Interactions with identified muscle cells break motoneuron equivalence in embryonic zebrafish

Two zebrafish motoneurons, CaP and VaP, are initially developmentally equiv alent; later, CaP innervates ventral muscle, whereas VaP dies. Current mode ls suggest that vertebrate motoneuron death results from failure to compete for limited, target-derived trophic support. In contrast, we provide evide nce that zebrafish ventral muscle can support both CaP and VaP survival. Ho wever, VaP's growth cone is prevented from extending into ventral muscle by CaP-dependent interactions with identified muscle fibers, the muscle pione ers; this interaction breaks the initial equivalence of CaP and VaP. Thus, the processes mediating VaP death are more complex than failure to compete for trophic support, and may be important for correct spatial patterning.