A lineage specific enhancer drives Otx2 expression in teleost organizer tissues

In mouse Otx2 plays essential roles in anterior-posterior axis formation and head development in anterior visceral endoderm and anterior mesendoderm. The Otx2 expression in these sites is regulated by VE and CM enhancers at the 5' proximal to the translation start site, and we proposed that these enhancers would have been established in ancestral sarcoptergians after divergence from actinopterigians for the use of Otx2 as the head organizer gene (Kurokawa et al., 2010). This would make doubtful an earlier proposal of ours that a 1.1 kb fragment located at +14.4 to +15.5 kb 3' (3'En) of fugu Otx2a gene harbors enhancers phylogenetically and functionally homologous to mouse VE and CM enhancers (Kimura-Yoshida et al., 2007). In the present study, we demonstrate that fugu Otx2a is not expressed in the dorsal margin of blastoderm, shield and early anterior mesendoderm, and that the fugu Otx2a 3'En do not exhibit activities at these sites of fugu embryos. We conclude that the fugu Otx2a 3'En does not harbor an organizer enhancer, but encodes an enhancer for the expression in later anterior mesendodermal tissues. Instead, in fugu embryos Otx2b is expressed in the dorsal margin of blastoderm at blastula stage and shield at 50% epiboly, and this expression is directed by an enhancer, 5'En, located at -1000 to -800 bp, which is uniquely conserved among teleost Otx2b orthologues.