Cell Body Contraction Of Spirostomum Does Not Involve Shortening Of Inter-Kinetosomal Distance Along Ciliary Lines

A heterotrichous ciliate Spirostomum is known to show characteristic rapid contraction of the cell body accompanied by twisting of ciliary lines which run longitudinally on the cell surface. The ciliary lines of Stenter, a closely related heterotrichous ciliate, are known to become shorter by themselves when the cell contracts, resulting in sliding between adjacent longitudinal microtubular sheets (LMSs) running just beneath the ciliary lines. In Spirostomum, in contrast, there is controversy over whether lengths of the ciliary lines alter or not. In this study, we examined changes of the ciliary line lengths by scanning electron microscopy, to measure the distance between the proximal ends of two neighboring cilia which are lining up along each ciliary line (inter-kinetosomal distances). We found that the inter-kinetosomal distance remained constant regardless of cell contraction and elongation, indicating that the cell body contraction of Spirostomum results from a cellular twisting motion with constant inter-kinetosomal distance. It can thus be concluded that the contraction of Spirostomum does not involve shortening of the inter-kinetosomal distance along ciliary lines, which is quite different from the case in Stentor.