Worm Breeder's Gazette 11(1): 64
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
As a joke, Stuart and I decided to put worms in an electric field. We did this by placing worms on an agarose mini-gel (0.3% in 1X M9 salts) with the running buffer just touching the sides of the gel. Interestingly, when we turned the power on, the worms rapidly moved toward the cathode. As a result of these observations, we have been following up this behavior in order to determine if it can tell us anything about worm biology. Galvanotaxis shows two distinct phases. The first occurs at relatively low voltage (~0.07 V/mm in M9 salts) and consists of animals moving somewhat haphazardly toward the cathode in a manner similar to that seen in chemotaxis assays. As the voltage increases, the response gets stronger until a new type of behavior is seen. At approximately 0.4 V/mm (in M9 salts) the worms move toward the cathode in a much more directed manner. They very quickly decide on the direction in which they are going and, once decided, rarely stop or change directions. Although the worms move in a straight line at the higher voltage, they do not move directly toward the cathode. Rather, worms initially placed in a single spot will move out of that spot in two streams, each of which makes an approximately 45 angle with a line between the initial spot and the cathode. An animal traveling along one of these angles will generally (but not always) have its ventral side closer to the cathode. Occasionally a worm will flip onto its other side causing it to change its angle of movement such that it now moves at an angle that is flipped relative to its starting angle (i.e. it makes a V-shaped pattern). The exact angle that a group of worms adopts is somewhat variable and appears to depend on the agarose concentration. Adult and L4 hermaphrodites as well as males show the strongest response. However, male worms tend to get distracted and make slower progress toward the cathode. L3 and L2 hermaphrodites show weaker responses in that younger animals spend a greater proportion of their time moving in a direction other than toward the cathode. L1 animals show little if any response to an electric field. Finally, dauer larvae tend to galvanotax fairly strongly over short periods of time. However, they often can be seen going in a direction other than toward the cathode. In order to determine if galvanotaxis might be a tractable genetic behavior, we have checked (and are continuing to check) existing mutations for the ability to galvanotax. These include a large number of unc, daf, che, lin, mec, and osm mutations. To date only three mutations appear to be defective (to varying extents) in galvanotaxis; lin-32(u282), 33), and n1754, a mutation isolated by L. Bloom as defective in thermal avoidance and also found to be chemotaxis deficient. All three of these mutations are quite pleiotropic in their defects and clearly are not specific for galvanotaxis. One possible explanation for this behavior could be that the worms are responding to a heat or salt gradient set up over time by the electric field. We do not believe this to be true for two reasons: (1) the response starts as soon as the power is turned on and stops immediately after the power is turned off, and (2) two mutations, n1937 and n1938, isolated by C. Bargmann as being completely chemotaxis deficient, are wild type for galvanotaxis. Therefore, it is likely that at least some components of galvanotaxis are distinct from chemotaxis. In order to find mutations that are specific for galvanotaxis, we have begun a mutant screen for animals that do not respond to an electric field. To date we have screened approximately 3000 haploid genomes without finding any galvanotaxis mutations. In order to try to increase the number of worms we can screen as well as make the assay easier to set up, we have adapted the assay to 9 cm tissue culture plates.