...for male moths, that is. Few students of zoology will be unaware of the remarkable mate-finding ability of certain male moths. As this occurs at night, the cues given by a receptive female are necessarily olfactory; she exudes from her abdomen tiny quantities of a volatile messenger, or pheromone. Using their hugely elaborate, sexually dimorphic antennae, males can detect females from (literally) miles downwind, apparently on the basis of receiving a signal from a single pheromone molecule. The male then closes in on the female with a simple strategy; he keeps flying upwind while the pheromone concentration increases, and breaks into zigzag hunting mode whenever he loses the signal. The success of this system is manifest; but how is it achieved at the molecular level? The female sex pheromone of the silk moth Bombyx mori is known to be a single molecule, the unsaturated long-chain alcohol bombykol. Sakurai and colleagues have now succeeded in identifying and characterising the male silk moth antennal receptor that responds to this sex pheromone.
Since the receptor responds to a female pheromone, the receptor mRNA was likely to be enriched in male antennae. So Sakurai et al. started with a B. mori adult male antennal cDNA library and screened it differentially, using male antennal cDNA as a positive probe and male body cDNA as a negative control. One of the 62 clones that the team sequenced was clearly for a G-protein-coupled receptor that closely resembles other known insect odorant receptors; its closest homologue in Drosophila is Or83b, an `orphan' odorant receptor (i.e. one for which no ligand has yet been found). The gene, dubbed BmOR-1 by the team, was found to be widely expressed only in chemosensory neurones of male silk moth antennae, and nowhere else in males or females, exactly as would be predicted if this receptor detects the female sex pheromone. To demonstrate that BmOR-1specifically responds to the female pheromone bombykol, the team expressed the receptor in Xenopus oocytes and exposed the oocytes to both bombykol and its natural oxidation product bombykal. They found that the oocytes were sensitive to bombykol, but not bombykal. This ties in well with physiology;although the female co-releases bombykal with bombykol, males only respond behaviourally to bombykol.
As a final flourish, the group took the work back to the organism, to prove that this was a bombykol receptor in vivo. They made a baculovirus (a natural virus for lepidoptera) encoding the receptor, and infected female antennae with it. Expression of the transgene was detectable by RT-PCR,suggesting that the receptor was expressed in female antennae, and these female antennae showed an electrophysiological response to bombykol –which of course they would not normally show, since females do not normally respond to their own pheromone. These results convinced the team that the receptor responds to bombykol in vivo.
The team then searched the emerging silkworm genome data, and identified 29 putative odorant receptor gene sequences. They expressed every one of these in Xenopus oocytes, and showed that none of them responded to bombykol. It thus seems clear that BmOR-1 is likely to be the only such sex pheromone receptor in Bombyx.
