Sea squirt's DNA makes a splash. (Evolution).To examine how animals with backbones arose, U.S. and Japanese biologists have sequenced nearly all the DNA DNA: see nucleic acid. DNA or deoxyribonucleic acid One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. 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 The DOE Joint Genome Institute (JGI) was created in 1997 to unite the expertise and resources in genome mapping, DNA sequencing, technology development, and information sciences pioneered at the DOE genome centers at Lawrence Berkeley National Laboratory (LBNL), Lawrence Livermore in Walnut Creek, Calif. For example, the sea squirt has the beginnings of a spinal cord spinal cord, the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column. , making it a so-called chordate chordate Any member of the phylum Chordata, which includes the most highly evolved animals, the vertebrates, as well as the marine invertebrate cephalochordates (see amphioxus) and tunicates. . It also has a rudimentary immune system immune system Cells, cell products, organs, and structures of the body involved in the detection and destruction of foreign invaders, such as bacteria, viruses, and cancer cells. Immunity is based on the system's ability to launch a defense against such invaders. . 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 See triangulation. 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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