Hard-wiring the fruit fly's visual system

These visual maps consist of millions of nerve cell contacts that need to be wired correctly during development in order for the adult animal to see normally. It is generally thought that the complexity of visual maps, like other brain regions, cannot only be genetically programmed but requires activity by neurons or nerve cells in the brain.

In a new study published in the journal Current Biology, Drs. P. Robin Hiesinger, R. Grace Zhai and co-workers in the laboratory of Dr. Hugo Bellen, director of the Program in Developmental Biology at Baylor College of Medicine, found that this neuronal activity is not required for the formation of the visual map in Drosophila melanogaster, the most common form of fruit fly used in laboratories around the world.

“There is a genetic component (to formation of the vertebrate visual system),” said Bellen, who is also a Howard Hughes Medical Institute investigator. “The neurons in vertebrates are born and are genetically programmed to project into a certain brain region. This is followed by a dynamic phase where neuronal activity refines the visual map. In contrast, in flies the system seems to be completely hard-wired and only rely on genetic inputs.”

“The most obvious difference between the insect and vertebrate brain is their size and the number of neurons and connections that need to be made. A possible explanation for the findings is that the fruit fly has many fewer neurons than vertebrates, and the system can therefore just rely on the genetic components in flies,” said Bellen.

“In vertebrates, complexity is added because of the challenge of millions of neurons having to make billions of precise connections. You have to work with a gross topological map first, and neuronal activity refines this map later,” he said.

The study adds to an ongoing debate about the extent to which brain wiring can be genetically programmed.

“We have to be careful when we interpret these results in light of the complexity of the human brain,” said Bellen.

However, he said, “It is astonishing though how only a few thousand genes can program billions of synaptic connections.”