RELATIONSHIP BETWEEN SPATIALLY RESTRICTED KROX-20 GENE-EXPRESSION IN BRANCHIAL NEURAL CREST AND SEGMENTATION IN THE CHICK-EMBRYO HINDBRAIN

Previous studies have suggested that the rostrocaudal patterning of br anchial arches in the vertebrate embryo derives from a coordinate segm ental specification of gene expression in rhombomeres (r) and neural c rest. However, expression of the Krox-20 gene is restricted to neural crest cells migrating to the third branchial arch, apparently from r5, whereas this rhombomere contributes cells to both the second and thir d arches. We examined in the chick embryo how this spatially restricte d expression is established. Expression occurs in precursors in both r 5 and r6, and we show by cell labelling that both rhombomeres contribu te to Krox-20-expressing neural crest, emigration occurring first from r6 and later caudally from r5. Krox-20 transcripts are not detected i n some precursors in rostral r5, presaging the lack of expression in c ells migrating rostrally from this rhombomere. After transposition of r6 to the position of r4 or r5, many Krox-20-expressing cells migrate rostral to the otic vesicle, whereas when r5 is transplanted to the po sition of r4, only a small number of migrating cells express Krox-20. These results indicate that, in the chick, Krox-20 expression in branc hial neural crest does not correlate with rhombomeric segmentation, an d that there may be intrinsic differences in regulation between the r5 and r6 Krox-20-expressing populations.