>Contents>The Evolution of Eyes
The vertebrate eye develops from a diverse collection of embryonic sources through a complex set of inductive events [6]. Whereas the neural retina is derived from the diencephalon and is a part of the brain, the lens comes from surface ectoderm and the iris and ciliary body arise primarily from the neural crest. Mapping the genes known to play a role in mouse eye development, for example, shows that some of these genes are present on every chromosome [6]. The apparent patchwork assembly of the eye makes it all the more surprising that common developmental programs seem to produce comparable outcomes across a broad phylogenetic divide [7]. Could we use the composition of lenses to gain insight into eye evolution?
Vertebrate lenses are formed from modified epithelial cells that contain high concentrations of soluble proteins known as crystallins because they are packed in a highly organized fashion. It is the change in relative concentration of these proteins from the periphery to the center of vertebrate lenses that produces the refractive index gradient necessary for a lens to be useful to the animal. In fact, the identity of the proteins seems not to be important since the crystallin proteins are not more transparent than others. Instead, the distribution of protein concentration as a function of radius is the key to a successful lens. Thus, the challenge in understanding lens evolution lies in discovering how the distribution of proteins within a lens is established and maintained.
Of the eleven lens crystallins now known, only three, alpha-, beta- and gamma-crystallins, are common to all vertebrates. In fact, until recently, all crystallins were thought to be unique to lens tissue and to have evolved for this special function. However, despite their apparently specialist role, most of the crystallins are neither structural proteins nor lens specific. There are two major groups of lens crystallins, those present in all vertebrates and those specific to a particular taxon. For example, in crocodiles and some bird species, the glycolytic enzyme lactate dehydrogenase B is a major protein in the lens. Indeed, 4 of the 8 taxon-specific crystallins are identical to metabolic enzymes and products of the same genes, suggesting these products share a gene.
Why might enzymes be recruited to make vertebrate lenses? Perhaps the robust regulation of enzyme production is advantageous for producing sufficient protein for a lens, but there is not much beyond speculation to support this notion. There may be some deeper reason, however, because this molecular opportunism seemed such a good idea, that certain invertebrates, e.g. mollusks, independently evolved the same strategy [8]. Squids have lenses whose protein content is nearly entirely the enzyme glutathione S-transferase. The common strategy of constructing lenses from different proteins seems to be a convergent evolutionary solution. This convergence of molecular strategy suggests that enzymes as lenses may have a functional meaning, or that it is easy to get lens cells to make a lot of enzyme, or there may be other as yet not understood reasons.
The Evolution of Eyes
Why Do We See What We See?
How Do Eyes Work and How Did They Evolve?
How Do Eyes Capture Photons?
Where Do Lenses Come From?
Eyes: Convergence or Homology?
Conclusions
References
Biography
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