Fig. 1: One of the elephant sharks used in a study showing that the elephant shark may be capable of color vision. The shark is held by A. S. Ibrahim from RMIT, Melbourne, Australia.
© 2009 Byrappa Venkatesh
In order to understand the human genome, scientists compare it to genomes of other species, a strategy termed ‘comparative genomics’. This process helps to identify ancient DNA sequences that are conserved across all vertebrates, and those that are unique to single species.
Now, Byrappa Venkatesh and co-workers at the Institute of Molecular and Cell Biology of A*STAR in Singapore and the UCL Institute of Ophthalmology in London have used a comparative genomics approach to show that the elephant shark, Callorhinchus milii, may possess color vision. The finding could help explain how color vision evolved in vertebrates, including humans, over the last 450 million years.
“Cartilaginous fishes, comprising sharks, rays, skates, and chimaeras, are the oldest group of living jawed vertebrates,” explains Venkatesh. “Thus, their genome sequences can give us useful insights into the origin and evolution of human and other vertebrate genomes.”
The researchers chose the elephant shark as a model because it has the smallest known genome among cartilaginous fish. Elephant sharks (Fig. 1) mainly live at depths of 200–500 meters on the continental shelves of South Australia and New Zealand, but move to shallow bays and estuaries to spawn.
The team began by searching the elephant shark genome for genes known to code for visual pigments called opsins. They confirmed that four opsins are expressed in the elephant shark’s retina.
“The main technical challenge was to measure the absorbance wavelengths of the opsins,” according to Venkatesh. “We overcame this challenge by collaborating with Prof. David Hunt’s group at UCL who are experts in this field.”
The analysis revealed that one of the four opsins is a rod opsin, commonly associated with deepwater fish because it functions well in low light levels. The other three, however, are cone opsins that could provide color vision.
The cone opsins probably arose from a gene duplication, which has been attributed with providing color vision in primates. Elephant sharks may have retained cones to adapt to the varying light conditions they encounter during their lifetimes; in contrast to other fish such as coelacanths, which never leave deep waters and have therefore dispensed with color vision.
Venkatesh hopes that future studies could explain other gaps in the evolution of color vision. “It would be interesting to investigate the color vision in sharks (the elephant shark is actually a chimaera) that live in shallow and deep waters, as well as the most primitive, jawless vertebrates such as hagfishes,” he says.
The A*STAR-affiliated authors in this highlight are from the Institute of Molecular and Cell Biology.
