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chicagotribune.com >> Technology
Genomes of a feather
Scientists say chicken DNA offers glimpse into past, even insight into human physiology
By Peter Gorner
Tribune science reporter
Published December 9, 2004
Scientists on Thursday published a detailed analysis of the newly mapped genome of the chicken--the first bird whose genes have been cataloged--which is expected to bring advances in human health, agriculture and evolutionary biology.
The genetic map also may help science understand the cockiness of a strutting rooster and a nesting hen's commitment to her young--and why such inborn gender differences might be shared by species as different as chickens and humans.
Since March, when the genome was made available to researchers, scientists have been comparing the chicken blueprint to the previously published genomes of humans, mice, rats and puffer fish.
It turns out that humans and chickens share more than half the same genes, the scientists reported Thursday in the journal Nature, but their DNA diverged in ways that may explain some of the differences between birds and mammals.
By looking at the genomes of a wide range of animals, scientists can better understand the structure and function of human genes and, ultimately, develop new strategies to improve human health, said Francis Collins, director of the Human Genome Research Institute, which paid for the $13 million chicken sequencing project.
"The chicken genome fills a crucial gap in our scientific knowledge," Collins said. "Located between mammals and fish on the tree of life, the chicken should provide us with new insights into genome evolution and human biology."
The sequencing project assumed new urgency with the emergence of bird flu epidemics in Asia, and among the important early findings is an immune system protein previously found only in humans. Studies of the chicken immune system could lead to better ways to control viruses that sometimes jump across species and infect humans.
"With both the human and chicken genome sequences completed, we can ask more questions about what types of resistance to viral infections and other pathogens are possible for humans and chickens," said project organizer Richard Wilson, director of the Genome Sequencing Center at Washington University School of Medicine in St. Louis.
Because chicken embryos develop in eggs, the bird long has been a favorite of researchers who study the earliest stages of life. The chicken genome also should help scientists identify the genetic basis for many important traits, including mutations in DNA linked to diseases, aging and death.
"Before the chicken genome was sequenced, we as researchers were essentially blind, but now we are able to see the genome more easily and explore the mechanisms by which it operates," said Mary Delany, a geneticist at the University of California at Davis and a co-author and coordinator for the analysis.
The sequenced genes belong to a 7-year-old female red jungle fowl known by her wing band number, 256. She lives in the lab of Jerry Dodgson, a professor of microbiology at Michigan State University in East Lansing, and is oblivious to her fame, he said.
A wild bird, the red jungle fowl (Gallus gallus) is the progenitor of the modern chicken, much as the wolf is the progenitor of the domesticated dog.
Offered to public domain
Nine months ago the genome was deposited into free public databases for use by researchers around the world. An international consortium of 175 scientists then set to work analyzing the genome and comparing it to organisms that already have been sequenced.
The chicken genome is expected to serve as a model for the estimated 9,600 species of birds.
Compared with humans, chickens have roughly the same number of genes but their genome is much more tidy and compact. It holds 20,000 to 23,000 genes formed from a billion DNA letters, compared with about 3 billion letters in humans.
The chicken genome contains far less smaller amount of repetitive "junk" DNA, indicating that its genes did not suffer mutations as often as those of humans.
The evolutionary lines leading to birds and mammals split from a common ancestor about 310 million years ago. Since then, chickens have gained genes that make proteins used for feathers, scales and beaks, while mammals have developed genes for hair proteins and lost genes related to egg albumen and yolk, the scientists said.
But the chicken and human genomes still hold 70 million DNA base pairs in common, suggesting that genetic material has been conserved since the species diverged in ways that may help explain why birds and mammals differ.
Comparing the genes for odor receptors, the scientists determined that chickens have a refined sense of smell--not comparable to dogs', but like humans'. That belied the common wisdom that birds have a poor sense of smell. Chicken genes do indicate they have a lesser sense of taste.
Understanding humans
Wilson compares the chicken genome and other genetic maps to Rosetta Stones that help scientists help understand the human genome.
"For every human gene, there's a gene in another species that's going to be most helpful in understanding it," he said. "For some human genes, we might have to look to the mouse; for others, we'll look to the chicken."
Although the chicken genome has obvious agricultural potential--higher egg production, better meat--Dodgson emphasized that it also will be important to biomedical research.
"Chickens and humans are infected, in some cases, by the same viruses, bacteria and parasites, and this research shows that chickens and humans share more than half of their genes," he said. "The chicken genome is expected to help us uncover genes that enhance natural resistance to disease in birds. Then we can see if those same genes are in humans."
The chicken also will become even more valuable as a model for human diseases, said LaDeana Hillier, senior research scientist at Washington University.
"The chicken serves as a model in embryology and gene regulation," Hillier said. "Because it's so easy to look at and manipulate gene activity as an embryo develops in the egg, researchers often use the chicken to study development. And it is being used by researchers in nervous-system development and in areas of study of developmental mutations, such as limb malformations, cleft palate, and dwarfism."
The chicken and its genes also may offer a peek at the prehistoric past, for, like all birds, chickens are thought to have descended from dinosaurs in the middle of the Mesozoic period.
"The chicken provides our first opportunity to look at the genetic sequences of the dinosaurs," Hillier said. "It's probably the best example we're going to get."
Manyuan Long, a University of Chicago member of the research team who studies the evolution of new genes, found that not much has changed in the chicken over the eons.
But people need only look to see remarkable things.
"When you see a bird flying around, you're actually seeing dinosaur meat," Long said. "Isn't that something?"
Copyright © 2004, Chicago Tribune
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