Hydra, a model system to trace the emergence of boundaries in developing eumetazoans

In developing embryos, boundary formation between neighbouring groups of cells is essential to establish compartments which later fulfil specialized functions. The ability to form such boundaries has likely developed early in animal evolution - due to functional requirements imposed by the necessity to separate tissues which protect the animal, take up food or ensure propagation. Essential for boundary formation are local cues which may be provided by the intersection of diffusible molecules or set locally by activation of membrane-bound receptors and transcription factors. In the simple diploblastic Hydra, a representative of the basally branching metazoan Cnidaria, tissue boundaries are morphologically detectable between the body column and terminally differentiated head and foot structures. In adult polyps, these borders correspond to sharp lines of differential gene expression. They form de novo during regeneration and budding of a young polyp. Functional studies strongly suggest the involvement of FGFR/Notch signalling in the establishment of the parent-bud boundary, and it is very likely that these pathways interact with the WNT and BMP systems. How boundaries in the head and foot regions are generated is still unclear. Expression patterns of transcription factors like Cngsc, HyAlx, HyBra, HyOtx, Prdl-a, CnNK2 and Manacle show strong position dependency and may be involved in regulating gene expression on either side of the boundaries, by interpreting positional information during their formation and maintenance. Due to its simplicity, the easy accessibility to pharmacological interference and, recently, transgenesis, Hydra is an interesting prebilaterian model system to study the emergence of boundary-forming mechanisms during evolution.