Abstract
Planarians are attractive animals to elucidate origin and evolution of the central nervous system (CNS). The planarian CNS is composed of two major parts, an anterior brain and a pair of ventral nerve cords that are distinct and overlapping structures in the head region. We found that the brain possesses distinct structural domains defined by the expression of evolutionarily conserved homeobox genes DjotxA, DjotxB and Djotp. DjotxA is expressed more medially, in the region containing the termini of the visual axons. DjotxB is expressed in a discrete region just lateral to the DjotxA-positive domain, but not in the more lateral branch structures, which in turn are characterized by the expression of Djotp. Systematic characterization of neural-specific genes through DNA microarrays and ESTs projects suggests that such distinct structural domains are further subdivided and also functionally diverse. Our findings suggest that the planarian brain is well-organized despite its morphologically simple structure.