Tails of Tetrahymena

SYNOPSIS. The source of force generation of beating cilia and flagella is an interaction between the doublet microtubules mediated by the dynein‐1 arms which cause the doublets to slide relative to one another. Previously, we demonstrated direct sliding of Tetrahymena ciliary axonemes by dark field light microscopy. In this paper, the results of such an experiment have been captured on a polylysine‐coated grid surface for whole‐mount electron microscopy. Images in which sliding between doublets has taken place can be identified. We conclude that doublets slide relative to one another with a constant polarity. To produce the observed displacement, the direction of the dynein‐1 arm force generation must be from base to tip, so that the doublet (n), to which the arms are attached, pushes the next doublet (n+ 1) toward the tip. In addition to the functional polarity, the dynein‐1 arms are found to have a structural polarity: they tilt toward the base when viewed along the edges of the A‐subfiber. A scheme is presented which reconciles the finding of a single polarity of active sliding with the geometry of microtubule tip displacement of bent cilia.