ИСТИНА

Soft tissue morphology and organisation of certain elements’ (modules) in colonies of thecate hydroids evidently display the predominance of radial symmetry. At the same time, the skeleton structures of shoot modules and entire shoots of the colonies demonstrate either absence of predominant type of symmetry or availability of different combinations of bilateral and translational symmetry (glide reflection symmetry, helical symmetry, etc.). The only colony module that is evidently bilateral is the stolon tip growing over the substrate. In most cases, the outer organisation of the morphogenetic shoot modules in thecate hydroids also displays bilateral symmetry. There is one plane of symmetry so it is possible to mark out left and right, adjacent and opposing (‘dorsal’ and ‘ventral’) sides of the module. The only module that possesses radial symmetry is the primary module developed from the settled larva. However, emerging secondary modules switches to bilateral symmetry that determines the shoot organisation. Transition to the bilateral symmetry of the morphogenetic modules in thecate colonial hydroids can be explained by the model of growth and development regulation in unitary and modular hydroids proposed by Berking (2003, 2006; Berking, S. & Herrmann, 2010) and based on the idea of positional information (Wolpert, 1971, 2011). The primary module developed from settled larva possesses radial symmetry. Development of its parts proceeds according to the positional information increasing during growing tip activity. The secondary growing tip emerging on the primary module is functionally bilateral starting from the origin: the adjacent to the parental hydranth side has higher value of positional information compared to opposing one. Henceforward all the secondary shoot and stolon modules are bilateral. Functional bilateralism stabilises morphogenesis in colony shoots providing regular spatial organisation without any need in new mechanism of morphogenesis regulation. Berking, S., 2003. A model for budding in hydra: pattern formation in concentric rings. J.Theor.Biol, 222(1), 37-52. Berking, S., 2006. Principles of branch formation and branch patterning in Hydrozoa. Int.J.Dev.Biol., 50, 123-134. Berking, S. & Herrmann, K., 2010. A model for tube formation and branching in Cnidaria. Central European Journal of Biology, 5(5), 710-723. Wolpert, L., 1971. Positional information and pattern formation. Curr.Top.Dev.Biol., 6, 183-224. Wolpert, L., 2011. Positional information and patterning revisited. Journal of Theoretical Biology, 269(1), 359-365.