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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.