Differential expression of two novel odorant receptors in the locust (Locusta migratoria)

Olfaction in animals is important for host localization, mating and reproduction in heterogeneous chemical environments. Studying the molecular basis of olfactory receptor neurons (ORNs) systems can elucidate the evolution of olfaction and associated behaviours. Odorant receptors (ORs) in insects have been identified, particularly in the holometabolous model Drosophila, and some of them have been functionally studied. However, ORs in the locust-a hemimetabolous model insect and the most important insect crop pest-have not yet been identified, hindering our understanding of locust olfaction. Here, we report for the first time four putative ORs in Locusta migratoria: LmigOR1, LmigOR2, LmigOR3 and LmigOR4.

These four putative OR genes encoded proteins with amino acids of 478, 436, 413 and 403 respectively. Sequence identity among them ranged from 19.7% to 35.4%. All ORs were tissue-specifically expressed in olfactory organs, without sex-biased characteristics. However, LmigOR1, LmigOR3 and LmigOR4 were only expressed in the antenna, while LmigOR2 could also be detected in mouthparts. In situ hybridization demonstrated that the LmigOR1antisense probe labelled olfactory receptor neurons (ORNs) in almost all segments of the antenna, but only a few segments housed ORNs expressing LmigOR2. The number of neurons labelled by LmigOR1 antisense probes in each antennal segment was much greater (>10 neurons/segment) than that labelled by LmigOR2 probes (generally 1-3 neurons/segment). Furthermore, some of the labelled neurons could be attributed to the basiconic sensilla, but LmigOR1 and LmigOR2 were expressed in different subtypes.

Our results strongly suggested that these newly discovered genes encode locust ORs and the differential expression patterns of LmigOR1 and LmigOR2 implied distinct functions. These results may offer insights into locust olfaction and contribute to the understanding of the evolution of insect chemoreception.