AGATA , K; UMESONO, Y; KOINUMA, S; OKAMOTO, K; ITOH, H; INOUE, T; Center for Dev. Biol., RIKEN Kobe; Center for Dev. Biol., RIKEN Kobe; Center for Dev. Biol., RIKEN Kobe; Center for Dev. Biol., RIKEN Kobe; Center for Dev. Biol., RIKEN Kobe; Center for Dev. Biol., RIKEN Kobe: New insight into brain evolution: viewpoint from planarian

Planarians have a more well-organized brain than we had expected. They possess an inverted U-shaped brain-like structure with nine branches on each outer side. Two eyes are located on the dorsal side of the 3rd branch and visual axons form an optic chiasma on the dorsal-inner region of the inverted U-shaped brain. The 6th-9th branches cluster more closely and form auricles on the surface which may function as the sensory organ of taste. We also found that such structurally distinct and functionally diverse domains are defined by the discrete expression of the three evolutionarily conserved homeobox genes DjotxA, DjotxB, Djotp. We found that the gross structure of the planarian CNS along the anterior-posterior (A-P) axis is strikingly similar to the distribution pattern of the primary neurons of vertebrate embryos which differentiate at the neural plate stage to provide a fundamental nervous system. These data suggest that the basic plan for the CNS development along the A-P axis might have been acquired at an early stage of evolution before conversion of the location of the CNS from the ventral to the dorsal side. We strongly believe that the fundamental genetic programs for complex brain development of the vertebrates may be understood by studying the genetic program for brain development in planarians. Recently, a gene-knock out method using RNAi has been developed in planarians. The availability of this method has greatly facilitated our ability to investigate the genetic programs underlying brain development by testing and observing the effects of specifically targeted RNAi during brain regeneration using molecular markers.