory system, however, have been challenged by defining the biologically relevant odorants, which is an essential knowledge when aiming to understand complex olfactory coding mechanisms. Many studies have revealed selective plant odor sensory neurons in both female and male moth. The most convincing results demonstrating highly sensitive and narrowly tuned neurons, come from linking chemical analyses to electrophysiological recordings of single units, which has allowed classification of neurons into functional types according to the molecular receptive range, appearing with one primary odorant and a few other structurally similar and less potent secondary odorants. In heliothine moth, the different neuron types barely overlap in their receptive range, though some variation is seen in species of other genera. A comparative study among heliothine species has demonstrated the presence of neuron types with identical specificity, indicating an evolutionary conservation of receptor proteins. Conservation of co-located neuron types in particular sensilla, within and across species, implies the sensillum as a functional unit. According to “the molecular logic of the sense of smell”, it is hypothesized that each of the approximately 70 glomeruli in the moth antennal lobe devoted to plant odor information receive projections from olfactory sensory neurons of the same functional type. It is hypothesized that the response profiles of antennal lobe projection neurons (uni- or multi-glomerular), reflect the molecular receptive ranges of the sensory neurons innervating the particular glomeruli, i.e. responding specifically to the primary odorant of these neurons. Furthermore, the antennal lobe network with local interneurons (predominantly inhibitory) and modulatory centrifugal neurons enhance complexity and flexibility of odor processing. Projection neurons convey antennal lobe output to higher olfactory areas in the protocerebrum mainly via three tracts terminating in the calyces of the mushroom bodies, the lateral horn and two less pronounced areas in the superior protocerebrum and around the peduncle. Plant odorant and pheromone information conveyed in parallel pathways into the antennal lobe, are kept separated in the calyces of the mushroom bodies (involved in learning), and the lateral protocerebrum (an integration area) indicating a functional organization according to behavioral relevance. Neuronal plasticity makes moths able to adjust behavior and adapt to a shifting environment. Altogether, the olfactory sensory system provides the moth with the ability to recognize a variety of plant odorants with high precision. However, the neuronal mechanism explaining how odorants finally generate/release the innate and learned behaviors of attraction, repellency, feeding and oviposition has yet to be resolved.