ABSTRACT
Amphioxus, the closest living invertebrate relative of the vertebrates, has a dorsal hollow nerve cord that has only a slight anterior swelling, the cerebral vesicle, and does not produce neural crest cells that migrate individually through tissues. However, the expression patterns of developmental genes together with detailed microanatomy have shown that the amphioxus nerve cord has likely homologues of two of the three genetic divisions of the vertebrate nerve cord: the forebrain plus anterior midbrain and the hindbrain plus spinal cord. The third division of the vertebrate nerve cord is the midbrain/hindbrain boundary (MHB) or the isthmo/ cerebellar region, which includes posterior midbrain and the anteriormost hindbrain and has organizer properties. Several genes are co-expressed in the vertebrate MHB including Wntl, Pax2, Pax5, Pax8, En1, and En2. We have, therefore, cloned and determined the embryonic expression of the amphioxus homologues of these genes: AmphiPax2/5/8, AmphiWnt1, and AmphiEn (Holland et al. 1997; Kozmik et al. 1999). AmphiWnt1 is not expressed anywhere in the developing central nervous system, while AmphiPax2/5/8 and AmphiEn are not expressed in any region that could be interpreted as homologous to the vertebrate MHB. Instead, the expression domains of these genes appear to correspond to the expression domains of their vertebrate homologues in the diencephalon and hindbrain. Thus, we suggest that the MHB as an organizer region is a vertebrate innovation.
