Expanded retina territory by midbrain transformation upon overexpression of Six6 (Optx2) in Xenopus embryos

During vertebrate eye development, the expression of the homeobox gene Six6 is restricted to the neural retina and is initiated later than Rx and Pax6 in the presumptive retina field. We show here that overexpression of mouse Six6 in Xenopus embryos can induce transformation of competent tissue of t he anterior neural plate into retinal tissue. In Six6 injected embryos, the molecular identity of the presumptive midbrain and rostral hindbrain regio ns was lost, as shown by the absence of XEn-2 and Xpax2 expression, being r eplaced by the ectopic expression of the retinal markers Xpax6 and Xrx. Whe n allowed to grow further, Six6 injected embryos developed ectopic eye-like structures in the rostral brain and showed a transformation of the midbrai n into retina. Similar results were obtained upon overexpression of Six3 or Xsix3, revealing a possible redundance of Six3 and Six6 activities. Taken together, results obtained suggest that during normal retina development, t he relatively late expressed Six6 gene becomes part of a network of retinal homeobox genes that are linked together by positive feedback loops. Furthe rmore, our results demonstrate that the primitive neural ectoderm of the fu ture midbrain and rostral hindbrain is competent to form retinal tissue. (C ) 2000 Elsevier Science Ireland Ltd. All rights reserved.