[Morphogenesis in colonial hydroids: pulsating rudiment splitting]

Two spatially separated processes underlie the growth and morphogenesis in hydroids (Cnidaria, Hydroidomedusa): (1) growth pulsations of the terminal growing tips and (2) cell proliferation and migration in more proximal parts of the colony soft tissues. Growing tips are morphogenetic elements of the colony that provide for the colony elongation and morphogenesis. In thecate hydroids (subclass Leptomedusae) with highly integrated colonies and monopodial shoot growth, the initiation of the lateral branches and hydranth rudiments looks like a periodic splitting of the growing tip into two or more rudiments. Published descriptions and proposed models of this process assume that the splitting results from the formation of the furrows running into the tip from its apical surface. In this study on a Sertulariidae species, we demonstrate that the visible process of the tip splitting into several rudiments begins in its proximal part. At the same time, the inner ridges are initiated at the skeleton lateral surfaces surrounding the growing tip. These ridges develop and grow along the proximodistal axis. Eventually, the opposite ridges fuse, which splits the tip into several rudiments. We propose that the tip splitting into several rudiments is impossible without the spatial regulation of the outer skeleton formation. This process explains many species-specific properties of the shoot spatial organization in thecate hydroids such as the partial or complete fusion of the zooid skeleton with the shoot stem skeleton, deflection of the distal parts of the zooid skeleton from the shoot stem axis, etc. The revealed mechanisms considerably supplements and corrects the models describing morphogenesis in colonial hydroids.