Hybrid Neurons in a MicroRNA Mutant Are Putative Evolutionary Intermediates in Insect CO ₂ Sensory Systems

Abstract

Carbon dioxide (CO 2 ) elicits different olfactory behaviors across species. In Drosophila, neurons that detect CO 2 are located in the antenna, form connections in a ventral glomerulus in the antennal lobe, and mediate avoidance. By contrast, in the mosquito these neurons are in the maxillary palps (MPs), connect to medial sites, and promote attraction. We found in Drosophila that loss of a microRNA, miR-279, leads to formation of CO 2 neurons in the MPs. miR-279 acts through downregulation of the transcription factor Nerfin-1. The ectopic neurons are hybrid cells. They express CO 2 receptors and form connections characteristic of CO 2 neurons, while exhibiting wiring and receptor characteristics of MP olfactory receptor neurons (ORNs). We propose that this hybrid ORN reveals a cellular intermediate in the evolution of species-specific behaviors elicited by CO 2 .In insects, both the position of CO 2 neurons and the behavior elicited by CO 2 differ among species. For example, olfactory detection of CO 2 through neurons positioned in or around the mouthparts of an insect, such as maxillary palps (MPs) and labial palps, correlates with feedingrelated behaviors. Indeed, in some blood-feeding insects such as mosquitoes and tsetse flies, these neurons are harbored in the MPs and are important in locating hosts via plumes of CO 2 that they emit (1-3). The hawkmoth, Manduca sexta, monitors nectar profitability of newly opened Datura wrightii flowers through CO 2 receptor neurons located in their labial palps (4,5). In these examples, CO 2 acts as an attractant. Conversely, in Drosophila CO 2 is a component of a stress-induced odor that triggers avoidance behavior (6). This repellent response is driven by antennal neurons expressing the CO 2 receptor complex 8). How did these diverse behavioral responses to CO 2 arise during insect evolution? We propose that this diversity emerged through multiple steps, including changes in cellular position (arising from elimination of CO 2 neurons in one appendage and generation of these neurons in another) and changes in circuitry.In the course of a genetic screen for mutants disrupting the organization of the olfactory system, we isolated a mutant (S0962−07) that resulted in the formation of ectopic Gr21a-expressing §To whom correspondence should be addressed.

Carbon dioxide receptor genes in cotton bollworm Helicoverpa armigera

“…Currently, most studies on insect CO2 receptors are focused on Drosophila or mosquitoes (Cayirlioglu et al 2008;Dahanukar et al 2007;Erdelyan et al 2012;Hartl et al 2011;Jones et al 2007;McMeniman et al 2014;Montell 2013;Turner and Ray 2009) and are aimed at discovering compounds for vector control that can modulate CO2 receptors in mosquitoes (Tauxe et al 2013;Turner and Ray 2009). Little attention has been paid to the CO2 receptors of agricultural pests.…”

“…For example, alternatives to heavy insecticide use, such as interfering with CO2 reception, would be helpful for controlling Helicoverpa armigera, which is one of the most destructive agricultural pests known (Sharm 2001). So far, insect CO2 receptors have only been functionally characterised in vivo using transgenic Drosophila, mutated mosquitoes or RNAi techniques (Cayirlioglu et al 2008;Dahanukar et al 2007;Erdelyan et al 2012;Hartl et al 2011;Jones et al 2007;McMeniman et al 2014;Montell 2013;Turner and Ray 2009). These approaches are not established for Lepidoptera.…”

Neurogenetic networks for startle-induced locomotion in Drosophila melanogaster

“…For example, both robo and lola are essential for axon guidance during development of the nervous system (17,18,27,28). Furthermore, mir-279 has been implicated in Drosophila in establishing the identity of CO 2 -detecting chemosensory neurons (29), and more generally, microRNAs have been implicated in neurodevelopment and morphogenesis (29)(30)(31)(32)(33). In addition, mir-317 is predicted to regulate ocelliless (34), which has been implicated in the development of the protocerebral bridge, a component of the central complex associated with locomotion (35).…”

Regulation of Odor Receptor Genes in Trichoid Sensilla of the Drosophila Antenna

“…We note that misexpression of an antennal receptor in the maxillary palp was found to alter the chemosensory behavior of a fly (Shiraiwa 2008), illustrating the possibility that a change in organ specificity of Or expression could confer a selective advantage to the animal. Moreover, CO 2 reception apparently moved from one organ to the other during the evolution of the Dipterans; it also moved following experimental mutation of a microRNA in Drosophila (Cayirlioglu et al 2008). …”