Passage I

Caenorhabditis elegans (roundworms about 1 mm long) are attracted to some chemical compounds. C. elegans can become desensitized to a compound, making them less likely to move toward that compound, if they are exposed to the compound over a period of time. Students conducted 2 experiments to study this process—called chemosensory adaptation—in C. elegans.

Experiment 1

To determine if C. elegans are attracted to Compounds X, Y, and Z, the students prepared 3 petri dishes (Dishes 1−3). On Dish 1, a drop of Compound X was placed on 1 edge of the dish, and a drop of Compound Y was placed on the opposite edge of the dish. A drop of Drug P, which temporarily paralyzes C. elegans upon contact, was placed next to each compound, and 100 C. elegans were placed in the middle of the dish (see Figure 1). This procedure was repeated for Dish 2, except with Compounds X and Z, and for Dish 3, except with Compounds Y and Z. The dishes were incubated for 1 hr. Table 1 shows, for each dish, the percent of C. elegans that moved toward each compound present.

Experiment 2

Six hundred C. elegans were divided into 3 equal groups. Each group was pre-exposed to Compound X for a different period of time: 0 min, 10 min, or 120 min. Then, each group was placed in the center of a separate dish that was identical to Dish 2 as prepared in Experiment 1. The dishes were incubated for 1 hr. Figure 2 shows the percent of C. elegans that moved toward each compound on each dish.