ROLE OF SONIC HEDGEHOG IN BRANCHIOMOTOR NEURON INDUCTION IN ZEBRAFISH

The role of zebrafish hedgehog genes in branchiomotor neuron developme nt was analyzed by examining mutations that affect the expression of t he hedgehog genes and by overexpressing these genes in embryos. In cyc lops mutants, reduction in sonic hedgehog (shh) expression, and elimin ation of tiggy-winkle hedgehog (twhh) expression, correlated with redu ctions in branchiomotor neuron populations. Furthermore, branchiomotor neurons were restored in cyclops mutants when shh or twhh was overexp ressed. These results suggest that Shh and/or Twhh play an important r ole in the induction of branchiomotor neurons in vivo. In sonic-you (s yu) mutants, where Shh activity was reduced or eliminated due to mutat ions in shh, branchiomotor neurons were reduced in number in a rhombom ere-specific fashion, but never eliminated. Similarly, spinal motor ne urons were reduced, but not eliminated, in syu mutants. These results demonstrate that Shh is not solely responsible for inducing branchiomo tor and spinal motor neurons, and suggest that Shh and Twhh may functi on as partially redundant signals for motor neuron induction in zebraf ish. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.