Sea squirt’s DNA makes a splash

From Boston, Mass., at the Genome Sequencing and Analysis conference

SQUIRT OF DNA. This sea squirt’s genes may reveal the origins of vertebrates. DOE

To examine how animals with backbones arose, U.S. and Japanese biologists have sequenced nearly all the DNA of Ciona intestinalis, a sea squirt.

Adult sea squirts consist largely of two connected tubes. Stuck to the ocean floor or to rocks, they suck in ocean water, from which they obtain nutrients before squirting the water back out. Despite this unassuming life, sea squirts have a unique suite of traits that has drawn the interest of evolutionary biologists.

Sea squirts such as C. intestinalis first appeared more than 500 million years ago. Although it lacks a true backbone, C. intestinalis bears many similarities to vertebrates, says Daniel S. Rokhsar of the Department of Energy’s Joint Genome Institute in Walnut Creek, Calif. For example, the sea squirt has the beginnings of a spinal cord, making it a so-called chordate. It also has a rudimentary immune system.

In April, several dozen biologists gathered in California to scrutinize the sea squirt’s DNA, which was sequenced by DOE and Japanese groups from Kyoto University in Tokyo and the National Institute of Genetics in Mishima.

The sea squirt has around 15,500 genes. About one-eighth of these have counterparts in people but not in invertebrates such as flies or worms. “These are candidates for genes specific to chordates,” says Rokhsar. By comparing the sea squirt’s genome with that of worms, fruit flies, and people, he adds, “we can start to triangulate when [specific] genes appeared during evolution.” A full analysis of the sea squirt’s genome is expected to be published later this year, notes Rokhsar.


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