Vegetal cell fate specification and anterior neuroectoderm formation by Hroth, the ascidian homologue of orthodenticle/otx.

To obtain insights into the mechanisms of gastrulation and neural tube formation, we studied the function and regulation of expression of Hroth, the ascidian homologue of orthodenticle/otx, during embryogenesis. Microinjection of synthetic Hroth mRNA into fertilized eggs led to embryos with an expanded trunk and a reduced tail. In these embryos, development of notochord and muscle was effected. Also, Hroth overexpression caused ectopic formation of anterior neuroectoderm, along with suppression of epidermis development, even in the absence of cell-cell interaction. Furthermore, we demonstrated that ectodermal expression of Hroth requires an inductive influence from the vegetal hemisphere cells. These data suggest roles of Hroth in both specification of mesoendodermal cells and anterior neuroectoderm formation.