Anteroposterior patterning of the epidermis

Patterning along the anteroposterior axis is a critical step during animal embryogenesis. In the vertebrate central nervous system (CNS), the patterned neural tube is characterized by regionspecific expression of many genes including homeobox genes such as dlx, otx, pax, Hox and cdx cognates (e.g., McGinnis and Krumlauf, 1992; Shimamura et al., 1995). The patterning of the vertebrate neural tube has been shown to be controlled by inductive signals emanating from the organizer tissue (Harland and Gerhalt, 1997; Kessler and Melton, 1994) as well as the anterior endoderm (Beddington and Robertson, 1998) and nonaxial mesoderm (Koshida et al., 1998; Woo and Fraser, 1997). Recently, comparative studies on regional specification in the CNS among the chordates have revealed that different members of the chordates (the vertebrates, amphioxus and ascidians) exhibit similar expression patterns of homeobox genes in the neural tube (Williams and Holland, 1998). For example, in the ascidian embryo, the ascidian homologue of orthodenticle/otx, Hroth (Wada et al., 1996) is expressed in the anteriormost region of the neural tube (the sensory vesicle) while the labial group Hox gene, HrHox-1 (Katsuyama et al., 1995) is expressed in a more posterior region of the neural tube. The single ascidian homologue of the Pax2, Pax5 and Pax8 genes is expressed in between (Wada et al., 1998). Expression pattern of these genes in the ascidian embryo resembles that of Otx1 and Otx2, Pax2, Pax5 and Pax8, and Hox genes in the vertebrate neural tube and thus suggests that regional specification of the CNS is conserved, at least in part, between the vertebrates and the ascidians (Katsuyama et al., 1996; Wada et al., 1998). The CNS patterning of the ascidian embryo also resembles that of the vertebrates with respect to cellular interactions involved in the process. The sensory vesicle of the ascidian embryo is induced by the vegetal hemisphere cells around gastrulation (Nishida, 1991; Nishida and Satoh, 1989; Okado and Takahashi, 1988, 1990; Reverberi et al., 1960; Rose, 1939). 4955 Development 126, 4955-4963 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV3994