CHARACTERIZATION AND DEVELOPMENTAL EXPRESSION OF AMPHINK2-2, AN NK2 CLASS HOMEOBOX GENE FROM AMPHIOXUS (PHYLUM-CHORDATA SUBPHYLUM CEPHALOCHORDATA)

The genome of amphioxus includes AmphiNk2-2, the first gene of the NK2 homeobox class to be demonstrated in any invertebrate deuterostome. A mphiNk2-2 encodes a protein with a TN domain, homeodomain, and NK2-spe cific domain; on the basis of amino acid identities in these conserved regions, AmphiNk2-2 is a homolog of Drosophila vnd and vertebrate Nkx 2-2. During amphioxus development, expression of AmphiNk2-2 is first d etected ventrally in the endoderm of late gastrulae. In neurulae, endo dermal expression divides into three domains (the pharynx, midgut, and hindgut), and neural expression commences in two longitudinal bands o f cells in the anterior neural tube. These neural tube cells occupy a ventrolateral position on either side of the cerebral vesicle (the pro bable homolog of the vertebrate diencephalic forebrain). The dynamic e xpression patterns of AmphiNFkx2-2 suggest successive roles, first in regionalizing the endoderm and nervous system and later during differe ntiation of specific cell types in the gut (possibly peptide endocrine cells) and brain (possibly including axon outgrowth and guidance).