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Zusammenfassung:
Cell fate specification and cell-cell signaling have been well studied during the development of C. elegans and provide a paradigm for evolutionary developmental biology. To identify the molecular basis of evolutionary alterations in development, we have developed Pristionchus pacificus as a satellite organism, in which i) forward and reverse genetic studies can be performed, ii) a 10 X coverage of the genome is available and iii) DNA-mediated transformation allows experimental manipulation (Schlager et al., 2009). One developmental process that has been studied intensively is vulva formation and functional differences were identified in the molecular mechanisms of vulva induction between C. elegans and P. pacificus. While in C. elegans EGF is the key initiation signal of vulva development, forward and reverse genetic experiments revealed that Wnt signaling is crucial for vulva induction in P. pacificus. A Ppa-bar-1/β-catenin mutant is completely vulvaless. Reverse-genetic analysis indicated that multiple Wnt ligands and Fz-like receptors play a redundant role in vulva induction, including Ppa-egl-20/Wnt. Two striking features are a negative role of Ppa-lin-17/Frizzled, mutations in which result in a multivulva phenotype, and the function of Ppa-egl-20, that when mutated can suppress the Ppa-lin-17 phenotype. Such genetic interactions suggest a role of this Frizzled receptor in ligand sequestration. Interestingly, Ppa-egl-20 is only expressed in the rectal region, similar to C. elegans, representing the first inductive ligand for vulva induction that is expressed outside the gonad. To further elucidate the molecular mechanisms of P. pacificus vulva induction, we are combining forward genetics and transformation technology. First, another multivulva mutant, formerly known as limu-1, has been cloned and displays a novel aspect of Wnt signaling function. Second, a transgenic approach using heat-shock induced Ppa-bar-1 and Ppa-egl-20 reveals the temporal aspects of Wnt signaling. These studies show that at least three distinct Wnt signaling mechanisms are at work, which control i) vulva induction, ii) the polarity of the secondary cell fates and iii) the fate of the epidermal cell P8.p. In particular Ppa-lin-17 has distinct functions in all three decisions, most of which are unknown from Cel-lin-17. This study provides the first molecular insight into vulva induction by a signaling center outside of the gonad and thereby extends our understanding of signaling mechanisms in nematodes. References: Schlager, B. et al. (2009) Molecular cloning of a dominant Roller mutant and establishment of DNA-mediated transformation in the nematode Pristionchus pacificus. Genesis in Press.
