Via Stranger Fruit, Insight into our sight: A new view on the evolution of the eye lens:
Fish, frogs, birds and mammals all experience image-forming vision, thanks to the fact that their eyes all express crystallins and form a lens; however, the vertebrates' nearest invertebrate relatives, such as sea squirts, have only simple eyes that detect light but are incapable of forming an image. This has lead to the view that the lens evolved within the vertebrates early in vertebrate evolution, and it raises a long-standing question in evolutionary biology: How could a complex organ with such special physical properties have evolved?
In their new work, Shimeld and colleagues approached this question by examining the evolutionary origin of one crystallin protein family, known as the ß?-crystallins. Focusing on sea squirts, invertebrate cousins of the vertebrate lineage, the researchers found that these creatures possess a single crystallin gene, which is expressed in its primitive light-sensing system. The identification of the sea squirt's crystallin strongly suggests that it is the single gene from which the vertebrate ß?-crystallins evolved.
The researchers also found that, remarkably, expression of the sea squirt crystallin gene is controlled by genetic elements that also respond to the factors that control lens development in vertebrates: The researchers showed that when regulatory regions of the sea squirt gene are transferred to frog embryos, these regulatory elements drive gene expression in the tadpoles' own visual system, including the lens. This strongly suggests that prior to the evolution of the lens, there was a regulatory link between two tiers of genes: those that would later become responsible for controlling lens development, and those that would help give the lens its special physical properties. This combination of genes appears to have then been co-opted in an early vertebrate during the evolution of its visual system, giving rise to the lens.
Common descent, testable evolutionary predictions and evaluating evolutionary hypotheses. Cool.