Motility and mechanosensitivity of macrocilia in the ctenophore Beroë

Abstract

Mechanical activation of the microtubule sliding mechanism in cilia and flagella^1–3 by local passive bending has been postulated to be essential for the initiation and propagation of bending waves along the axoneme^4,5. In addition, responsiveness of cilia to hydrodynamic forces imposed externally by their neighbours is thought to be responsible for metachronal coordination of ciliary activity^6–10, as well as for synchronal beating of component cilia within compound ciliary organalles^11,12. Direct tests of the mechanosensitivity of motile cilia are limited, but generally support these views^{7,10,11,13–18}. It remains problematical, however, whether mechanical interaction between cilia operates continuously during both the effective and recovery phases of the asymmetrical beat cycle. Moreover, the directional sensitivity and temporal responsiveness of motile cilia to mechanical stimuli have been explored in only a few cases^7,10.14. Finally, the continuous nature of the ciliary beat cycle has hindered investigation of the ‘switch point hypothesis’ in which doublet sliding is assumed to be activated sequentially on the two halves of the axoneme to produce bends in opposite directions¹⁹. Here we report that macrocilia²⁰ on the ctenophore Beroë beat discontinuously with separate effective and recovery strokes, resulting in ‘split-cycle’ waves of metachronal coordination. This new pattern of ciliary beating is used to investigate the motile responses of cilia to controlled mechanical stimuli during each phase of the beat cycle.