MOTONEURON FATE SPECIFICATION REVEALED BY PATTERNED LIM HOMEOBOX GENE-EXPRESSION IN EMBRYONIC ZEBRAFISH

In zebrafish, individual primary motoneurons can be uniquely identifie d by their characteristic cell body positions and axonal projection pa tterns, The fate of individual primary motoneurons remains plastic unt il just prior to axogenesis when they become committed to particular i dentities, We find that distinct primary motoneurons express particula r combinations of LIM homeobox genes. Expression precedes axogenesis a s well as commitment, suggesting that LIM homeobox genes may contribut e to the specification of motoneuronal fates. By transplanting them to new spinal cord positions, we demonstrate that primary motoneurons ca n initiate a new program of LIM homeobox gene expression, as well as t he morphological features appropriate for the new position. We conclud e that the patterned distribution of different primary motoneuronal ty pes within the zebrafish spinal cord follows the patterned expression of LIM homeobox genes, and that this reflects a highly resolved system of positional information controlling gene transcription.