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公開日:2008年10月 1日 多細胞生物の行動に関する研究



■連携機関 東北大学教養学部情報科学科菅原研究室

Effects of electrictaxis and phototaxis in Volvox carteri


The behavior of microorganism has been described under the influence of a single kind of stimulation. But, normally in nature the conditions are as a rule more complex than this; the animal is affected by several sets of stimuli at once. "What is the behavior under such conditions?" if the animal is exposed to two types of stimuli a and b at the same time, "Which types of stimuli deose the animal select to resonce?". Or will it, react in a new way, different from the usual reactions to either a or b.

We would like to consider this responces of livning creatures from evolutionally primitive organisms. Volvox is a primitive multicellular organism evolved from single-cell organism, cryptomonas. The cells of Volvox carteri algal swim toward a light source or away from it. The direction of algal phototaxis is revesed by enbironmental factors. Halldal([8], [9]) found that the sighn of phototaxis in Platymonas could be controlled by changing the concenrations of magnesium, calcium and potasium ions in the medium. Further study of modification of the tactic sign by external ions, pH and chemicals was studied by Sakaguchi([4]). In the case of single-cell organism, cryptomonas, study of effect of chemical environment on phototaxis have been perfomed by Kaneda and Furuya([7]).

The sign of phototaxis in Volvox carteri is temperature-dependent; positive at room temperature and negative at low temperature([3]). As multile stimuli, when two factors in environment, ion concentration and temperature were changed, it was found that an increase in the potassium or hydrogen ion concentration raised the reversal temperature of the sign. Sakaguchi and Kozo([3]) concluded that the sign of phototaxis was determined by membrane polarization; on depolarization of the membrane the sign of phototaxis changes from positive to negative.

On the other hand, from celluler point of view, Volvox carteri is a spherical multicellular alga with many features that recommend it as a model for studying the process of cytodifferentiation ([2]) and the early development of photoreception in eucaryotes. Individuals of this species contain only two distinct cell types, 16 large reproductive cells (gonidia) and from 2000 to 4000 somatic cells that cannot divide. The somatic cells are arranged in a single layer at the surface of the transparent sphere, whereas the 16 gonidia are located below the surface, where they have no direct contact with the external medium. All somatic cells are flagellated and possess eyes, and they are responsible for guiding the colony to places of light conditions that are optimal for photosynthetic growth([5]).

The orientation of the individual somatic cells within the spheroid, combined with the three-dimensional pattern in which their flagella beat, cause the spheroid to rotate in a counterclockwise direction. The two flagella of each cell beat synchronously and in an almost precisely parallel fashion. The flagella of all cells beat toward the posterior of the spheroid and slightly to the right, causing the spheroid to rotate to the left as it moves foreward ([?, ?] [11]). Whether cells accelerate or decelerate in response to on and off stimuli depends on the light intensity, its illumination history and other environmental factors. Thus, in other words, colonial algae orient in light by a complex differential response of the cells at different sides of the colony and not by a differential response of the two flagella in an individual cell. Because algal colonies rotate more slowly than single-cell species, light-mediated signaling in an alga that exists in colonies is also expected to be slower than signaling in a single-celled alga.

Figure 1: Schematic picture of the system

Negative electrotaxis has been known for Paramecium([1]). organisms orient themselves with respect to the direction of the direction of current and moves toward negative electrode. Volvox carteri also have electrotaxis. In the present study, as a single stimuli, we first examined the phototactic sign of Volvox as a function of light intensity. Secondly, electric-field was applied, and the responce of the swimming cells was studied.

In order to answer the question, "Which types of stimuli deose the animal select to resonce?", photostimili and electric-stimili are applied at the same time to solution of Volvox carteri perpendicularly, and the responce of the swimming cells was discussed.


  • [1] Jennings HS, Behavior of the Lower Organisms (Indiana University Press, Bloomington, 1906)
  • [2] D. L. Kirk and J. F. Harper, Int. Rev. Cytol. 99 217 (1986)
  • [3] H. Sakaguchi and K. Iwasa, Plant and Cell Physiol. 20, 909 (1979)
  • [4] H. Sakaguchi, Plant and Cell Physiol. 20, 1643 (1979)
  • [5] F. Braun and P. Hegemann, Biophysical Journal 76 1668 (1999)
  • [6] E. Ebnet, M. Fischer, W. Deininger, and P. Hegemann, The Plant Cell 11 1473 (1999)
  • [7] H. Kaneda and M. Furuya, Plant and Cell Physiol 23 1377 (1982)
  • [8] P. Halldal, Nature 179 215 (1957)
  • [9] P. Halldal, Physiol. Plant 12 742 (1959)
  • [10] J. H. Hoops, J. Cell Sci. 104 105 (1993)
  • [11] J. H. Hoops, 1997. . Protoplasma. 199 99 (1997)



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