Fig. 6. Wild-type, or val^-, cells can partially rescue r5 krox-20 expression in the fgf3-MO; fgf8^- hindbrain. (A) The transplantation technique used for the mosaic analyses. Naïve animal pole cells are excised from fluorescein-labeled donor embryos at shield stage and transplanted into the presumptive hindbrain region (Woo and Fraser, 1997) of shield stage fgf3-MO; fgf8^- embryos. In some cases, fgf3-MO; fgf8-MO embryos were used as hosts. 100% (n=45) of the fgf3-MO; fgf8^- or fgf3-MO; fgf8-MO control embryos (not receiving donor cells) showed no r5 krox-20 expression. (B) fgf3-MO; fgf8^- control embryo (18 s) showing only r3 krox-20 expression. (C-C'') A host fgf3-MO; fgf8^- embryo (18 s) showing wild-type donor cells (C), krox-20 expression (C') and merge of the two labels (C''). Red r5 cells (arrow, C'') are rescued fgf3-MO; fgf8^- host cells expressing krox-20. Yellow r5 cells (arrowhead, C'') are wild-type donor cells expressing krox-20. (D-D') A host fgf3-MO; fgf8^- embryo (18 s) showing krox-20 expression (D) and a merge (D') showing wild-type donor cells (green) and krox-20 expression (red). Donor cells populate the caudal hindbrain and anterior spinal cord and no rescue of r5 krox-20 is observed. (E-E') A host fgf3-MO; fgf8-MO embryo (2 s) showing krox-20 expression (E) and an overlay (E') of wild-type donor cells (green) and krox-20 expression (black) (E'). Rescue of r5 ege2 expression (arrowhead) has occurred adjacent to unilateral donor cells in r4 (arrow). Four out of nine embryos showed similar rescue at about 2 s. (F-F'') A host fgf3-MO; fgf8-MO embryo (18 s) showing val^- donor cells (F), krox-20 expression (F') and merge of the two labels (F''). val^- cells are excluded from the rescued r5 region on the right side of the hindbrain (arrow). val^- cells are not excluded from the ‘r5’ level on the left, non-rescued side (arrowhead). Dorsal views show anterior towards the left. Scale bars: in B, 50 µm for B-D',F-F''; in E, 50 µm for E-E'.

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