Fig. 6. Mis-expression of hoxb1a, hoxa1a or AmphiHox-1 causes extensive posteriorizing transformations, but mis-expression of hoxc1a causes reduced transformations. The concentration of mRNA injected (in ng/µl) is indicated in the top right-hand corner of each panel. (A,B) hoxb1a-injected embryos, retrograde labelling from the spinal cord, rhombomeres as numbered, ectopic neurones are indicated by '. (A) Bilateral transformation of r2 to an r4-like character, note duplications of M, Mi and nV (vestibular nuclei) neurones. (B) Example with a more extensive posteriorizing transformation, multiple Mauthner neurones have formed at the r2-r4 levels; there are also unilateral ectopic Mi cells extending through r1-r4. (C-E) hoxb1a-injected embryos assayed for expression of endogenous hoxb1a (blue) and krox20 (red, C,D only), 19 h, anterior towards left. (C) Example with hoxb1a expression at the r2 level; note also expansion of the krox20 positive r3 territory. (D) Example with more extensive ectopic expression of endogenous hoxb1a, expression is present in r1-r3 in addition to the r4 domain. (E) Example with ectopic hoxb1a expression in r2, midbrain (mb), forebrain (fb) and eyes (e). (F) hoxa1a mis-expression; example of bilateral ectopic Mauthner neurones (M') in r2 revealed with 3A10 antibody at 28 h. (G,H) hoxa1a-injected embryos assayed for expression of hoxb1a. (G) Note ectopic expression of hoxb1a at the r2 level and enlargement of the r3 territory. (H) Note extensive anterior truncation accompanied by expansive ectopic hoxb1a expression (asterisk). (I) AmphiHox-1 mis-expression, example of a unilateral ectopic Mauthner neurone (M') in r2 revealed with 3A10 antibody. (J) hoxc1a-injected embryo assayed for expression of hoxb1a, note small, lateral domains of ectopic expression of hoxb1a at the anterior r2 level (arrowhead). Scale bars: 50 µm.