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The mesoderm of the amphibian embryo arises through an inductive interaction in which blastomeres of the vegetal hemisphere of the embryo act on overlying equatorial cells

The mesoderm of the amphibian embryo arises through an inductive interaction in which blastomeres of the vegetal hemisphere of the embryo act on overlying equatorial cells (Nieuwkoop, 1969). Once mesoderm is induced, gastrulation takes place through coordinated morphological movements of the three germ layers. These movements, which include convergent extension and involution, are driven predominantly by cells of the mesoderm (Keller et al., 1992; Keller, 1986). This is particularly obvious during convergent extension, when polarised axial mesoderm cells intercalate in radial and mediolateral directions to cause dramatic elongation of the dorsal marginal zone (Shih and Keller, 1992; Wilson and Keller, 1991). The molecular mechanisms by which mesodermal cells become polarised and drive these movements are, however, poorly understood. The mesoderm-inducing factor activin causes prospective ectodermal cells to undergo convergent extension (Symes and Smith, 1987) and also to activate the expression of mesodermspecific genes in an immediate-early fashion. Of these genes, Xenopus Brachyury (Xbra), which is expressed in a widespread fashion throughout the mesoderm (Smith et al., 1991), is of some interest. Brachyury function is required for posterior mesoderm and notochord differentiation in mouse, zebrafish and Xenopus embryos (Conlon et al., 1996; Herrmann et al., 1990; Schulte-Merker et al., 1994), and mis-expression of Xbra in prospective ectodermal tissue of Xenopus causes ectopic mesoderm formation in a dose-dependent fashion (Cunliffe and Smith, 1992; O’Reilly et al., 1995). Of particular relevance to this study, we note that chimeric genetic studies in mice demonstrate that Brachyury is necessary for normal morphogenetic movements during gastrulation (Wilson et al., 1995). Furthermore, the morphogenetic movements of activintreated animal caps are inhibited by a dominant-negative Xbra construct (Conlon and Smith, 1999). Together, these results indicate that Brachyury plays a key role not only in mesoderm formation but also in morphogenesis. Since Xbra functions as a transcription activator (Conlon et al., 1996; Kispert et al., 1995), it is likely that Xbra targets include genes that are required for convergent extension. To investigate this issue, we have turned to a subtracted library in which Brachyury-inducible genes are highly enriched (Tada et al., 1998). This screen has allowed us to isolate Xwnt11, which was originally identified as a maternally expressed Wnt gene (Ku and Melton, 1993). Expression of Xwnt11 in the marginal zone of the Xenopus embryo requires Xbra function, and induction of Xwnt11 by hormone-inducible Xbra does not require protein synthesis (Saka et al., 2000), suggesting that it is activated by Xbra in a direct fashion. To investigate the function of Xwnt11 during gastrulation, we have constructed a C-terminally truncated form of the 2227 Development 127, 2227-2238 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV1490