Exercise 4.1

Mutations Degrade Worm Behavior

(This exercise is based on Ajie, B. C., S. Estes, M. Lynch, and P. C. Phillips. 2005. Behavioral degradation under mutation accumulation in Caenorhabditis elegans. Genetics 170: 655–660.)

(Note: The reference above links directly to the article on the journal’s website. To access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

Mutations are a relatively rare phenomenon, and individual mutations often have rather small effects. For these reasons, measuring mutational effects on fitness and other phenotypic traits is a challenge. For more than 40 years, evolutionary biologists have used mutation accumulation (MA) studies to determine the effect of spontaneous mutations on some phenotypic character. In these MA experiments, mutations are allowed to accumulate in lines that are maintained for several to hundreds of generations. The large number of generations permits the researcher to detect relatively small effects. In these experiments, a small number of individuals (or sometimes a single individual) are randomly selected each generation to continue the line; this process ensures that the effects of natural selection are minimized, allowing mutation and random genetic drift to be the main evolutionary processes that alter the genetic composition of the lines.

The nematode worm Caenorhabditis elegans has several features that make it well suited for MA studies. Its small size and large number of progeny per individual make this worm easy to study in laboratory experiments in general. Because it has a short generation time (roughly 3 days), many generations can be produced in a fairly short period of time. These nematodes also can be frozen for storage and revived later; this permits one to measure ancestors and distant descendants at the same time and under the same set of conditions.

A team of researchers then at the University of Oregon investigated behavioral changes of worms that occur as mutations accumulate. Michael Lynch has been performing MA studies in a variety of organisms to examine mutational components of fitness and phenotypic traits. Patrick Phillips and his postdoctoral fellow Susanne Estes have been investigating the evolutionary genetics and chemosensory behavior of C. elegans. Beverly Ajie, at the time an undergraduate at the University of Oregon, examined the responses of worms from the MA lines to chemical stimuli, as part of her honors’ thesis.

In the MA experiment, a single, randomly-selected individual was permitted to start the next generation. One of the advantages of using C. elegans for mutation accumulation studies is that these worms are hermaphrodites, capable of fertilizing themselves.

The MA experiment started with 100 replicate lines; of these, 67 survived by the time of the behavioral study 370 generations later.

QUESTIONS

Question 1. What potential bias does this loss of lines introduce?

Figure 1 The responses of a representative worm from control lines (in panel A) and an individual from a mutation accumulation line that showed poor response to the repellent (in panel B). The asterisk shows the location of the repellent, and the arrow shows the final position of the worm. The numbers represent distance (in mm) on the grid; note the different scales.

Question 2. The behavioral assay in these experiments was the response to linoleic acid, a repellent chemical. In response to linoleic acid, non-mutant worms will steadily move away from the repellent. Describe the behavior of the worms shown in Figure 1. As measured by distance from the starting point (as the crow flies), about how far did the worms from the respective panels travel? Use the Pythagorean Theorem to calculate the distance.

Figure 2 The directness, velocity (in mm/s × 100), and turn rate (number per minute). The shaded bars represent the average of the unmutated control and the solid bars represent the average of the MA lines.

Question 3. To quantify the behavior of the worms, the researchers used three measures: directness, velocity, and turn rate. Directness is the beeline distance divided by the total path length. Velocity is the measured in millimeters per second. The turn rate is the number of turns of more than 90 degrees in a minute. Based on the data in Figure 2, describe the differences between the unmutated controls and the MA lines with respect to directness and velocity.

Question 4. Which turn more often, worms from the MA lines or worms from the unmutated lines?

Question 5. The mean directness values for the unmutated controls and for worms from the MA lines are 0.264 and 0.213 respectively. By what percent did mean directness decline during the 370 generations of mutation accumulation?

Question 6. By what percent did directness decline each generation on average?

Question 7. The mean turn rates of unmutated controls and of worms from the MA lines are 4.66 and 6.70 respectively. By what percent did turn rates increase each generation on average?

Figure 3 The average directness for individual lines: control lines are shaded and MA lines are solid.

Question 8. Based on the data in Figure 3, is the variance of MA lines greater, smaller, or about the same as the control lines?