Description: Mutations are a relatively rare phenomenon, and individual mutation
ID: 260682 • Letter: D
Question
Description:
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.
Question 1. What potential bias does this loss of lines introduce? 20 ? 15 4 g10 2 15 -10 -5 0 51 3 Distance (mm) Figure 1 The responses of a representative worm from control lines (in panel A) and an individual from a mutation accumulation line that showed numbers represent distance (in mm) on the grid; note the different scales.Explanation / Answer
In the given graph (A) the representative worm from control line (that is without mutation) showed greater response to repellent in comparison with the mutation line. In both the graphs, there is difference in the scale which clearly shows that normal worm (from control) move away from the repellent which is not for mutated worm and hence survival of mutated worms in presence of repellent is less due to which only 67 survived out of 100 which shows normal worms are favoured (by natural selection) over mutated worms.