The chemotaxis index is a calculation of the deliberate movement of organisms toward a specific chemical relative to random movement. Use the formula below to calculate the chemotaxis index for both the wildtype and mutant C. elegans. Chemotaxis index: C_elegans on salt side / C_elegans on control side Wildtype chemotaxis index: Mutant chemotaxis index: If the index is positive, the organisms moved toward the chemical NaCl (salt) in this case and therefore showed chemotaxis. If the index is zero, there was nearly equal movement toward the chemical as away from it, so no chemotaxis was observed. A negative index means that the organisms actively moved away from the chemical, which was not part of this experiment. b) Based on the chemotaxis index, did the experiment answer the hypothesis that “C. elegans with a daf-18 mutation cannot learn to associate salt with food”? Briefly explain why you answered as you did in b): 2.a) What do you think would happen during the chemotaxis assay if C. elegans were fed without salt prior to the chemotaxis assay? b) Design a brief experiment to test this hypothesis:
The Correct Answer and Explanation is:
To calculate the chemotaxis index, you use the formula: Chemotaxis Index=C. elegans on salt sideC. elegans on control side\text{Chemotaxis Index} = \frac{\text{C. elegans on salt side}}{\text{C. elegans on control side}}Chemotaxis Index=C. elegans on control sideC. elegans on salt side
Wildtype chemotaxis index:
- C. elegans on salt side (wildtype): The number of wildtype organisms that moved toward the NaCl side of the dish.
- C. elegans on control side (wildtype): The number of wildtype organisms that moved toward the control side.
For wildtype C. elegans, you would calculate the index by dividing the number of organisms on the salt side by the number of organisms on the control side. Wildtype chemotaxis index=C. elegans on salt side (wildtype)C. elegans on control side (wildtype)\text{Wildtype chemotaxis index} = \frac{\text{C. elegans on salt side (wildtype)}}{\text{C. elegans on control side (wildtype)}}Wildtype chemotaxis index=C. elegans on control side (wildtype)C. elegans on salt side (wildtype)
If the chemotaxis index is positive, this indicates that the organisms moved toward the salt (NaCl), suggesting that they can detect and are attracted to the salt.
Mutant chemotaxis index:
- C. elegans on salt side (mutant): The number of mutant organisms that moved toward the NaCl side of the dish.
- C. elegans on control side (mutant): The number of mutant organisms that moved toward the control side.
You would calculate the index for the mutant in the same manner as for the wildtype: Mutant chemotaxis index=C. elegans on salt side (mutant)C. elegans on control side (mutant)\text{Mutant chemotaxis index} = \frac{\text{C. elegans on salt side (mutant)}}{\text{C. elegans on control side (mutant)}}Mutant chemotaxis index=C. elegans on control side (mutant)C. elegans on salt side (mutant)
b) Answering the hypothesis:
The hypothesis to test is: “C. elegans with a daf-18 mutation cannot learn to associate salt with food.”
- If the wildtype C. elegans show a positive chemotaxis index (attracted to salt), and the mutant has a chemotaxis index closer to zero or negative, it suggests that the daf-18 mutation might interfere with the ability to associate salt with food or detect it in the same way as the wildtype.
- A positive index in both wildtype and mutant C. elegans could suggest that the mutation does not affect chemotaxis, which would challenge the hypothesis.
- A negative or near-zero index for the mutant, however, supports the hypothesis that the mutation impairs learning or detection of salt.
Thus, if the mutant chemotaxis index is significantly lower than the wildtype, the experiment would likely support the hypothesis that “C. elegans with a daf-18 mutation cannot learn to associate salt with food.”
2a) Effect of feeding C. elegans without salt prior to the chemotaxis assay:
Feeding C. elegans without salt before the chemotaxis assay would likely reduce their ability to associate the presence of salt with a food source. Without exposure to salt as a cue for food, they may not show a strong preference for the salt side during the chemotaxis assay. This could result in a chemotaxis index closer to zero, implying a lack of chemotaxis, since the organism would have no prior experience with salt being a food-associated stimulus.
2b) Designing a brief experiment to test this hypothesis:
To test the hypothesis that feeding C. elegans without salt affects their chemotactic response to salt, the following experiment can be designed:
- Control group: Feed one group of C. elegans with salt present in their food.
- Experimental group: Feed a second group of C. elegans without salt in their food for a set period (e.g., 24 hours).
- Chemotaxis assay: After the feeding period, perform the chemotaxis assay for both groups by placing them in a petri dish with a gradient of NaCl on one side and a control substance (e.g., water) on the other.
- Measure chemotaxis index: Count the number of C. elegans on the salt side and on the control side for both groups and calculate the chemotaxis index.
By comparing the chemotaxis index of both groups, you can determine if feeding C. elegans without salt affects their movement toward salt in the assay. A lower chemotaxis index in the experimental group would support the hypothesis.
