Abstract
Among the millions of invertebrate species with visual systems, the genetic basis of eye development and function is well understood only in Drosophila melanogaster. We describe an eye transcriptome for the planarian Schmidtea mediterranea. Planarian photoreceptors expressed orthologs of genes required for phototransduction and microvillus structure in Drosophila and vertebrates, and optic pigment cells expressed solute transporters and melanin synthesis enzymes similar to those active in the vertebrate retinal pigment epithelium. Orthologs of several planarian eye genes, such as bestrophin-1 and Usher syndrome genes, cause eye defects in mammals when perturbed and were not previously described to have roles in invertebrate eyes. Five previously undescribed planarian eye transcription factors were required for normal eye formation during head regeneration. In particular, a conserved, transcription-factor-encoding ovo gene was expressed from the earliest stages of eye regeneration and was required for regeneration of all cell types of the eye.
Highlights
The vertebrate eye is a structure of striking complexity in form and function
Embryonic development of multiple eye types involves Pax-6, Sine oculis, Eyes absent, and Otx gene family members. These observations have led to the suggestion that photoreceptor cell types were already present prior to the existence of the Bilateria, and that the common ancestor of the Bilateria utilized transcription factors for eye development still commonly used in extant eyes (Nilsson, 2009)
We describe several conserved transcription factors expressed in the planarian eye that are functionally required for eye regeneration, including a central role for the conserved gene ovo in formation of all eye progenitors
Summary
The vertebrate eye is a structure of striking complexity in form and function. understanding the evolution and development of eyes has been a classic challenge. Genetic studies in model invertebrates have the potential to advance the understanding of eye evolution and development, and of the functions of conserved genes associated with eye disorders. The relevance of invertebrate models to vertebrate systems depends in part on whether homology exists between most bilaterian eyes, a difficult point to establish based on morphological studies alone. Embryonic development of multiple eye types involves Pax-6, Sine oculis, Eyes absent, and Otx gene family members. These observations have led to the suggestion that photoreceptor cell types were already present prior to the existence of the Bilateria, and that the common ancestor of the Bilateria utilized transcription factors for eye development still commonly used in extant eyes (Nilsson, 2009)
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