Flight drove insect olfaction evolution
How did insects come by their sense of smell? They actually have two different smell systems - an ancient one that they share with their crustacean cousins like crabs and a newer one that they've evolved separately. Chris Smith spoke to Christine Missbach...
Christine - Insects mainly use two receptive families. One, the family of so-called insect olfactory receptors - these are specific to the insects. The closest relatives of the insects, the crabs, do not have this kind of receptors. It's the hypothesis that insect olfactory receptors have lost when the insect had to transition from water to land, and detect airborne chemicals.
Chris - So, how did you try to understand whether that was the case that as soon as the insects left the water and came on to land, they responded by producing a new class of olfactory receptors?
Christine - Most studies on the insect olfactory receptors were done in flying insects. So, nobody has ever looked into taxa that are older, for example, the jumping bristletails or the firebrats. And by comparing the results we got from those two higher insects or the closely related crustaceans, we hope to get new insights into the evolution of this insect olfactory genes.
Chris - So, how did you study those insect species which are these more primitive non-flying insects to find out how they respond to smells?
Christine - First of all, of course, we had to show that this insect have a functional olfactory system. Using electrophysiology, the insects have very long antenna that are covered by small cuticular structures called sensilla and the neurons are inside the sensilla and we can contact these sensilla and record from the neurons inside the sensilla.
Chris - So, you were making electrical recordings from the olfactory apparatus of these insects and presenting them with different chemicals to see if they all responded and if they responded in the same way to those chemicals across the different classes of insects you studied.
Christine - Yeah. First of all, we can use this technique to show that they can smell and also, try to categorise the different neurons in a sensilla and count also the number of types of neurons that are present.
Chris - And that will presumably give you a handle on that evolutionary timeline because presenting these chemicals to the different groups of insects, I presume what you're able to do is to show that with evolutionary time, the smelling ability becomes more diverse.
Christine - Yes. First of all, the numbers of neurons we identified for this primitive insect were much smaller and also, rather broadly tuned.
Chris - So, did you then, having identified the responses to the chemicals go to the genome and find the genes corresponding to these receptors?
Christine - We used RNA-Seq, looking which genes are active in the olfactory tissue and then searched into this data for the receptors.
Chris - So, putting all of the findings together, what do you conclude?
Christine - We think the insect type of olfactory receptor is not an adaption to the terrestrial conditions and the detection of volatile, air-borne chemicals but evolved later during the evolution of insects. We think that they could be an adaption to the insect fly because during flight, also, much higher temporal resolution and higher sensitivity is necessary.