THE genome of the sea squirt, a distant cousin of animals with backbones, has been sequenced. It is helping reveal how the genome of vertebrates like us evolved.
Sea squirts, with their leathery, filter-feeding tubes, do not look much like long-lost relatives. But the larval form of sea squirts reveals their true ancestry. These free-swimming tadpoles have a stiffened rod, or notochord, running down their back, which in a developing vertebrate is the forerunner to the backbone. They also have a simple brain and heart.
The creature that gave rise to both sea squirts and vertebrates appeared on the planet during the Cambrian explosion, an orgy of evolutionary experimentation about 550 million years ago. And as modern sea squirts are thought to be similar to this common ancestor, comparing their genome with our own reveals how the vertebrate genome evolved.
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The species sequenced, Ciona intestinalis, has about 17,000 genes – about half as many as us – according to the joint American and Japanese sequencing team. But the sea squirt genome is just a twentieth of the size of ours, with 160 million letters (Science, vol298, p 2157).
Our extra genes are mostly duplicates of ones that already existed as single copies in Ciona. No one was sure when these genes were duplicated, but thanks to this study we now know that it probably occurred in an early vertebrate.
The extra functions that the duplicated genes can take on may be what allowed vertebrates to become more complex than other animals. “If your genome is big and floppy, maybe you have more flexibility,” says team member Dan Rokhsar at the US Department of Energy Joint Genome Institute in Walnut Creek, California.
One surprise is that Ciona seems to have stolen at least one gene from bacteria: a gene that enables it to make a cellulose-like compound. It uses the tough substance, not found in other animals, to make a leathery tunic. “It’s the most dramatic example of gene transfer into an animal,” says Rokhsar. When the human genome was published, it was claimed we had acquired about 200 genes from other organisms, but later research has suggested this is not the case.
Genomes are now coming in thick and fast. Ciona is the seventh animal to have its genome sequenced, along with the mouse, fruit fly, mosquito, nematode worm, pufferfish and human.