14 DAY TRIAL // Analyzing the newly sequenced genome of the sea urchin, scientists were surprised to see many similarities with the vertebrates. "The sea urchin is surprisingly similar to humans," said George Weinstock, of Baylor College of Medicine.
"Sea urchins don't look any more like humans than fruit flies, but about 70 % of sea urchin genes have a human counterpart whereas only about 40 % of fruit fly genes do."
Sea urchin's look comes from a round outer shell of its skin covered with spikes for deterring predators and many tiny tube feet like tentacles used for creeping and getting food along the seafloor. Sea urchins, together with starfishes, sea cucumbers, and sea lilies form the phylum Echinodermata ("thorny skinned"), while vertebrates belong to the phylum Chordata ("animals with spine").
But the echinoderms and chordates belong to a larger group called the deuterostomes, opposed to proterostomes, grouping worms, mollusks, arthropods (crustaceans, spiders, scorpions, insects, millipedes). Thus, sea urchin is more related to humans than other species favored by biologists for research, like fruit flies and C. elegans roundworms. Sea urchins could serve as a model for understanding how the vertebrates split off, developing different traits.
The research team studied the genome of the California purple sea urchin, Strongylocentrotus purpuratus (photo), a kelp eater living along the west coast of North America from Baja to Alaska (this is the first genome sequencing made on echinoderms). Geneticists identified 23,300 genes but - surprisingly - 7,077 of them had human counterparts.
The sea urchin's genome showed a surprisingly unique and complex immune system, which surpasses by far the human one and which could explain their lengthy life spans of up to 100 years. The immune system differs greatly between species (humans show great differences from chimpanzees, for example) and the sea urchin's immune system could shed a light on understanding how the human immune system evolved. Like humans, sea urchins have many immune genes active in the gut, helping to cope with gut symbiotic bacterial flora. "It is one thing to be able to recognize all bacteria and get rid of all of them. It is another thing to maintain that in a complex way that you don't kill all of them," said Jonathan Rast of the University of Toronto.
The immune system of vertebrates has an innate and an adaptive part: the innate immune system attacks invaders without necessarily recognizing them, but the adaptive one changes and responds to new types of germs. Urchins appear to have the primal genes of the adaptive immune system (formed by the antibodies and T-cells). "They are probably acting in a different way but it was a real surprise to find them," Rast said. "If we can understand how they are acting in a sea urchin, it will help us understand how the system emerged in vertebrates."
"They live as long as we do, maybe longer, and thus must protect themselves," Weinstock told.
"So an elaborate set of defense genes would be necessary. What was absolutely unexpected was the finding that they had expanded the innate immunity branch of the immune system."
The human adaptive immune system must learn how to attach and destroy new germs, while sea urchins are hard wired to detect foreign bacteria and viruses and begin an attack. This means they possess a lot of genes for detection, and they could lead to development of new drugs for infectious diseases. Genes associated with many human diseases, like muscular dystrophy and Huntington's disease, were detected in sea urchins and could be studied to understand these diseases in humans.
Sea urchins do not possess visual or hearing organs, yet they carry about 979 genes for proteins linked in vertebrates to taste, smell, hearing, sight and balance. "Nobody would've predicted that sea urchins have such a robust gene set for visual perception," said Gary Wessel of Brown University in Rhode Island.
Some of the visual proteins group on the tube feet and must process some information. "It is remarkable that the same sensory proteins are used in organs with such different structures in sea urchins and man," Weinstock said.
But scientists are very puzzled by how the sea urchins use the genes linked to senses, and especially to sight. "There is not a lot of light at the bottom of the ocean so it is not clear what they might be 'seeing,'" Weinstock said.