BIOL 5240 SPRING 2005
ECOLOGICAL GENETICS
3/4/05 STUDY QUESTIONS:
RG Shaw, DL Byers, and E Darmo. 2000. Spontaneous mutational effects on
reproductive traits in Arabidopsis thaliana. Genetics 155:369-378.
This paper can get dense at some points. In particular, you may want to lightly read the
“analysis” section of MATERIALS AND METHODS. Here’s some guidance through that
section:
VL =
variance between lines. All lines were originally genetically identical. So,
the variance among lines at the beginning of the experiment should be
zero (VL0  0). However, as mutations accumulate and the lines drift
away from each other, the variance among lines should increase (0 < VL8
< VL17). VL includes all sources of variance (VMat, VE, and VM - described
below).
VMat = variance due to the maternal environment. For example, if a plant was
particularly good at providing nutrients to it seeds, all of those seeds might
have higher fitness. However, this would be an environmental effect, not
a genetic effect due to the accumulation of mutation.
VE =
variance due to general environmental effects (i.e. plants growing near a
window or fan or something). These effects also have nothing to do with
the accumulation of mutations.
VM = variance due to the accumulation of spontaneous mutation alone. This is
what we are really interested in.
rL and CovL = correlation and covariance among lines. Correlation and
covariance are closely related statistics. Covariance is in the units of
measurement and correlation is dimensionless. Both of them indicate the
extent to which mutations have similar effects in the different lines. For
example, if rL = 1, then mutation effected the lines all in exactly the same
way. If rL = 0, there is no similarity between how mutation effected the
lines.
Important things that you should pay attention to are the change in the means and
variances of the traits over time. You can see these patterns in Table 1 and Figs. 1 and 2.
Statistical tests that show whether significant variation was introduced to the lines by the
accumulation of spontaneous mutation are in Table 2.
QUICK QUIZ QUESTIONS
1. What is the goal of a mutation accumulation experiment? Why to you think it is
important to estimate the rate of spontaneous mutation?
2. Briefly describe the basic experimental design of a mutation accumulation
experiment.
3. What is the big advantage of doing a mutation accumulation experiment in plants
as opposed to flies or nematodes?
4. It is often assumed that most mutations have negative effects on fitness. If this
were true, how would you expect the mean of reproductive traits to change as
mutations accumulate? How does this expectation compare to the results of this
study?
5. As stated above, it is commonly held that mutations have negative effects on
fitness. If this were true, how would you expect the variance in reproductive traits
to change as mutations accumulate (i.e. what should the shape of the distribution
look like – you can draw a picture)? How does this expectation compare to the
results of this study?
6. How do the results of this study differ from mutation accumulation experiments
done on different organisms?