Plasticity in zebrafish hox expression in the hindbrain and cranial neuralcrest

The anterior-posterior identities of cells in the hindbrain and cranial neu ral crest are thought to be determined by their Hox gene expression status, but how and when cells become committed to these identities remain unclear . Here we address this in zebrafish by cell transplantation, to test plasti city in hox expression in single cells. We transplanted cells alone, or in small groups, between hindbrain rhombomeres or between the neural crest pri mordia of pharyngeal arches. We found that transplanted cells regulated hox expression according to their new environments. The degree of plasticity, however, depended on both the timing and the size of the transplant. At lat er stages transplanted cells were more likely to be irreversibly committed and maintain their hox expression, demonstrating a progressive loss of resp onsiveness to the environmental signals that specify segmental identities. Individual transplanted cells also showed greater plasticity than those lyi ng within the center of larger groups, suggesting that a community effect n ormally maintains hox expression within segments. We also raised experiment al embryos to larval stages to analyze transplanted cells after differentia tion and found that neural crest cells contributed to pharyngeal cartilages appropriate to the anterior-posterior level of the new cellular environmen t. Thus, consistent with models implicating hox expression in control of se gmental identity, plasticity in hox expression correlates with plasticity i n final cell fate. (C) 2001 Academic Press.