FIGURE 3.

Morphogen gradients and their role in positional information in animals and ciliates. (A) In a simplified model of the freshwater Hydra, a gradient of “head activator” is established, with its high value at the head and low value at the base (green gradient). High levels of this rapidly‐diffusing morphogen lead to differentiation of tentacles, stinging cells and a mouth. Low levels are correlated with the expression of an adhesive disk at the base of the animal. An equally important morphogen gradient involves the head inhibitor (pink) that also emanates from the head region, but has a more limited ability to diffuse. The head inhibitor suppresses secondary head formation along the body axis. As the hydra grows in length, the head inhibitor gradient still covers the same limited physical range, so that at some point, tissues in the proximity of the base become exposed to supra‐threshold levels of activator, and sub‐threshold levels of inhibitor, and a new head forms (budding occurs) [after (Vogg et al., 2019; Webster & Wolpert, 1966); see Mercker et al. (2021) for recent review]. A more complete model would include the description of a “foot activator” and “foot inhibitor” emanating from the basal disk. (B) A simplified model of gradients involved in cortical patterning of Tetrahymena. A posterior‐high gradient of the oral inhibitor (pink = elo1) prevents initiation of oral assembly too close to the posterior. A purely hypothetical posterior‐high gradient of oral activator (green) stimulates oral development near mid‐body (arrow) as the cell launches predivision development