Growing with the flow: On the maintenance and malleability of colony form in the hydroid Hydractinia

The extent of genetic predetermination and hence the limits of developmental plasticity of phenotypes are largely unknown. In this context, we address a prominent theme of morphological variation in clonal organisms: the relative investment in vascular versus ramet tissue. The colonial hydroid Hydractinia is characterized by colonies with widely spaced polyps and long stolonal connections (runners) and colonies with closely packed polyps and short stolonal connections (sheets). Surgical manipulations of gastrovascular architecture were used to alter flow patterns within developing runner and sheet colonies, and subsequent morphological development of colonies was assessed. Reducing within-colony variation in patterns of gastrovascular how of runner colonies resulted in a lower rate of production of peripheral stolons and a sheet-type morphology. Experimentally increased variation in flow patterns within sheet colonies resulted in the development of peripheral stolons-hence, the initiation of a runnerlike growth trajectory. In both experiments, control colonies, in which stolonal architecture was not manipulated; maintained the runner or sheet morphology characteristic of their respective clones. The results imply that for this clonal species, morphological differences between genotypes can be controlled by changing gene expression in response to the local physiological conditions.