The homeobox gene otx2 is a key regulator for specifying the rostral part of the vertebrate head. In Xenopus, otx2 directly controls the differentiation of the cement gland, the anterior-most organ formed in the tadpole. Since embryos of a direct developing frog, Eleutherodactylus coqui, lack a cement gland, we are interested in whether altered expression of the otx2 gene is involved in this evolutionary change. We have cloned the E. coqui homologue of otx2, Ecotx2. The homeodomain of the Ecotx2 protein is identical to the mouse and zebrafish Otx2 proteins and differs by a single amino acid from the Xenopus Otx2 protein. Study of the spatiotemporal expression pattern shows that Ecotx2 RNA is progressively restricted to the anterior region of the embryo during gastrulation and becomes further restricted to the future forebrain and midbrain during neural development. In Xenopus, in addition to the conserved expression in the anterior neuroectoderm, the expression in ectoderm expands more anteriorly to the cement gland primordium. This anterior expansion of otx2 expression is not found in E. coqui, correlating with the loss of a cement gland. When misexpressed in Xenopus laevis ectoderm, Ecotx2 can activate expression of the cement-gland-specific genes XCG and XAG1, indicating that the function of activating the pathway of cement gland formation is retained by the Ecotx2 protein. Our results indicate that there are modifications in the pathway of cement gland formation, upstream of otx2 expression, in the development of E. coqui.
Copyright 1999 Academic Press.