Biology of Learning and Memory in the Fruit fly

The fruit fly Drosophila, like the Brahmin, is born twice. First it emerges from the egg as a maggot, then after a week or so of metamorphosis, as a two-winged fly from the pupal case. When Prof. Siddiqi began to study its sense of smell some 25 years ago, he was inclined to believe that the chemosensory behaviour of the fly is largely stereotyped, governed by its genes. Over the years experiments have forced him to modify this view.

Prof Siddiqi's group now has a fairly detailed view of the organization of Drosophila's olfactory brain which, much like our own, is highly structured. It undergoes dramatic changes after birth and adapts the individual fly to the environment in which it is born. These adaptive changes arise from a subtle interaction between genes and environment. Odorants are detected by olfactory receptors, a set of sinusoidal (snake-like) proteins embedded in the membranes of sensory neurons. The receptors coupled to a highly sensitive amplification system send electrical signals to the brain. The signals encode the information about the olfactory world which is analyzed in three principal regions of the fly's brain, the antennal lobe (AL), the mushroom body (MB) and the central complex (CC). The representations of the odour world in these brain centres are not fully understood.

The fruit fly is capable of different types of learning and associative conditioning. Its small, but relatively tractable brain, provides an excellent system for studying the neurobiology of learning and memory. Prof. Siddqi and his associates have been studying olfactory learning in Drosophila.
They have discovered a novel form of adaptive conditioning which is akin to odour imprinting in birds and mammals. Most odorants elicit a dual response from the fly. At low concentration, these chemicals are attractive but at higher concentrations they become aversive. Attraction and aversion are however not inborn but acquired characteristics. The odorants which are encountered early after birth become attractants. Chemicals which have not been experienced become aversive.

Imaginal conditioning is an associative process. In order to become attractive, an odour has to be associated with feeding. It seems to be a form of reward learning. Olfactory conditioning is partly peripheral and partly central. The neural substratum of olfactory learning is partly peripheral and partly
central. The neural substratum of olfactory learning and how its representation in the brain changes is currently a subject of exciting research in neurobiology. Some insights into this fascinating subject have been gained by studying learning and memory in the fruit fly.