Whereas typically technology demands furnaces, so that the glass for a lens is produced at hundreds of degrees Celsius and then requires most careful grinding, so nature calls upon proteins known as crystallins.

How on earth do you make a tissue transparent, such as is needed in the lens of an eye? The story is a classic of both biology and evolution. First, because nature manages to synthesize compounds at room temperature whereas typically technology demands furnaces, so that the glass for a lens is produced at hundreds of degrees Celsius and then requires most careful grinding. So nature calls upon proteins known as crystallins. They did not, however, evolve as part of the origin of the eye. Quite the contrary, because they have been recruited from organisms, typically microbes, where they performed quite different functions. In other words they have been co-opted.

Why then are they so suitable? Principally because typically they are involved with stress resistance, which pre-adapts them for long-term stability so that the lens can remain transparent, perhaps for more than a century. This, in itself, opens a very interesting evolutionary question, because many of the stress-resistant proteins act as enzymes and therefore need to be “turned off” because by remaining metabolically active they would jeopardize their new function in the lens. This may involve very subtle changes, and has interesting implications for what is called “gene sharing”. In addition, crystallin proteins are relatively small and soluble in water. This means they can be precisely aligned in an aqueous medium, again a pre-requisite for transparency. The final point is that there is not one crystallin, but many distinct crystallin proteins, that have independently been recruited whenever a lens (or for that matter a reverse eye) is required.

Crystallins provide a very striking example of molecular convergence. Even more extraordinarily in at least one case the convergence extends to the so-called promoter region, that is the genetic sequence that precedes the actual expression of the crystallin genes. Specifically it involves the crystallins of the scallop (mollusc) eye, in itself a fascinating example because it has evolved a mirror system, and the eye of some vertebrates (chicken, mouse). The scallop crystallin is recruited from an aldehyde dehydrogenase protein, whereas the vertebrate is a so-called αA-crystallin and has a completely separate evolutionary origin as a heat-shock protein. Despite this, the promoter sequence is “surprisingly similar” (Carosa et al. 2002). A similar story may apply also to the way crystallins are regulated in the squid and chicken.