AmphiPax3/7, an amphioxus paired box gene: insights into chordate myogenesis, neurogenesis, and the possible evolutionary precursor of definitive vertebrate neural crest

Amphioxus probably has only a single gene (AmphiPax3/7 ) in the Pax3/7 subf amily. Like its vertebrate homologs (Pax3 and Pax7 ), amphioxus AmphiPax3/7 is probably involved in specifying the axial musculature and muscularized notochord. During nervous system development, AmphiPax3/7 is first expresse d in bilateral anteroposterior stripes along the edges of the neural plate. This early neural expression may be comparable to the transcription of Pax 3 and Pax7 in some of the anterior neural crest cells of vertebrates. Previ ous studies by others and ourselves have demonstrated that several genes ho mologous to genetic markers for vertebrate neural crest are expressed along the neural plate-epidermis boundary in embryos of tunicates and amphioxus. Taken together, the early neural expression patterns of AmphiPax3/7 and ot her neural crest markers of amphioxus and tunicates suggest that cell popul ations that eventually gave rise to definitive vertebrate neural crest may have been present in ancestral invertebrate chordates. During later neuroge nesis in amphioxus, AmphiPax3/7, like its vertebrate homologs, is expressed dorsally and dorsolaterally in the neural tube and may be involved in dors oventral patterning. However, unlike its vertebrate homologs, AmphiPax3/7 i s expressed only at the anterior end of the central nervous system instead of along much of the neuraxis; this amphioxus pattern may represent the los s of a primitive chordate character.