Among the cartilaginous fishes, the elephant shark was selected for sequencing because of its compact genome, which is one-third the size of the human genome.
By analyzing the elephant shark genome and comparing it with other genomes, the scientists discovered a family of genes that is absent in the elephant shark but present in all bony vertebrates, including the chicken, cow, mouse and human.
In a surprise finding, the team found that the elephant shark appears to lack special types of immune cells that are essential to mounting a defense against viral and bacterial infections and for preventing autoimmune diseases such as diabetes and rheumatoid arthritis.
Elephant shark is a member of cartilaginous fishes, which are the oldest living group of jawed vertebrates that diverged from bony vertebrates about 450 million years ago.
An unexpected finding was that elephant shark appears to lack special types of immune cells previously considered essential for defence against viral/bacterial infections and preventing autoimmune reactions such as diabetes and rheumatoid arthritis(2).
The study also found that the elephant shark genome is the slowest evolving among all vertebrates, including the coelacanth, popularly known as a "living fossil", whose genes were recently shown to be evolving slower than those of other bony vertebrates(3).
It was unexpected that a 'primitive' vertebrate like the elephant shark had the potential for color vision like humans," said Byrappa Venkatesh, a scientist at Singapore's Institute of Molecular and Cell Biology (IMCB), who with David Hunt, from the Institute of Ophthalmology at University College London (UCL), headed the research team responsible for this surprising discovery.
The research team found that the elephant shark had three cone pigments for color vision and, like humans, it accomplished this through gene duplication.