Ciliary reversal without rotation of axonemal structures in ctenophore comb plates.

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Abstract
A newly discovered reversal response of ctenophore [Pleurobrachia pileus, Mnemiopsis leidyi and Lampea pancerina] comb plates was used to investigate the structural mechanisms controlling the direction of ciliary bending. High K+ concentrations cause cydippid larvae of the ctenophore Pleurobrachia to swim backward. High-speed cine films of backward-swimming animals show a 180.degree. reversal in beat direction of the comb plates. Ion substitution and blocking experiments with artificial seawaters demonstrate that ciliary reversal is a Ca2+-dependent response. Comb plate cilia possess unique morphological markers for numbering specific outer-doublet microtubules and identifying the sidedness of the central pair. Comb plates of forward- and backward-swimming ctenophores were frozen in different stages of the beat cycle by an instantaneous fixation method. Analysis of transverse and longitudinal sections of instantaneously fixed cilia showed that the assembly of outer doublets does not twist during ciliary reversal. This directly confirms the existence of a radial switching mechanism regulating the sequence of active sliding on opposite sides of the axoneme. The axis of the central pair remains perpendicular to the plane of bending; the ultrastructural marker showed that the central pair does not rotate during a 180.degree. reversal in beat direction. The orientation of the central pair does not control the direction of ciliary bending (i.e., the pattern of active sliding around the axoneme). The validity of this finding for 3-dimensional and 2-dimensional ciliary beat cycles is discussed. Models of central-pair function based on correlative data alone must be re-examined in light of these new findings on causal relations.

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