The evolution of complex and physiologically remarkable structures suchas the vertebrate eye has long been a focus of intrigue and theorizingby biologists. In work reported this week in Current Biology, theevolutionary history of a critical eye protein has revealed apreviously unrecognized relationship between certain components ofvertebrate eyes and those of the more primitive light-sensing systemsof invertebrates. The findings help clarify our conceptual frameworkfor understanding how the vertebrate eye, as we know it, has emergedover evolutionary time.
The work is reported by Sebastian Shimeld at the University ofOxford and colleagues at the University of London and RadboudUniversity in The Netherlands.
Our sight relies on the ability of our eye to form a clear,focused image on the retina. The critical component in focusing is theeye lens, and the physical properties that underlie the transparency ofthe lens, as well as its ability to precisely refract light, arise fromthe high concentrations of special proteins called crystallins found inlens cells.
Fish, frogs, birds and mammals all experience image-formingvision, thanks to the fact that their eyes all express crystallins andform a lens; however, the vertebrates' nearest invertebrate relatives,such as sea squirts, have only simple eyes that detect light but areincapable of forming an image. This has lead to the view that the lensevolved within the vertebrates early in vertebrate evolution, and itraises a long-standing question in evolutionary biology: How could acomplex organ with such special physical properties have evolved?
In their new work, Shimeld and colleagues approached thisquestion by examining the evolutionary origin of one crystallin proteinfamily, known as the �?-crystallins. Focusing on sea squirts,invertebrate cousins of the vertebrate lineage, the researchers foundthat these creatures possess a single crystallin gene, which isexpressed in its primitive light-sensing system. The identification ofthe sea squirt's crystallin strongly suggests that it is the singlegene from which the vertebrate �?-crystallins evolved.
The researchers also found that, remarkably, expression of thesea squirt crystallin gene is controlled by genetic elements that alsorespond to the factors that control lens development in vertebrates:The researchers showed that when regulatory regions of the sea squirtgene are transferred to frog embryos, these regulatory elements drivegene expression in the tadpoles' own visual system, including the lens.This strongly suggests that prior to the evolution of the lens, therewas a regulatory link between two tiers of genes: those that wouldlater become responsible for controlling lens development, and thosethat would help give the lens its special physical properties. Thiscombination of genes appears to have then been co-opted in an earlyvertebrate during the evolution of its visual system, giving rise tothe lens.
The researchers include Sebastian M. Shimeld, University of Oxford,Oxford, United Kingdom; Andrew G. Purkiss, Orval A. Bateman, andChristine Slingsby of Birkbeck College University of London, UnitedKingdom; Ron P.H. Dirks and Nicolette H. Lubsen of Radboud University,Nijmegen, The Netherlands. This work was supported by the MedicalResearch Council, UK and the Biotechnology and Biological SciencesResearch Council.
Shimeld et al.: "Urochordate gamma-crystallin and theevolutionary origin of the vertebrate eye lens." Publishing in CurrentBiology, Vol. 15, pages 1684-1689, September 20, 2005. DOI10.1016/j.cub.2005.08.046 www.current-biology.com
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