We show that, in addition to a role in mesoderm induction during blastula stages, FGF signalling plays an important role in maintaining the properties of the mesoderm in the gastrula of Xenopus laevis. eFGF is a maternally expressed secreted Xenopus FGF with potent mesoderm‐inducing activity. However, it is most highly expressed in the mesoderm during gastrulation, suggesting a role after the period of mesoderm induction. eFGF is inhibited by the dominant negative FGF receptor. Embryos overexpressing the dominant negative receptor show a change of behaviour of the dorsal mesoderm such that it moves around the blastopore lip instead of elongating in an antero‐posterior direction. In such embryos there is a reduction in Xbra expression during gastrulation. We show that during blastula stages eFGF and Xbra are able to activate the expression of each other, suggesting that they are components of an autocatalytic regulatory loop. Moreover, we show that Xbra expression in isolated gastrula mesoderm cells is maintained by eFGF, suggesting that eFGF continues to regulate the expression of Xbra in the blastopore region. In addition, overexpression of eFGF after the mid‐blastula transition results in the up‐regulation of Xbra expression during gastrula stages and causes suppression of the head and enlargement of the proctodeum, which is the converse of the posterior reductions of the FGF dominant negative receptor phenotype. These data suggest an important role for eFGF in regulating the expression of Xbra and for the eFGF‐Xbra regulatory pathway in the control of mesodermal cell behaviour during gastrula stages.

The caudal gene codes for a homeodomain transcription factor that is required for normal posterior development in Drosophila. In this study the biological activities of the Xenopus caudal (Cdx) family member Xcad3 are examined. A series of domain-swapping experiments demonstrate that the N-terminus of Xcad3 is necessary for it to activate Hox gene expression and that this function can be replaced by the activation domain from the viral protein VP16. In addition, experiments using an Xcad3 repressor mutant (XcadEn-R), which potently blocks the activity of wild-type Xcad3, are reported. Overexpression of XcadEn-R in embryos inhibits the activation of the same subset of Hox genes that are activated by wildtype Xcad3 and leads to a dramatic disruption of posterior development. We show that Xcad3 is an immediate early target of the FGF signalling pathway and that Xcad3 posteriorizes anterior neural tissue in a similar way to FGF. Furthermore, Xcad3 is required for the activation of Hox genes by FGFs. These data provide strong evidence that Xcad3 is required for normal posterior development and that it regulates the expression of the Hox genes downstream of FGF signalling.

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Convergent extension behaviour is critical for the formation of the vertebrate body axis. In Xenopus, components of the Wnt signaling pathway have been shown to be required for convergent extension movements but the relationship between cell fate and morphogenesis is little understood. We show by loss of function analysis that Xnr3 activates Xbra expression through FGFR1. We show that eFGF activity is not essential in the pathway, and that dishevelled acts downstream of Xnr3 and not in a parallel pathway. We provide evidence for the involvement of the EGF-CFC protein FRL1, and suggest that the pro-domain of Xnr3 may be required for its activity. Since Xnr3 is a direct target of the maternal βcatenin/XTcf3 signaling pathway, it provides the link between the initial, maternally controlled, allocation of cell fate, and the morphogenetic movements of cells derived from the organizer.

Using Xenopus tropicalis, we present the first analysis of the developmental effects that result from knocking down the function of the three Cdx genes present in the typical vertebrate genome. Knockdowns of individual Cdx genes lead to a similar range of posterior defects; compound Cdx knockdowns result in increasingly severe posterior truncations, accompanied by posterior shifts and reduction of 5′ Hox gene expression. We provide evidence that Cdx and Wnt3A genes are components of a positive feedback loop operating in the posterior axis. We show that Cdx function is required during later, but not early stages of development, for correct regional specification of the endoderm and morphogenesis of the gut. Our results support the hypothesis that during amphibian development the overall landscape of Cdx activity in the embryo is more important than the specific function of individual Cdx proteins. Developmental Dynamics 238:835–852, 2009. © 2009 Wiley-Liss, Inc.

Cdx homeodomain transcription factors play important roles in the development of the vertebrate body axis and gut epithelium. Signaling involving FGF, wnt and retinoic acid ligands has been implicated in the regulation of individual Cdx genes. In this study we examine the requirement for FGF-dependent signal transduction pathways in the regulation of Cdx gene expression. In the amphibian Xenopus laevis the earliest expression of Cdx1, Cdx2 and Cdx4 is within the developing mesoderm. We show that a functional FGF signaling pathway is required for the normal expression of all three amphibian Cdx genes during gastrula stages. We show that FGF stimulation activates signaling through both the MAP kinase pathway and the PI-3 kinase pathway in Xenopus tissue explants. However, our analysis of these pathways in gastrula stage embryos indicates that the MAP kinase pathway is required for Cdx gene expression, whereas the PI-3 kinase pathway is not. We show that FGF and wnt signaling can interact in the regulation of Cdx genes and during gastrula stages the normal expression of the Cdx genes requires the activity of both pathways. Furthermore, we show that wnt mediated Cdx regulation is independent of the MAP kinase pathway.

rom the identification of genes to the characterization of their functions and interactions. Developmental biologists have long used whole mount in situ hybridization (WISH) to determine gene expression patterns, as a vital tool for formulating and testing hypotheses about function. This paper describes the application of WISH to the study of gene expression in larval and adult schistosomes. Fixed worms were permeablized by proteinase K treatment for hybridization with digoxygenin-labelled RNA probes, with binding being detected by alkaline phosphatase-coupled anti-digoxygenin antibodies, and BM Purple substrate. Discrete staining patterns for the transcripts of the molecules Sm29, cathepsin L, antigen 10.3 and chorion were observed in the tegument cell bodies, gut epithelium, oesophageal gland and vitelline lobules, respectively, of adult worms. Transcripts of the molecules SGTP4, GP18-22 and cathepsin L were localized to tegument cell bodies and embryonic gut, respectively, of lung schistosomula. We also showed that Fast Red TR fluorescent substrate can refine the pattern of localization permitting use of confocal microscopy. We believe that method of WISH will find broad application, in synergy with other emerging post-genomic techniques, such as RNA interference, to studies focused at increasing our molecular understanding of schistosomes