Morphological and molecular development of the eyes during embryogenesis of the freshwater planarian Schmidtea polychroa.

Photoreception is one of the most primitive sensory functions in metazoans. Despite the diversity of forms and components of metazoan eyes, many studies have demonstrated the existence of a common cellular and molecular basis for their development. Genes like pax6, sine oculis, eyes absent, dachshund, otx, Rx and atonal are known to be associated with the specification and development of the eyes. In planarians, sine oculis, eyes absent and otxA play an essential role during the formation of the eye after decapitation, whereas pax6, considered by many authors as a master control gene for eye formation, does not seem to be involved in adult eye regeneration. Whether this is a peculiarity of adult planarians or, on the contrary, is also found in embryogenesis remains unknown. Herein, we characterize embryonic eye development in the planarian species Schmidtea polychroa using histological sections and molecular markers. Additionally, we analyse the expression pattern of the pax6-sine oculis-eyes absent-dachshund network, and the genes Rx, otxA, otxB and atonal. We demonstrate that eye formation in planarian embryos shows great similarities to adult eye regeneration, both at the cellular and molecular level. We thus conclude that planarian eyes exhibit divergent molecular patterning mechanisms compared to the prototypic ancestral metazoan eye.