Fruit flies similarly have the ability to continue to perceive their environment in times of sensory loss. For example, they can determine the nutritional value of food, even when their ability to taste has been impaired. Now, a study in Nature Neuroscience has uncovered details as to how this might be possible.
On top of this, flies that have had their ability to taste restored, but are still chemically unable to absorb glucose into their haemolymph, actively choose the sweetest foods they can find, even if those sugars are the hardest to digest and therefore the worst nutritional option. They’re like toddlers presented with the option of cake with blue icing, or Ryvita and hummus - sweetest wins, regardless of whether it's any good for them.
If they can taste, but not absorb sugar into the blood, flies make bad food decisions. If they can't taste, and can't absorb sugar into the blood, flies can't decide anything, and eat whatever they can find. Clearly, there’s something about the presence of glucose in the haemolymph that activates a taste-independent pathway that selects nutritional food.
To try to work out the molecular causes of this, the researchers looked for mutations in genes that caused normal, happy healthy flies to suddenly lose the ability to distinguish different sugars. They found one and, obviously, named the gene it was in ‘cupcake’. Cupcake is similar to a human gene called, boringly, SGLT1, which is responsible for the absorption of glucose into the blood from the small intestine. So is cupcake responsible for glucose movement into the haemolymph, and therefore subsequent food selection based on nutritional value?
The answer, confusingly, is no.
|The curiously beautiful fruit fly |
ellipsoid body (right)
Deep in the fly brain, a tiny bundle of cupcake nerves somehow fire electrical impulses when, after hours of starvation, sugar reaches the gut and passes into the haemolymph. This then changes the fly's behaviour to seek out more of the good quality food that has triggered this reaction. Whether the sugar directly triggers the nerves or other, indirect processes are at play remains to be seen.
All of this makes me wonder — what processes, decisions and impulses are going on in our brains, independently of the information we feed them? Could we make good food choices even without the ability to taste? If people can achieve spatial awareness without the ability to see, perhaps this is not so far fetched. Brains are the most remarkable of things.
Dus, M., Ai, M. & Suh, G. S. Taste-independent nutrient selection is mediated by a brain-specific Na(+)/solute co-transporter in Drosophila. Nature Neuroscience 16, 526-528 (2013)