ified ciliary stick. It has been suggested that gravity is perceived by bending the ciliary
complex, which induces changes in membrane potential, regulating the activity of the
body cilia (Fenchel and Finlay 1984, 1986).
7.9 Susception in the protoplast-based systems of EuglenaandParamecium
Starting in the nineteenth century, different hypotheses have been proposed to explain the
mechanism of gravitaxis in unicellular organisms either involving a pure physical mech-
anism, a physiological process, or a combination of both (for reviews, see Bean 1984;
Machemer and Bräucker 1992; Häder et al. 2005). Although the physical mechanisms as-
sume a passive alignment of the cell in the water column caused by, for example, the cell
being tail-heavy, physiological mechanisms predict the existence of an active gravirecep-
tor. To summarize the results from decades of experimentation—some of which will be
mentioned below—a physiological gravity signal transduction pathway exists in unicel-
lular systems and, thus, the existence of a gravity-sensing mechanism can be predicted in
free-swimming organisms. For example, after immobilizing Parameciumcells (Figure
7.1), Kuznicki (1968) observed not only sedimentation of the cells but also their variable
orientation, in contradiction to a purely physically determined mechanism (buoyancy
principle) of gravitaxis (Fukui and Asai 1985).
In contrast to the results obtained from Ceratodonand Loxodes, increasing the density
of the medium impaired graviorientation of Euglena(Figure 7.1) (density 1.046 g/ml_1)
and Paramecium(density 1.054 g/ml_1). The capacity for orientation was completely dis-
turbed under isodensity conditions. In addition to indications for a physiologically guided
mechanism of graviperception, these results support the hypothesis of protoplast-based
graviperception in Parameciumand Euglena. It was speculated that membrane-located
gravisensors are early inventions of evolution, whereas the Müller bodies of Loxodesare
later acquisitions due to adaptation to special living conditions (e.g., living in the sedi-
ment of lakes).
7.10 Graviperception in the statolith-based systems of Chara
For graviperception to occur in characean rhizoids, statoliths must be fully settled on the
gravisensitive subapical plasma membrane area (Figures 7.2 and 7.4; see also Braun
2002). Recent experiments that have been performed during parabolic flights of the A300
Zero-G aircraft elucidated the mode of gravireceptor activation in characean rhizoids
(Limbach et al. 2005). The final curvature angles of flight samples, which experienced
several short phases of microgravity, were compared to controls in a 1 greference cen-
trifuge which had experienced the same conditions of the flight, except for the excursions
of microgravity. The data revealed that statoliths, which were weightless but still in
contact with the plasma membrane during the microgravity phases, still activated the
membrane-bound gravireceptor. It therefore could be ruled out that the pressure exerted
by the weight of statoliths is required for gravireceptor activation. This finding was sup-
ported by control experiments on the ground which demonstrated that increasing the