Dose-response relationships for experimental heterochrony in a colonial hydroid

Hydractiniid hydroids display a range of morphological variation. At one end of the spectrum, the colonies grow in a sheet-like configuration with their polyps close together and short stolons. At the other extreme, the colonies have a runner-like form in which the polyps are farther apart and connected by long stolons. These patterns exemplify the heterochronic variation found in many colonial animals and correspond to changes in the timing of the production of polyps and stolen tips relative to rates of stolen growth and colony maturation. Experimental studies of clonal replicates of a Podocoryne carnea colony demonstrated a dose-response relationship between these heterochronic traits and within-colony gastrovascular flow to peripheral stolons. A dose-response relationship was found whether flow was perturbed by manipulating the amount of food consumed by the colonies or by treating the colonies with 2,4-dinitrophenol, an uncoupler of oxidative phosphorylation. In colonies in which flow was highly perturbed by either treatment, a similar rate of flow produced a similar morphological response. These data support the hypothesis of a causal relationship between flow rate and heterochronic variation. Nevertheless, flow was diminished by two clearly different mechanisms. Feeding manipulation altered flow relative to the size of the stolen by altering stolen thickness, without affecting the absolute quantity of Bow. Uncoupling with dinitrophenol diminished the absolute quantity of flow, but did not affect the size of the stolen. A plausible assumption is that feeding manipulation affects the resistance of the stolen tissue to flow, or the fluid absorption of this tissue, or both; whereas uncoupling affects the amount of energy available to drive the flow. At the level of cellular metabolism, on the other hand, feeding manipulation and uncoupling again have similar effects, triggering metabolic activation (e.g., an increase in oxygen uptake and a shift in the mitochondrial redox state in the direction of oxidation). In the context of theories suggesting the metabolic control of development, a direct effect of feeding and uncoupling on colony development thus cannot be ruled out. Further, there may be an interaction between flow rate and metabolism, since gastrovascular flow distributes food throughout the colony, and since such substrate affects metabolic state. Both within-colony flow rate and metabolism may affect heterochronic variation in these hydroids, and methods appropriate to distinguish these two effects are discussed.