In order to orient and survive in their environment, animals must develop a concept of their own size. Researchers from the Johannes Gutenberg University of Mainz (JGU) showed that the Drosophila Drosophila melanogaster developed a very stable long-term memory for its own size and reach of its extremities after the pupa case outbreak . The fruit fly acquires this memory through a visual feedback obtained when walking, but during the two hours following the workout, the memory is still sensitive to the effects of stress and is not yet firmly anchored.
Once the memory is consolidated, it appears from our observations that it remains intact for life. The insects seem to be calibrated for the rest of their lives. "
Professor Roland Strauss of the Institute of Developmental Biology and Neurobiology of JGU
However, it is still difficult to understand why they can only access the acquired knowledge 12 hours after the training. Researchers still do not know what's going on in the brain in the meantime.
The research group of Professor Roland Strauss uses Drosophila melanogaster to study the processes of memory retention and consolidation that are known to occur to some extent also in humans. Previous studies had shown that the short term memory of fruit flies declined with age and that a protein involved in this process was similar to that playing a role in humans.
In the new study, Tammo Krause and Laura Spindler analyzed the memory of Drosophila's body size. Fruit flies are insects that metamorphose completely. They undergo three larval stages during which they grow. Finally, at the end of pupation, the mature fruit fly emerges. Due to the hard exoskeleton of Drosophila, the size of the body can not change anymore; however, it can vary because the actual size of a fly is determined by the availability and quality of food during the larval stages.
Leg-to-head behavior indicates intention to climb
"In our recent study, we wanted to know how insects acquired information about their size and memorize them later," explained Tammo Krause and Laura Spindler. They observed how, in various conditions, the insects were trying to overcome a small gap exceeding their step size. Drosophila displays stereotyped behavior in such situations: In order to start the climbing attempt, the insect begins by doing research movements with its forelegs raised on the head. If the gap is much too big, this typical behavior is not observed. The fly turns away.
Knowledge of body size is gained through visual feedback
The results show that Drosophila learns to estimate the distance across a gap and the reach of its legs by linking visual information from the environment, such as a pattern with stripes, to the size of their body. Newly hatched fruit flies raised in the dark overestimate their size and attempt to fill in far too large gaps much more frequently than animals reared in the usual light-dark cycle. The parallax of motion created on the retina by the surrounding structures when walking is used for the learning process. Another experiment in which motion parallax has been manipulated and artificially reduced during the learning process has confirmed this. As a result, flies underestimated the size of their bodies and undertook fewer attempts to climb.
"When fruit flies hatch from the pupa case and move in space, the resulting movement is picked up by the eye and the fly can calibrate itself," explained Professor Roland. Strauss. "Once this calibration is done, the knowledge is kept for life." The research team proved it by an additional experiment in which fruit flies had to spend 21 days in the dark after three days under normal lighting conditions. Even after this long period, they have always made the same number of climbing attempts as the three-day flies. "We therefore assume that the memory of their own size is the most permanent form of reminder identified in Drosophila to date," concluded the two lead authors, Tammo Krause and Laura Spindler. Current biology.
The body's memory is only available after 12 hours
Neurobiologists have discovered additional mechanisms that raise new questions. Memory consolidation within two hours of training is always stress sensitive. This means that exposure to stress can result in its suppression during this period. Once the acquired information is anchored, it takes 12 hours after the end of the training so that fruit flies can access it permanently. Researchers would now like to know what is happening in the meantime and how the information acquired is permanently consolidating in the brain. "We are particularly interested in the epigenetic factors involved in the development of long-term memory, which is a topic on which we will continue to work," Strauss said.
Johannes Gutenberg Universitaet Mainz
Krause, T. et al. (2019) Drosophila acquires a large memory of durable body from visual commentary. Current biology. doi.org/10.1016/j.cub.2019.04.037.