19 October 2009
Fish vision evolution observed at molecular level
by Kate Melville
Emory University researchers have identified the first fish known to have switched from ultraviolet vision to violet vision (the ability to see blue light). The discovery links molecular evolution to functional changes in the organism and the environmental factors that drive those changes. Evolutionary geneticist and study leader Shozo Yokoyama says the findings, reported in the Proceedings of the National Academy of Sciences, strengthen the case for the importance of adaptive evolution.
Vision serves as a good study model, says Yokoyama, since it is the simplest of the sensory systems. For example, only four genes are involved in human vision. "It's amazing, but you can mix together this small number of genes and detect a whole color spectrum," he said. "It's just like a painting." He adds that our ancient common vertebrate ancestor possessed UV vision, but many species, including humans, have switched from UV to violet vision.
Fish provide important clues for how environmental factors can lead to such vision changes, since the available light at various ocean depths is well understood. All fish previously studied have retained UV vision, but the Emory researchers found that the scabbardfish has not. To tease out the molecular basis for this difference, they used genetic engineering, quantum chemistry and theoretical computation to compare vision proteins and pigments from scabbardfish and lampfish. The results indicated that scabbardfish shifted from UV to violet vision by deleting the molecule at site 86 in the chain of amino acids in the opsin protein.
Scabbardfish spend much of their life at depths of 25 to 100 meters, where UV light is less intense than violet light, which could explain why they made the vision shift, Yokoyama theorizes. Lampfish also spend much of their time in deep water, but they may have retained UV vision because they feed near the surface at twilight on tiny, translucent crustaceans that are easier to see in UV light.
"Normally, amino acid changes cause small structure changes, but in this case, a critical amino acid was deleted," Yokoyama explains. "The finding implies that we can find more examples of a similar switch to violet vision in different fish lineages. Comparing violet and UV pigments in fish living in different habitats will open an unprecedented opportunity to clarify the molecular basis of phenotypic adaptations, along with the genetics of UV and violet vision."
Yokoyama believes such a nuts-and-bolts understanding of evolutionary processes is critical as he contends that evolutionary biology is filled with arguments that are misleading. "To make a strong case for the mechanisms of natural selection, you have to connect changes in specific molecules with changes in phenotypes, and then you have to connect these changes to the living environment," he said.
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Source: Emory University