Characterization of an amphioxus paired box gene, AmphiPax2/5/8: developmental expression patterns in optic support cells, nephridium, thyroid-like structures and pharyngeal gill slits, but not in the midbrain-hindbrain boundary region

On the basis of developmental gene expression, the vertebrate central nervo us system comprises: a forebrain plus anterior midbrain, a midbrain-hindbra in boundary region (MHB) having organizer properties, and rhombospinal doma in. The vertebrate MHB is characterized by position, by organizer propertie s and by being the early site of action of Wnt1 and engrailed genes, and of genes of the Pax2/5/8 subfamily. Wada and others (Wada, H., Saiga, H., Sat oh, N. and Holland, P.W.H. (1998) Development 125, 1113-1122) suggested tha t ascidian tunicates have a vertebrate-like MHB on the basis of ascidian Pa x258 expression there. In another invertebrate chordate, amphioxus, compara ble gene expression evidence for a vertebrate-like MHB is lacking, We, ther efore, isolated and characterized AmphiPax2/5/8, the sole member of this su bfamily in amphioxus, AmphiPax2/5/8 is initially expressed well back in the rhombospinal domain and not where a MHB would be expected. In contrast, mo st of the other expression domains of AmphiPax2/5/8 correspond to expressio n domains of vertebrate Pax2, Pax5 and Pax8 in structures that are probably homologous - support cells of the eye, nephridium, thyroid-like structures and pharyngeal gill slits; although AmphiPax2/5/8 is not transcribed in an y structures that could be interpreted as homologues of vertebrate otic pla codes or otic vesicles. In sum, the developmental expression of AmphiPax2/5 /8 indicates that the amphioxus central nervous system lacks a MHB resembli ng the vertebrate isthmic region, Additional gene expression data for the d eveloping ascidian and amphioxus nervous systems would help determine wheth er a MHB is a basal chordate character secondarily lost in amphioxus, The a lternative is that the MHB is a vertebrate innovation.