The Two Main Olfactory Receptor Families in Drosophila, ORs and IRs: A Comparative Approach

Abstract

Most insect species rely on the detection of olfactory cues for critical behaviors for the survival of the species, e.g., finding food, suitable mates and appropriate egg-laying sites. Although insects show a diverse array of molecular receptors dedicated to the detection of sensory cues, two main types of molecular receptors have been described as responsible for olfactory reception in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). Although both receptor families share the role of being the first chemosensors in the insect olfactory system, they show distinct evolutionary origins and several distinct structural and functional characteristics. While ORs are seven-transmembrane-domain receptor proteins, IRs are related to the ionotropic glutamate receptor (iGluR) family. Both types of receptors are expressed on the olfactory sensory neurons (OSNs) of the main olfactory organ, the antenna, but they are housed in different types of sensilla, IRs in coeloconic sensilla and ORs in basiconic and trichoid sensilla. More importantly, from the functional point of view, they display different odorant specificity profiles. Research advances in the last decade have improved our understanding of the molecular basis, evolution and functional roles of these two families, but there are still controversies and unsolved key questions that remain to be answered. Here, we present an updated review on the advances of the genetic basis, evolution, structure, functional response and regulation of both types of chemosensory receptors. We use a comparative approach to highlight the similarities and differences among them. Moreover, we will discuss major open questions in the field of olfactory reception in insects. A comprehensive analysis of the structural and functional convergence and divergence of both types of receptors will help in elucidating the molecular basis of the function and regulation of chemoreception in insects.

Transgenerational inheritance of learned preferences for novel host plant odors inBicyclus anynanabutterflies

“…; Gomez‐Diaz et al. ). Although B. anynana larvae do not feed on fruit, odor receptor genes for fruit odors might be expressed in the appropriate organs before the adult stage of development (Gerber and Stocker ), when these receptors are actually useful to detect ripe fruit, which is the adult food of these butterflies.…”

“…Although we used 2% solutions for all odors, we were unable to obtain information about the exact molecular composition and concentration of the chemicals in these food odors from the company that made them, and it is possible that the banana and mango solutions had higher concentrations of chemicals than the coffee or almond solutions. In addition, for an odor to be detected by a larva, the adequate chemo receptor genes for the different components of the odor have to be present in the genome, and these genes have to be expressed in sensory cells such as in the olfactory neurons that enervate the sensory sensilla in the larvae antennae or in the mouthparts (Hallem and Carlson 2006;Popescu et al 2013;Gomez-Diaz et al 2018). Although B. anynana larvae do not feed on fruit, odor receptor genes for fruit odors might be expressed in the appropriate organs before the adult stage of development (Gerber and Stocker 2007), when these receptors are actually useful to detect ripe fruit, which is the adult food of these butterflies.…”

Neurophysiological and Behavioral Responses ofIxodes scapularisto host odors

“…Recent insights into the foreleg transcriptome of the I. scapularis offer insights: there were no odorant receptors (ORs) present, however a variety of gustatory and ionotropic receptors (GRs and IRs) were apparent (Josek et al 2018b). Lack of ORs however does not preclude the ticks from sensing semiochemicals, since phenols, carboxylic acids and aldehydes we found here to be eliciting electrophysiological activity (Table 1) are detected by multiple IRs (Rytz et al 2013, Gomez-Diaz et al 2018. Carbon dioxide, a key chemostimulus in tick biology (Wilson et al 1972, Eisen and Paddock 2020) is detected by GRs in insects (Jones et al 2007).…”

Context-Dependence and the Development of Push-Pull Approaches for Integrated Management of Drosophila suzukii

“…In addition, D. melanogaster is strongly attracted to odours of yeast and fermentation [83]. Many gene families that code for olfactory receptors (Odorant Receptors and Ionotropic Receptors) or taste receptors (Gustatory Receptors) have been characterized, including the developmental stage or organs in which they are expressed [84,85]. Also the neuronal circuitry for sensing and perceiving olfactory cues has been extensively investigated (e.g., [86][87][88]).…”