In Search for Pheromone Receptors: Certain Members of the Odorant Receptor Family in the Desert Locust Schistocerca gregaria (Orthoptera: Acrididae) Are Co-expressed with SNMP1

Abstract

Under given environmental conditions, the desert locust (Schistocera gregaria) forms destructive migratory swarms of billions of animals, leading to enormous crop losses in invaded regions. Swarm formation requires massive reproduction as well as aggregation of the animals. Pheromones that are detected via the olfactory system have been reported to control both reproductive and aggregation behavior. However, the molecular basis of pheromone detection in the antennae of Schistocerca gregaria is unknown. As an initial step to disclose pheromone receptors, we sequenced the antennal transcriptome of the desert locust. By subsequent bioinformatical approaches, 119 distinct nucleotide sequences encoding candidate odorant receptors (ORs) were identified. Phylogenetic analyses employing the identified ORs from Schistocerca gregaria (SgreORs) and OR sequences from the related species Locusta migratoria revealed a group of locust ORs positioned close to the root, i.e. at a basal site in a phylogenetic tree. Within this particular OR group (termed basal or b-OR group), the locust OR sequences were strictly orthologous, a trait reminiscent of pheromone receptors from lepidopteran species. In situ hybridization experiments with antennal tissue demonstrated expression of b-OR types from Schistocerca gregaria in olfactory sensory neurons (OSNs) of either sensilla trichodea or sensilla basiconica, both of which have been reported to respond to pheromonal substances. More importantly, two-color fluorescent in situ hybridization experiments showed that most b-OR types were expressed in cells co-expressing the “sensory neuron membrane protein 1” (SNMP1), a marker indicative of pheromone-sensitive OSNs in insects. Analyzing the expression of a larger number of SgreOR types outside the b-OR group revealed that only a few of them were co-expressed with SNMP1.In summary, we have identified several candidate pheromone receptors from Schistocerca gregaria that could mediate responses to pheromones implicated in controlling reproduction and aggregation behavior.

Expression of odorant‐binding proteins in mouthpart palps of the desert locust Schistocerca gregaria

“…OBPs are supposed to be involved in solubilizing and transferring the hydrophobic odorants, but may have additional functions as well (Pelosi, ; Xu et al , ; Pelosi et al , ; ). Although most of our knowledge about OBPs is based on studies concentrating on holometabolous dipteran and lepidopteran species (Vogt et al , ; Robertson et al , ; Harada et al , ; Schultze et al , ; Pelosi et al , ), progress has also recently been made in analysing OBPs of hemimetabolous locusts, notably the desert locust S. gregaria and the migratory locust L. migratoria (Jin et al , ; Xu et al , ; Zhang et al , ; Wang et al , ; ; Zhang et al , ; Jiang et al , ; ; Pregitzer et al , ; Zhang et al , ; Li et al , ).…”

“…Total RNA was extracted from maxillary and labial palps of adult males and females using Extrazol reagent (Blirt, Gdańsk, Poland) following the manufacturer’s protocol. Poly(A) + RNA was isolated and converted into cDNA as described earlier (Pregitzer et al , ). Nonquantitative RT‐PCR utilizing SgreOBP‐specific sense and antisense primers (for primers, see Jiang et al , ; ) were performed with the following conditions: 94 ℃ for 90 s followed by 20 cycles with 94 ℃ for 30 s, 50–60 ℃ for 30 s (thereby, the annealing temperature was decreased by 0.5 ℃ per cycle) and 72 ℃ for 90 s. Subsequently, 20 cycles with an annealing temperature of 40–50 ℃ were performed followed by incubation at 72 ℃ for 15 min.…”

“…Digoxigenin (DIG)‐labelled or biotin‐labelled antisense probes were generated as described by Jiang et al (; ). Two‐colour FISH on tissue sections was performed as described earlier (Jiang et al , ; Pregitzer et al , ). WM‐FISH was performed with freshly prepared labial and maxillary palps, which were directly transferred into fixation solution [4% paraformaldehyde in 0.1 M sodium bicarbonate (NaHCO 3 ), pH 9.5] and incubated overnight (12–14 h) at 4 ℃.…”

“…Overview of the expression of OBPs in either sensilla basiconica and/or sensilla chaetica of the antennae Two-colour FISH on tissue sections was performed as described earlier Pregitzer et al, 2017…”

“…OBPs are small water‐soluble proteins of ~110–200 amino acids, which are supposed to solubilize the mostly lipophilic odorous compounds and may also be involved in signal transduction or termination (Vogt et al , ; Sandler et al , ; Tegoni et al , ; Große‐Wilde et al , ; ; Leal, ; Suh et al , ). Analysing the repertoire of potential OBPs of S. gregaria , we have identified 14 antennal OBPs from an antennal transcriptome (Jiang et al , ), which is a relatively small number compared with the large repertoire (119) of odorant receptors (ORs) (Pregitzer et al , ). In the closely related migratory locust Locusta migratoria , 16 OBPs have been identified, and 142 ORs were found in genome and antennal transcriptome analysis (Ban et al , ; Xu et al , ; Yu et al , ; Wang et al , ; ).…”