The zebrafish: heritable disorders in transparent embryos.

elegant embryological work on early development of frog or chick, and targeted gene mutation in mice? It is true that similarities abound between Drosophila and human. Despite separation from a presumptive common ancestor over the last 400 million years, there is conservation of genetic programs which direct gastrulation, axial determination, and metameric regionalization. However, several new features characterize the vertebrate which are not present in invertebrates. For example, the notochord, the transient embryonic backbone which defines chordates, is not only a support system but also generates signals which are required for the differentiation of adjacent nervous system, bone, and muscle. The neural crest, the migratory cells from the nervous system which contribute to the face, pharyngeal arches, heart, peripheral nervous system, and melanocytes, also appears to be a vertebrate invention. The vertebrate heart, vasculature, kidney, and gut derivatives such as pancreas, have no clear Drosophila cognates, although elements may have emerged in invertebrates by convergent evolution. Therefore, it is important to turn to a vertebrate, and to one established as a genetic system. The mouse is ideal in this regard for study of late-acting genes, but the opacity of mother and uterus makes early development less accessible, and large-scale screens are prohibitively expensive.