Week In Science

An international team of 240 scientists, that included researchers from the Human Genome Sequencing Center (HGSC) at Baylor College of Medicine in Houston, Texas, has announced the completion of the sequencing of the genome of the sea urchin (strongylocentrotus purpuratus).

The work has taken three years, and already resulted in the submission of more than 40 papers.

So far, whilst the raw sequence of base pairs has been figured out, not all the genes have been identified. It is estimated that there are around 23,300 genes in the sea urchin genome; a similar number to that of human beings.

Like human beings, sea urchins are deuterostomes. This is opposed to animals such as flies and worms which are protostomes. Both superphyla of animals develop firstly as a ball of cells called a blastula. But in deuterostomes, the first opening becomes the anus, whereas in protostomes, the first opening becomes the mouth.

However, beyond that, sea urchins are quite different from human beings. They lie in the echinodermata phylum; animals with fivefold radial symmetry at some point in their lives and a hard exoskeleton made of calcified plates and spines.

Humans however, belong to the chordate phylum - animals which during development have a notochord (which essentially becomes the backbone in humans), dorsal nerve chord (which becomes the brain and spinal cord in humans), pharyngeal pouches (which become, amongst other things, the middle ear, tonsils and certain glands in humans) and a muscular tail that extends beyond the anus (well, we have one before we are born, but this usually just refers to the coccyx in humans).

So, in full, the significance of the sequencing of the sea urchin genome is that it is the first of an echinoderm.

Some interesting things have been discovered. The sea urchin has an innate immune system, meaning that it is designed to provide a fixed set of immunological responses. This is quite different from the immune system of a human, which is able to adapt over our lifetimes to new threats.

However the immune system of a sea urchin is by no means simple. Jonathan Rast, an immunologist at the University of Toronto, says of it, "The complexity is unprecedented."

The genes sequenced include many that code for proteins involved in the detection of bacteria. At least two kinds of these have analogues in humans, but of these, the sea urchin has ten times as many.

Most amazingly, certain genes found in the sensory systems of humans (in particular those coding for proteins involved in the sense of sight and smell) are found in the tube feet of sea urchins. And those genes are actually switched on, implying that they have a function there!! Of course the urchins have no sense of sight or smell, so the function of these genes is completely unknown in the urchin, at present.

Other examples of genes that are found in humans turning up in unrelated contexts in other animals are known, but as with the presumed sensory genes of the urchin, the function of these genes in other animals is not always known.

Approximately 70% of the genes found in the urchin are estimated to have counterparts of one kind or another in human beings. As with comparative anatomy, comparative genomics is expected to provide scientists with interesting new discoveries for many years to come.