Role of the skeleton in determination of the branching points in hydroid colonies

Colonial hydroids of the suborder Thecaphora have rigid outer skeleton that possesses species-specific shape of the colony elements. Organisation of the elements within a colony shows strict spatial patterning. The points of branching (emergence of the new growing tip) within shoots are strictly determined and show positive correlation with the place of the most pronounced curvature of the elements skeleton. As it was shown earlier, the shoot growing tip, after emergence, performs its program of functioning independently from the colony condition. Several modifications of experiments with grafting of the growing tip together with rotation around its longitudinal axis were fulfilled. As the result of such grafting the normal orientation of the formed skeleton was altered. In overwhelming cases of further tip growth and shoot development the new growing tip emerged in correspondence with new orientation of the formed skeleton. The orientation of the hydranth of the maternal shoot element had no effect upon the orientation of the new tip emergence after grafting. It is supposed that the place of the new tip emergence is regulated hierarchically. At first level, the interaction of the soft tissues with the skeleton has priority. Anisotropy of mechanical tensions within cell layers due to interaction with the skeleton at the point of its most curvature serves as a primary stimulus in the chain of events leading to the initiation of the new growing tip. If this does not work the intrinsic tissue property (polarity) determines the place of the tip emergence. Such two-level mechanism of determination of the place of the new tip emergence in sympodial shoots stabilises spatial organisation of the entire shoot and decreases the probability of epigenetic mistakes.