It may appear somewhat astonishing that the small fruit fly Drosophila melanogaster is used to gain insights into how the human brain functions. Is this unmanned flying object really able to solve complex calculations? Can it write? Read? Can it talk? "Using a model organism, one implicitly takes something for granted," said Prof. Dr. Karl-Friedrich Fischbach from the Department of Neurobiology and Genetics at the University of Freiburg's Institute of Biology III. "And this assumption is that there exist general principles in nature which can be transferred from simple organisms to organisms of greater complexity, including humans." Fruit flies reproduce sexually and hence underlie the laws of classical genetics. In the first half of the 20^th century, the Nobel Laureate Thomas Hunt Morgan used fruit flies to demonstrate that genes are carried on chromosomes and are the basis of heredity. The same principle is also found in humans. Therefore, one may also ask whether there is a general similarity between the human and fruit fly brain.

More than ten years ago, Fischbach and his team discovered the gene irregular Chiasm C (irreC) in one of the fly mutants. irreC is necessary for the correct projection of visual fibres in the optic chiasm. Defective irreC genes lead to malformations in the optic area in the fly brain, which are referred to as outer and inner optic chiasms. The axons, originating from the eye, take in this case a long detour before eventually finding their final destination in deeper brain areas. Further research has shown that the protein Irre C has a sister, the protein Kirre. These two proteins are located in the membrane of axon endings and extend into the extracellular space where they bind to defined proteins on the surface of other cells. "That is how the growing axons recognise their target cells," explained Fischbach. The researchers also found protein partners on the membranes of the target cells, which are bound by IrreC and Kirre. The entire functional unit was subsequently named "irre cell recognition module" (IRM). All these molecules are members of the immunoglobulin family, and are similar to the antibodies of the immune system that are found in humans. And this is a long way from being the only similarity between flies and humans.