| Abstract: | Valve morphogenesis in Proboscia alata Sundstrom was followed in living cells and during treatment with antiactin and antimicrotubule drugs. Once cleaved, sibling cells rounded up and retracted. Soon, a granular organizing center (OC) appeared adjacent to the stub of the initiated valve. Silicification started within a silicon deposition vesicle (SDV) adjacent to the OC. The elongating valve was initially tubular and sealed at one end, creating the proboscis of the conical valve. The edge of the SDV and thinnest region of the forming valve was lined by a sleeve of bundled microtubules (MTs) that terminated short of the older more rigid part of the valve. The growing proboscis of living cells treated with the anti‐MT drug oryzalin became grossly distorted. EM revealed dense material lining the growing edge of the SDV; immunofluorescence microscopy showed a ring of actin here. Applied to living cells, the antiactin drug cytochalasin D caused the very young proboscis to collapse; in older valves, the base of the proboscis expanded. Thus, valve morphogenesis appeared controlled by the MT cytoskeleton, keeping the proboscis straight while actin molded its conical outline. At the tip of the proboscis was a slit resembling a labiate process. Its morphogenesis involved striated fibers and two MTs, reminiscent of the fibers and MTs associated with raphe morphogenesis. In contrast to spine‐like processes that elongate by tip growth, the tip of the proboscis was formed first, and the consequent “antitip growth” suggests the tip was originally the center of the valve face. |
