An evolutionary conserved complex network of different signaling pathways and nuclear factors regulates brain and eye

An evolutionary conserved complex network of different signaling pathways and nuclear factors regulates brain and eye specification (Kurata et al., 2000; Callaerts et al., 2001; Kumar and Moses, 2001). One striking illustration of such genetic conservation is the Pax6 gene, which is structurally and functionally characterized in a large number of triploblastic metazoans from planarians to humans (Quiring et al., 1994; Chisholm and Horvitz, 1995; Czerny and Busslinger, 1995; Zhang and Emmons, 1995; Loosli et al., 1996; Glardon et al., 1997; Tomarev et al., 1997; Glardon et al., 1998; Callaerts et al., 1999). Pax6 represents one of the nine members of the Pax class, characterized by the presence of two conserved DNAbinding domains, the paired domain and the homeodomain (Callaerts et al., 1997). The comparative molecular characterization of Pax6 in different animal phyla supports the hypothesis that this protein has a conserved function in a variety of developmental processes, ranging from regionalization to cell type specification, in eye and central nervous system development (Quiring et al., 1994; Kurusu et al., 2000; Pratt et al., 2000). In some vertebrates, Pax6 is also related to the development of head sensory organs or other structures such as pituitary or pancreas (Walther and Gruss, 1991; Turque et al., 1994). Pax6 genes from various animal phyla are capable of inducing formation of ectopic eyes in Drosophila (Gehring and Ikeo, 1999). In frogs, Pax6 misexpression can also induce ectopic eyes (Chow et al., 1999). This finding indicates that Pax6 is a master control gene for eye development, and also suggests a common genetic program for making eyes, despite the great variety of visual structures found in the animal kingdom. Two Pax6 genes involved in eye development have been found in zebrafish (Nornes et al., 1998). The presence of a second Pax6 gene arisen by gene duplication during insect evolution is also described in holometabolous insects, the two Drosophila genes eyeless (ey) and twin of eyeless (toy) being the best-studied examples. ey and toy share a similar pattern of expression in the developing visual system, and have nonredundant functions, indicating that they have been recruited with different roles into the eye genetic pathway (Czerny et al., 1999). Planarians are flatworms well known for their exceptional regenerative capabilities. They are free-living members of the Platyhelminthes, considered a basal phylum of the Lophotrocozoa clade (Aguinaldo et al., 1997; Carranza et al., 1997; Bayascas et al., 1998). The study of the role of Pax6 genes during eye and central nervous system (CNS) regeneration and maintenance will be of interest for 1423 Development 129, 1423-1434 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV2827