Ascidians are sea squirts — hermaphrodite marine invertebrates that live in shallow ocean waters, and which are thought to exhibit characteristics of the evolutionary ancestors of chordates and vertebrates. In the December 13 Science, Paramvir Dehal and colleagues report a draft sequence of the genome of the ascidian Ciona intestinalis, one of the smallest of the chordates (Science, 298:2157-2167, December 13, 2002).

Most of the DNA came from a single individual in Half Moon Bay, California, and was sequenced using a whole-genome shotgun approach. Dehal et al. observed a high rate of allelic polymorphism, nearly 15-fold higher than that seen in humans. The genome assembly spans 116.7 Mb and contains 95% of the protein coding genes. Japanese researchers have also collected over 480,000 ESTs (expressed sequence tags) that were used to find genes in the Ciona genome sequence. An annotation workshop came up with a prediction...

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