Meeting Abstract
Widely shared across metazoan neural development is a transcription factor network implicated in anterior-posterior brain patterning, including OTX anteriorly, the HOX genes posteriorly and an intercalated cassette of GBX, PAX2/5/8 and ENGRAILED. In vertebrates, this cassette identifies the midbrain-hindbrain boundary, the site of a prominent signaling center known as the isthmic organizer (IO), which expresses the signaling ligands FGF8 and WNT1. Outside of the vertebrates, soft-bodied cephalopods have the largest brains. We explored the role of these highly conserved developmental control genes with in situ hybridization experiments on Octopus bimaculoides embryos. We found that anterior OTX, posterior HOX1 and an intercalated IO transcription factor cassette are readily demonstrated in octopus embryogenesis. Moreover, the IO cassette coincides with a morphological depression in the ectoderm (the “cleft”). The cleft in turn marks out the major structural transition in the prospective cephalopod brain, that between the subesophageal and supraesophageal masses. We found that the cleft, like the vertebrate midbrain-hindbrain boundary, is a major signaling center. It harbors most of the major developmental signaling molecules, including HH, DPP and multiple WNT genes. FGF8 and WNT1 are, however, absent. Our findings with this lophotrochozoan indicate that, even at the same stage of animal development, anterior-posterior signaling centers can show comprehensive transcription factor conservation in the face of marked signaling ligand lability.
