Behavioral Mechanisms of Chemotaxis

Gradient-stimulated modulation of the direction of movement (classical, narrow definition of chemotaxis) Amoebae (e.g., Dictyostdium discoideum), cells with amoeboid movement (e.g., tumor cells, white blood cells), spermatozoa (of, e.g., hydroids and tunicate) 5,6,7 

Gradient-stimulated modulation of the linear velocity followed by a directional change (phobic response) Bacteria (e.g., Rhodobactersphaeroides, Sinorhizobium melUoti), archaea (e.g., some cells of Halobacterium salmarium) 3 

Some generalizations can be made about all moleeular mechanisms of chemotajds. The first component of the signal-transduction pathway is a detector—a receptor that interacts with the stimulant. Often, the receptor is membrane-bound it senses changes in the eoncentration of 

Beyond this universal, general framework, the molecular mechanisms of chemotaxis involve both commonalities and diversities. Generally speaking, the molecular mechanisms of prokaryotes and eukaryotes are different. Within the prokaryotes, however, commonality seems to govern. Within the eukaryotes, both commonality and diversity appear to dominate (Table 3). 

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Table 2 summarizes the behavioral mechanisms of chemotaxis in the systems reviewed in this book. If these systems faithfully represent the mechanisms of chemotaxis in nature, it appears that prokaryotes mainly employ trial-and-error chemotaxis mechanisms— the phobic response and klinokinesis, defined in Table 2 and, in more detail, in Chapter 1. In contrast, eukaryotes appear to primarily employ the more direct mechanism—modulation of the direction of movement according to the stimulant gradient. A number of non-mutually-exclusive reasons may account for this difference. First, the mechanisms employed by bacteria... 

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