The sushi genome project
Sea urchin genes reveal surprising similarities with humans.
Researchers have determined the genetic sequence of the purple sea urchin, Strongylocentrotus purpuratus. Spines and sushi aside, Helen Pearson investigates what researchers find to like about these creatures.
What's so special about sea urchins?
Few people would name sea urchins as their favourite animal. They are the pincushions whose spines pierce the feet of unsuspecting paddlers. They are also members of the echinoderms, a family of sea-dwelling organisms that includes sea cucumbers and starfish. These organisms have five-fold symmetry but no obvious brain.
So why do scientists study them?
Some researchers still cherish them because it is possible to collect bucketfuls of eggs and extract large quantities of DNA and protein for analysis. It is also very easy to find out how specific genetic sequences function, allowing researchers to understand how networks of genes control each other to form certain tissues.
"It's a wonderful system and those that work on it don't want to work on anything else," says Eric Davidson at the California Institute of Technology, Pasadena, who has studied sea urchins for over a quarter century. "They're unbelievably forgiving, as long as you don't cook them."
How do they relate to humans?
More closely than you might think. Sea urchins, humans and other vertebrates are all members of a group called the deuterostomes. This means that they shared a common ancestor over 540 million years ago and there are still similarities in the way our embryos develop. Like us, for example, but unlike insects and molluscs, urchins and starfish have an internal skeleton.
Sea urchins, then, are more closely related to humans than other widely used model organisms such as flies and nematode worms, but far more distant than mice, chickens or other vertebrates. So their sequence fills a gap for geneticists, revealing which genes are unique to vertebrates and which are ancient genes shared by deuterostomes.
What does the genome reveal?
Researchers are particularly excited about the sea urchin's immune system, and certain families of genes that are involved in recognizing bacteria and other invaders. Humans have tens of these genes; the sea urchin has several hundred, the genome sequence published in Science this week reveals1. This may be because the animal which can live for over a century uses them to fend off bacteria and viruses.
Some genes assumed to be unique to vertebrates have also turned up in sea urchins. For example, the eyeless, earless animal has genes that, in us, are involved in detecting sight and sound. Urchins might use them to sense light or heat.
Do researchers eat them?
Sea urchin reproductive organs are a delicacy in certain cuisines, such as Japanese sushi. But it is an acquired taste. "I have a rule that I never eat what I do research on," Davidson says.
"This is really the sushi genome project," adds George Weinstock of Baylor College of Medicine in Houston, Texas, who led the sequencing effort. In 2002, researchers determined the genome of another Japanese treat, the pufferfish Fugu rubripes.
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- Weinstock G. M., et al. Science, 314 . 941 - 952 (2006).