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Urochordate beta gamma-crystallin and the evolutionary origin of the vertebrate eye lens

Shimeld, Sebastian M. and Purkiss, Andrew G. and Dirks, R.P.H. and Bateman, Orval A. and Slingsby, Christine and Lubsen, N.H. (2005) Urochordate beta gamma-crystallin and the evolutionary origin of the vertebrate eye lens. Current Biology 15 (18), pp. 1684-1689. ISSN 0960-9822.


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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 [1]. The vertebrate beta gamma-crystallin genes encode abundant structural proteins critical for the function of the lens [2]. 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 beta gamma-crystallin. The crystal structure of Ciona beta gamma-crystallin is very similar to that of a vertebrate beta gamma-crystallin domain, except for paired, occupied calcium binding sites. The Ciona beta gamma-crystallin is only expressed in the palps and in the otolith, the pigmented sister cell of the light-sensing ocellus. The Ciona beta gamma-crystallin promoter region targeted expression to the visual system, including lens, in transgenic Xenopus tadpoles. We conclude that the vertebrate beta gamma-crystallins evolved from a single domain protein already expressed in the neuroectoderm of the prevertebrate ancestor. The conservation of the regulatory hierarchy controlling beta gamma-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.

Item Type: Article
Keyword(s) / Subject(s): sponge geodia-cydonium, ciona-intestinalis, brain, proteins, photoreceptors, superfamily, chordate
School or Research Centre: Birkbeck Schools and Research Centres > School of Science > Biological Sciences
Depositing User: Sandra Plummer
Date Deposited: 24 Jan 2006
Last Modified: 11 Oct 2016 11:58

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