A team led by Helena Bailes at the School of Biomedical Sciences, University of Queensland, Brisbane, Australia, analysed retinas from Australian lungfish (Neoceratodus forsteri), thought to be the closest living relative to the first terrestrial vertebrates. The researchers then compared these to other fish and amphibian retinas. The DNA of five visual pigment (opsin) genes in the retinas of lungfish reveals that these have more in common with four-legged vertebrates (tetrapods) than with fish retinas.
Although lungfish mainly take in oxygen through their gills like most fish, they can also breathe air if water quality is poor. Lungfish were previously thought to have poor eyesight due to their small eyes, low spatial resolving power, sluggish behaviour in captivity and ability to detect prey using electroreception. N.forsteri inhabits a brightly lit, shallow freshwater habitat similar to the environment from which terrestrial evolution probably occurred. This prompted the team to investigate the complement of opsins expressed in N. forsteri, to trace photoreception's evolution in ancestral tetrapods.
The study paves the way for behavioural work with lungfish to see if they can discriminate between objects based on colour.
"The genus Neoceratodus, of which N. forsteri is the sole survivor, is found in the fossil record from the Lower Cretaceous period 135 million years ago and therefore N. forsteri lays claim to being the oldest surviving vertebrate genus," says Bailes. "The visual system of N. forsteri may represent an evolutionary design most closely reflecting that present just prior to the emergence of land vertebrates in the Devonian period."
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