Friday, October 16, 2009

How to See Deep Sea Blue: Move a Molecule

Speaking of fish and their vision, research just published in the Oct 13th issue of the Proceedings of the National Academy of Sciences, reports that the Scabbardfish has evolved from UV vision, like most fish that are surface feeders, to seeing blue in deeper water. What is remarkable about this finding is that the change did not occur gradually nor was it performed through a sequence of evolutionary steps. It occurred by the elision of a critical amino acid molecule from the fish's opsin. In other words, a single, simple change at the molecular level altered a phenotype. That's like deleting one byte of a program's code in memory and, rather than crashing, it exhibits a new functionality!

By contrast, adaptive changes often occur via a small number of amino acid substitutions, but most such substitutions do not lead to functional changes. In the case of the Scabbardfish, a mutation that deleted a molecule at site 86 in the chain of amino acids in the opsin protein, changed its vision capability. UV light is very short wavelength with high energy (E = hν). It does not propagate to great depths in the ocean because it can couple to the electronic states in the water atoms. IR and reddish light are long wavelength (lower energy) and couple to the vibrational modes of the water molecule. Blue light is more intermediate in energy and therefore doesn't couple very efficiently to water molecules. That's why it penetrates to a greater depth and also why the ocean often looks blueish to us (and presumably Scabbardfish).

The likley evolutionary advantage was that it allowed the Scabbardfish to feed at greater depths with less competition. The lead author, Prof. Shozo Yokoyama, theorizes that the Scabbardfish spends much of its life at depths of 25 to 100 meters, where UV light is less intense than violet light, which could explain why it made the vision shift. Conversely, Yokoyama thinks the Lamprey, which also spend much of their time in deep water, may have retained UV vision because they feed near the surface at twilight on tiny, translucent crustacea that are easier to see in UV light.

For Yokoyama, vision serves as a good model of molecular evolution because 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. It's just like a painting," he says. [An oblique reference to trichromatic color theory?]

The common vertebrate ancestor possessed UV vision. However, many species, including humans, have switched from UV to violet vision or the ability to sense the blue color spectrum. All fish previously studied have retained UV vision, but the Scabbardfish has not.

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