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http://www.sciencemag.org/cgi/content/full/314/5801/908
GENETICS:
Sea Urchin Genome Confirms Kinship to Humans and Other Vertebrates
Elizabeth Pennisi
Looking like old-fashioned pincushions and lacking legs, eyes, and even an obvious brain, sea urchins seem nothing like humans. But looks can be deceptive.
On page 941, George Weinstock of Baylor College of Medicine in Houston, Texas, and his colleagues describe the 814 million DNA bases that make up the genome of the purple sea urchin, Strongylocentrotus purpuratus. Its 23,500 genes suggest that these algae-eating invertebrates have more complex immune and sensory systems than researchers had appreciated.
The genome also includes many genes essential to humans and other vertebrates, although notably missing are numerous genes typical of flies and worms.
The genome "casts in concrete the reality" that sea urchins and other echinoderms really are closer kin to humans and other chordates than to beetles, flies, crabs, and clams, says Eric Davidson, a developmental geneticist at the California Institute of Technology in Pasadena.
Even if the sea urchin didn't share an ancestor with the chordates, its genome warranted deciphering. The animal has been a boon to biologists and biomedical researchers for more than a century. In the late 1870s, researchers observed for the f irst time the fusion of the egg's and sperm's nuclei by studying sea urchin eggs. Twenty-five years later, in 1902, Theodor Boveri used urchins to show that development was impossible unless each embryonic cell had the full complement of chromosomes.
Female sea urchins spew out millions of eggs at a time, which are easy to modify genetically, making them a perennial favorite of developmental and cell biologists. More recently, researchers have begun piecing together gene networks by tracing gene interactions during the sea urchin's development.
Now this organism's contributions to genetics, developmental biology, immunology, and other fields should explode even further, and not just because of the genome sequence. For the first time, a new genome sequence is accompanied by a comprehensive analysis (p. 960) of when and where genes are expressed--the so-called transcriptome. Other reports on pages 939, 940, and 956, and in the 1 December issue of Developmental Biology, help define the sets of sea urchin genes associated with specific functions such as biomineralization, as well as unravel the evolution of the genome.
Researchers have found unexpected sophistication in the urchin genome--particularly among its immune system genes--and in how the genes are employed for reproduction, development, and sensing the outside world.
This complexity shows that "evolution was pretty successful in developing most of the major [genetic] building blocks of a very complex organism quite a long time ago," says Francis Collins, director of the National Human Genome Research Institute in Bethesda, Maryland.
The sea urchin "provides a global view of the genes necessary for evolution to a human," adds Gary Wessel, a developmental biologist at Brown University.