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Fig. 6. Comparison of expression domains between chordate neural tubes and proposed
homology of brain regions. An underlying tripartite organisation is depicted
by brackets, expression of transcription factors by shading and the organiser
activity of FGF8/17/18 proteins by arrows (not proven in ascidia). In all
three chordate taxa, the most anterior of the three regions is marked by
expression of Otx. This region is divided into two regions in
vertebrates: an anterior region without Dmbx expression (forebrain)
and a posterior region with Dmbx expression (midbrain). Amphioxus and
ascidia do not possess the Dmbx-expressing domain (midbrain) anterior
to Pax2/5/8 expression (MHB). In vertebrates and ascidians,
expression of Pax2/5/8 family genes fills the gap between the
Otx-expressing region and the posterior region expressing Hox genes;
this is not seen in amphioxus. Dmbx has a later expression domain in
the hindbrain of vertebrates (shown by longitudinal stripes), coincident with
Hox gene expression (up to rhombomere 2); co-localisation of Hox
(CiHox3) and Dmbx also occurs in the ascidian anterior visceral
ganglion.
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