SCIENCE: Hard-wiring the fruit fly's visual system
EcoPolis
Hard-wiring the fruit fly's visual system
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Both vertebrate and fruit fly have so-called visual maps in the brain
that represent the world they see. 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."
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Others who participated in this research include Drs. Yi Zhou, Tong-Wey
Koh, Sunil Q. Mehta, Karen L. Schulze, Yu Cao and Patrik Verstreken,
all of BCM; Thomas R. Clandin of Stanford University; Karl-Friedrich
Fischbach of the University of Freiburg in Germany; and Ian A.
Meinertzhagen at Dalhousie University in Halifax, Nova Scotia.
Hiesinger, who is first author, is now with The University of Texas
Southwestern Medical Center in Dallas.
Funding for this research comes from the Howard Hughes Medical
Institute, the National Institutes of Health and the Deutsche
Forschungsgemeinschaft.
(Source: Baylor College of Medicine)