We share 80% of sea squirt's 16, 000 genes Credit: JGI

Scientists have sequenced the sea squirt1. This latest genome promises insights into how animals develop, and how those with backbones evolved from those without.

Sea squirts (Ciona intestinalis) are among our closest spineless relatives. Their hearts and nervous systems, for example, are like simple version of ours. Larvae look like tadpoles, with a rod of tissue stiffening their backs. Adults live attached to rocks, and look a bit like sponges.

At 160 million DNA letters, the sea squirt genome is relatively tiny. It is less than half the size of the smallest known vertebrate genome - that of the pufferfish - and 20 times smaller than the human.

The animal has about 16,000 genes, half as many as most vertebrates. We share about 80% of these. But where humans or mice have families of many similar genes, such as for the immune system, the sea squirt typically has just one for each function.

"Vertebrates increased their complexity by having multiple genes," says Dan Rokhsar of the Joint Genome Institute in Walnut Creek, California, where much of the sequencing work was done.

The squirt's simple genetics will help us to understand how and when genes switch each other on and off, he says.

"If you combine [the sea squirt] with other organisms, it's very useful," says physiologist Alex McDougall of the University of Newcastle upon Tyne, UK. "You can do the early experiments with this simple organism, then move up a level, maybe to the mouse, and eventually human."

You can do the early experiments with this simple organism Alex McDougall , University of Newcastle upon Tyne

McDougall's team studies fertilization and cell division. His team is already using the draft sequence to hunt down the genes that control the fertilized egg's first cell division.

The squirt can also do things that we can't. It stiffens its adult body with cellulose, for example. Plants and bacteria use this tough molecule in their cell walls, and possess similar genes for making and breaking it.