Male silkmoths sniff out mates with neurons tuned to detect just the right amount of scent.
Forget elaborate dances, sweet serenades or complicated foreplay. And throw away the Spanish fly and oysters. If you’re a female silkmoth, chances are that your would-be mate is already drunk on your very own elixir of love if he’s within whiffing distance. It takes just 170 molecules of the sex pheromone bombykol to put a male silkmoth in the mood.
Farmers have been using bombykol for years to bamboozle love-sick moths in their fields. But how so few molecules elicit such a strong behavioural response has been somewhat of a mystery to scientists. But in a new study, a team of Japanese researchers have cleverly deciphered the neural circuitry of the Bombyx mori olfactory system as it responds to the sex pheromone.
The silkmoth sex pheromone bombykol is detected by specialised sensory organs called sensilla. These tiny hair-like projections adorn the elaborate antennae of male moths to help them detect extremely dilute concentrations of bombykol emitted by females. Each sensillum is enervated with an olfactory receptor neuron, which is devoted to sniffing out chemicals like bombykol that happen to waft past.
The olfactory receptor neurons that respond to bombykol do so because of specialised receptor proteins that detect the pheromone. This then triggers a nerve impulse that gets relayed through nerves in the moth antennae and into the neural networks of the brain.
With such a low concentration of bombykol required to make male moths amorous, the Japanese researchers reasoned that there must be some kind of signal amplification happening in the moth neural networks. A single olfactory receptor nerve could be relaying its message to several nerve cells in the antennae, thereby boosting the signal that is transmitted to the brain.
Another way that the signal could be amplified is by the moths getting repeated small sniffs of bombykol over a short period of time, all adding up to a large signal relayed to the brain. This temporal integration has been notoriously challenging to study because of the difficulty in controlling the dose of bombykol that the moths receive.
To overcome these difficulties, the team used a nifty technique called optogenetics to replace the spurts of pheromone with flashes of blue light to trigger the moths’ olfactory receptor neurons. The first step was to genetically engineer silkmoths to produce a light-sensitive ion channel protein from green algae. This protein, called channelrhodopsin-2 (ChR2) was engineered to only be produced in the olfactory receptor neurons that detect bombykol. When the researchers flashed a blue light onto a moth’s antennae, the ChR2 channels opened, generating a nerve impulse as though their bombykol receptors had been triggered. The moths flashed with a blue light would start to vigorously flap their wings, just as they would if they had sniffed the presence of a female in the vicinity.
When moths were repeatedly flashed with the blue light, flashes less than 80 milliseconds apart increased how much the moths responded to the fake bombykol signals. This confirmed the researchers’ hypothesis that temporal integration was occurring to amplify the pheromone signal. The researchers were also able to determine that this integration of signals was occurring in neurons in the moth antennae.
Interestingly, the moths were more sensitive to shorter bursts of light, equivalent to lower concentrations of bombykol. If the signals were too strong, temporal integration was blocked in the antennae neurons and the moths failed to respond to the cue.
The findings fit well with how moths experience bombykol in the wild – they are optimised to detect low concentrations that drift past in plumes of air. The more they sniff, the greater their excitement. But only up to a point, it seems.
Reference: Tabuchi M, Sakurai T, Mitsuno H, Namiki S, Minegishi R, Shiotsuki T, Uchino K, Sezutsu H, Tamura T, Haupt SS, Nakatani K & Kanzaki R. (2013). Pheromone responsiveness threshold depends on temporal integration by antennal lobe projection neurons. Proceedings of the National Academy of Sciences USA doi:10.1073/pnas.1313707110