Genetic patterning of ectoderm and endoderm in amphioxus: from homeobox genes to hormones

The cephalochordates, typified by amphioxus, are close relatives of the vertebrates. With a few exceptions, for every set of two or more related genes in vertebrates, amphioxus has just one. This facilitates recognition of the primitive roles of particular genes. We show that Hox genes are an ectoderm-specific patterning system in amphioxus and that other spatially expressed homeobox genes are endoderm-specific. It seems that different sets of genes interpret positional information in different germ layers, and translate this into position-specific cell fate. We also present new data on the spatial expression of one of the putative downstream effector genes in endoderm: an amphioxus insulin-related gene.