SEQUENCE AND DEVELOPMENTAL EXPRESSION OF AMPHIDII, AN AMPHIOXUS DISTAL-LESS GENE TRANSCRIBED IN THE ECTODERM, EPIDERMIS AND NERVOUS-SYSTEM - INSIGHTS INTO EVOLUTION OF CRANIATE FOREBRAIN AND NEURAL CREST

The dynamic expression patterns of the single amphioxus Distal-less ho molog (AmphiDll) during development are consistent with successive rol es of this gene in global regionalization of the ectoderm, establishme nt of the dorsoventral axis, specification of migratory epidermal cell s early in neurulation and the specification of forebrain, Such a mult iplicity of Distal-less functions probably represents an ancestral cho rdate condition and, during craniate evolution, when this gene diversi fied into a family of six or so members, the original functions eviden tly tended to be parcelled out among the descendant genes, In the amph ioxus gastrula, AmphiDll is expressed throughout the animal hemisphere (presumptive ectoderm), but is soon downregulated dorsally (in the pr esumptive neural plate), During early neurulation, AmphiDll-expressing epidermal cells flanking the neural plate extend lamellipodia, appear to migrate over it and meet mid-dorsally, Midway in neurulation, cell s near the anterior end of the neural plate begin expressing AmphiDll and, as neurulation terminates, these cells are incorporated into the dorsal part of the neural tube, which forms by a curling of the neural plate, This group of AmphiDII-expressing neural cells and a second gr oup expressing the gene a little later and even more anteriorly in the neural tube demarcate a region that comprises the anterior three/four ths of the cerebral vesicle; this region of the amphioxus neural tube, as judged by neural expression domains of craniate Distal-less-relate d genes, is evidently homologous to the craniate forebrain, Our result s suggest that craniates evolved from an amphioxus-like creature that had the beginnings of a forebrain and possibly a precursor of neural c rest - namely, the cell population leading the epidermal overgrowth of the neural plate during early neurulation.