(A–F) Parapineal (pp) cells from an H2B:RFP mRNA-injected Tg(foxD3:GFP);(flh:eGFP) donor embryo transplanted to the left (A, D) or right (B, E) side of a sox1a-/- Tg(foxD3:GFP);(flh:eGFP) recipient at 32 hpf send projections to the habenula at 50 hpf (white arrows in B and E), as shown by live-imaging. Transplanted pineal cells (C, F) do not send projections to the habenula and locate to the midline (C) or reincorporate into the pineal (F) by 50 hpf. Scale bars 25 µm. (A’–F’) By 4 dpf, the habenula adjacent to the transplanted wild-type parapineal cells acquires a left habenula phenotype in sox1a-/- mutants as shown by kctd12.1 mRNA expression (A’,B’) and anterograde labelling of habenula-IPN projections (dorsal and lateral views) (D’,E’). sox1a-/- larvae with pineal cell transplant have double-right habenulae (C’,F’). Solid boxes show the transplanted parapineal cells at 4 dpf, sending out long projections (arrows in B’ and E’) to the adjacent habenula. The whole pineal complex is shown for the pineal cell transplanted larvae (C’,F’). lHb – left habenula, rHb – right habenula, dIPN – dorsal interpeduncular nucleus. Scale bars 25 µm.