The putative Notch ligand HyJagged is a transmembrane protein present in all cell types of adult Hydra and upregulated at the boundary between bud and parent

We have cloned a cDNA from Hydra, which encodes a bona-fide Notch ligand with a conserved domain structure similar to that of Jagged-like Notch ligands from other animals. Hyjagged mRNA is undetectable in adult Hydra by in situ hybridisation but is strongly upregulated and easily visible at the border between bud and parent shortly before bud detachment. In contrast, HyJagged protein is found in all cell types of an adult hydra, where it localises to membranes and endosomes. Co-localisation experiments showed that it is present in the same cells as HvNotch, however not always in the same membrane structures.The putative Notch ligand HyJagged is conserved in Cnidarians. Together with HvNotch it may be involved in the formation of the parent-bud boundary in Hydra. Moreover, protein distribution of both, HvNotch receptor and HyJagged indicate a more widespread function for these two transmembrane proteins in the adult hydra, which may be regulated by additional factors, possibly involving endocytic pathways.Early embryonic development is regulated in all animals by common signalling pathways such as the Wnt-, FGF-, Hedgehog-, TGFβ/BMP- and Notch pathways. They have been shown to be present in basal metazoans such as Cnidarians [1-5]. The fresh water polyp Hydra consists of a simple tube-like body made of two epithelial layers, the inner endoderm and the outer ectoderm separated by an acellular mesoglea. It further possesses a hypostome and a ring of tentacles at the apical end and a foot at its basal end. Interstitial cells are located in the spaces between the epithelial cells of both cell layers. They constitute a pluripotent stem cell lineage that differentiates nerve cells, gland cells, germ cells and nematocytes. The latter carry nematocyte capsules used for catching prey and are unique to Cnidarians [6-9].The Notch signalling pathway has recently been shown to be involved in nematocyte and female germ cell differentiation, as well as in budding [5,10]. Whilst an