A recent article has filled another gap in our ignorance (and created two new gaps) about eye evolution. As reported in
How complex and physiologically remarkable structures such as the human eye could evolve has long been a question that has puzzled biologists. But in research reported this week in Current Biology, the evolutionary history of a critical eye protein has revealed a previously unrecognized link between certain components of sophisticated vertebrate eyes - like those found in humans - and those of the primitive light-sensing systems of invertebrates. The findings, from researchers at the University of Oxford, the University of London and Radboud University in The Netherlands, put in place a conceptual framework for understanding how the vertebrate eye, as we know it, has emerged over evolutionary time.
Ref: Current Biology, Vol. 15, pages 1684-1689, September 20, 2005. DOI 10.1016/j.cub.2005.08.046
Urochordate βγ-Crystallin and the Evolutionary Origin of the Vertebrate Eye Lens Sebastian M. Shimeld, Andrew G. Purkiss, Ron P.H. Dirks, Orval A. Bateman, Christine Slingsby, and Nicolette H. Lubsen
Abstract:A refracting lens is a key component of our image-forming camera eye; however, its evolutionary origin is unknown because precursor structures appear absent in nonvertebrates . The vertebrate βγ-crystallin genes encode abundant structural proteins critical for the function of the lens . We show that the urochordate Ciona intestinalis, which split from the vertebrate lineage before the evolution of the lens, has a single gene coding for a single domain monomeric βγ-crystallin. The crystal structure of Ciona βγ-crystallin is very similar to that of a vertebrate βγ-crystallin domain, except for paired, occupied calcium binding sites. The Ciona βγ-crystallin is only expressed in the palps and in the otolith, the pigmented sister cell of the light-sensing ocellus. The Ciona βγ-crystallin promoter region targeted expression to the visual system, including lens, in transgenic Xenopus tadpoles. We conclude that the vertebrate βγ-crystallins evolved from a single domain protein already expressed in the neuroectoderm of the prevertebrate ancestor. The conservation of the regulatory hierarchy controlling βγ-crystallin expression between organisms with and without a lens shows that the evolutionary origin of the lens was based on co-option of pre-existing regulatory circuits controlling the expression of a key structural gene in a primitive light-sensing system.
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