Fruit flies that are nearly genetically identical raised in the same environment still learn at different rates, suggesting that random differences in brain development may have evolved to produce variation in a species
2 February 2022
Genetically similar fruit flies raised in an identical environment still learn in different ways, suggesting that individuality may not be influenced by nature or nurture alone, but might have a third key factor: randomness.
Despite having essentially the same genetic code and being raised in exactly the same experimental conditions, individual fruit flies (Drosophila melanogaster) learn to avoid negative experiences, such as electric shocks and bad tastes, at different rates. Random factors – or possibly extremely slight differences in experience – might therefore contribute to a species’ ability to adapt to a changing world by ensuring a healthy variety of individual traits, says Benjamin de Bivort at Harvard University.
“If the world is unpredictable in how reliable learning cues are, it may be a good strategy to produce some flies that are early adopters – which would be helpful when cues are reliable – alongside sceptics, which are helpful when cues are unreliable,” says de Bivort. “That way, no matter the state of the world, some of the progeny will have the right learning strategy.”
In previous studies, fruit flies that have been genetically modified or bred in order to have essentially identical genomes, and then raised in identical laboratory settings, have shown individual preferences for light, temperature, postures and turning left or right. De Bivort and his colleagues wondered whether the flies would also behave differently in a learned, rather than innate, context.
They tested the flies’ abilities in learning tasks using 1-week-old female fruit flies that had been genetically modified to have nearly identical genomes, housed together and fed the same cornmeal diet. The researchers placed each fly in a testing arena with two tunnels, each with a different odour. One odour was initially associated with either an electric shock or a bitter taste – and the next day, the scientists switched which odour was associated with these negative stimuli.
The flies had widely varying responses to a single training session, reflecting clear individual differences, says de Bivort. And those differences remained consistent for each individual: the ones that quickly learned to avoid the shock were the same ones that quickly learned to avoid the bitter taste, for example.
“This sort of pushes against the idea of nature versus nurture, or genes and the environment [alone], as explanatory factors for variation,” he says.
Technically speaking, the differences could possibly be related to fine details like whether a fly spent more time touching the plastic of the tube it grew up in, compared with other flies, or whether it had more or less light than the others depending on where in the crowd of flies it generally stayed, he says. There might even be atomic-level thermal fluctuations within the fly’s brain molecules – if they affect the structure of key neural circuits. But these factors are practically impossible to measure, he says.
A more “convenient” explanation is just pure randomness during brain development, says de Bivort.
“Our best guess is that small random developmental differences in learning circuits account for the different learning styles,” he says. “The adult fly brain builds itself during the animal’s pupal phase. The ‘program’ that constructs it likely has a slightly different outcome every time, and we hypothesise that those differences matter for the behaviour.”
Journal reference: Biology Letters, DOI: 10.1098/rsbl.2021.0424
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