EXPRESSION PATTERN OF 2 OTX GENES SUGGESTS A ROLE IN SPECIFYING ANTERIOR BODY STRUCTURES IN ZEBRAFISH

We isolated two zebrafish sequences containing a homeobox related to o rthodenticle (otd), a gene expressed in the developing head of Drosoph ila. One of these is clearly homologous to Otx1, a homeobox gene previ ously reported to be expressed in the developing rostral brain of the mouse. We termed this zebrafish gene otx1. The second gene is not as c losely related to Otx1 and is equally divergent from Otx2, a second ho meobox gene expressed in the developing rostral brain of the mouse. We termed it otx3, even if a corresponding murine Otx3 gene has not been reported yet. Both genes are expressed in early-gastrula zebrafish em bryos in the involuting presumptive anterior mesendoderm. With the ext ension of the body axis, the expression domain of both genes extends t o neuroectodermal regions fated to become fore- and mid-brain. From th is stage the expression domains of the two genes differ slightly from each other but both cover the rostral brain with a sharp posterior bou ndary coinciding with that between midbrain and hindbrain. This late e xpression closely corresponds to that of the murine Otx1 gene, whereas the earliest expression of both zebrafish otx genes is different from that of Otx1 and reminiscent of that of Otx2 in the mouse. In this li ght, the zebrafish otx1 and otx3 genes appear to share some expression features of both murine Otx1 and Otx2. It will be of considerable int erest to study the specific role of the various genes of the otx famil y in the development of the zebrafish brain regions. The peculiar spat io-temporal pattern of these genes during early zebrafish gastrulation suggests a role of this gene family in interactions between anterior mesendoderm and neuroectoderm.