Expression of a retinal homeobox (Rx) gene during planarian regeneration.

Retinal homeobox (Rx) genes, with representatives in vertebrates and invertebrates, encode fundamental regulators of early eye and brain formation. Here we describe the spatio-temporal expression profile of a candidate planarian orthologue of Rx during regeneration in Dugesia japonica and Schmidtea mediterranea. Although low levels of Rx transcripts were found throughout the body of intact planarians, high levels of Rx expression were specific to regenerating tissue in both head and tail fragments. We also observed that Rx was never expressed in the simple rhabdomeric planarian eyes, supporting the notion that only formation of eyes that use the ciliary type of photoreceptors requires Rx function.