Odour, ardour and anger

04 September 2014
By Ximena Ibarra-Soria

Odorants and pheromones that provide information about the context and instruct how a mouse should react in a particular situation. Credit: Wellcome-Library, London

Odorants and pheromones that provide information about the context and instruct how a mouse should react in a particular situation. Credit: Wellcome-Library, London

Males and females behave very differently, especially when it comes to mice. For example, if a female and a male find each other, they will show courtship behaviours and will try to mate; but if it is a male that comes across another male, they will fight and try to establish the territory as their own.

So, how is it that males and females can react so differently to the same situation?

Mice, and many other animals, use their sense of smell to identify other animals around them. It is odorants and pheromones that provide information about the context and instruct how a mouse should react in a particular situation. These are detected by receptor genes present in neurons that are in the nose.

Receptors work like a lock-and-key: when an odorant matches the shape of the receptor, it can bind and activate that neuron. Active neurons then send a signal to the brain where the animal interprets the meaning of the smell and instructs the appropriate behaviour. In order to be able to detect the trillions of odours that exist, the mouse genome contains more than 1,500 receptor genes.

These genes are present in the genomes of both male and female mice, so I wanted to understand if the great differences in behaviour could be explained by how the receptors are turned on, or expressed, in their noses. Surprisingly, I found that they all look almost identical. This suggests that the different responses between the sexes are not provoked by their capacity to detect distinct smells. Instead, the difference could be encoded in how the detected odour signals are processed in the brain.

The methodology used for this study was RNAseq, which is a very powerful technology that allows probing all the genes in the genome in a single experiment. This means that we can get a global picture of the tissue of interest, and analyse every gene that is turned on. Despite not finding any sexual differences in the expression of the receptors, being able to study the more than 1,500 receptor genes simultaneously revealed some interesting things. I found that a few unusual receptors are turned on at much higher levels than others. I am now working on understanding what it means to have so much more expression of a specific receptor gene, and if this impacts our sensitivity to certain smells.

Interestingly, when I compared the expression of the receptor genes between different animals, I found an identical pattern: the highly expressed receptors are always the same. In this case, all the animals tested have the same genome, and this may explain why the expression is conserved across them.

I am now studying whether changes in the genome or in the environment can alter the expression of these genes. All this information together will help us better understand how our sense of smell works, and why animals (and eventually humans) sense and react to the world differently.

Ximena Ibarra-Soria is currently a PhD student in the Genetics of Behaviour group where she works under the supervision of Darren Logan on understanding the expression of olfactory receptor genes and their relationship to perception and behaviour

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