We will study the change in frequency of an allele for eye color in replicated small populations of the fruit fly Drosophila melanogaster. Each student will have their own deme of flies that will be independent of all the other demes in the entire ensemble of demes (i.e., a metapopulation with no gene flow). You will be responsible for the maintenance of these fly-vial demes over a few generations. At each generation you will record the frequencies of eye phenotypes and convert these to genotype and allele frequencies. By putting together the data from each studentÕs vial, we use our collective metapopulation to demonstrate the changes in allele frequencies that are due to the combined action of natural selection and genetic drift. Your vial of flies contains 6 white-eyed females (homozygous w/w at the white locus) who have been mated to white eyed males (the females are carrying sperm), plus 4 red-eyed females (homozygous wild-type = w+/w+) who have been mated to wild-type males. These 10 females have been laying eggs in the food (corn meal, sugar, yeast, agar). The flies were added to the vials on 9/5/97, and have about a two week generation time. Follow these maintenance instructions carefully: To keep your flies happy, leave them on a shelf in a look-warm part of your room (ideally 25û C or 77û F). If they are too hot, they may dry out, and if too cold they will develop slowly.
| Date | Do this |
| 9/12 | Shake the adults into a glass of water to get rid of them (don't just release them into your room). This "clears" the adults so they will not be combined with the offspring when the latter hatch on 9/17 - 9/19. |
| 9/19 | Transfer all newly hatched adults to a new vial of food (this will be provided). To do this without losing all your flies, 1) take the cotton plug out of the fresh vial and, with one hand, hold it upside down over the old vial (you will be holding both the old and the new vial with one hand). 2) Gently tap the vials on your desktop to force the live flies down to the bottom of the old vial. 3) With your free hand, pull the cotton plug out of the old vial and then close the new vial down on the opening of the old vial. 4) Turn your hand over so the new vial is now below the old vial and gently tap the vials so all the flies fall into the new vial. 5) Put the new cotton plug into the new vial. |
| 9/22 | Transfer all adults to an empty vial (this will be provided) and put it in a freezer to kill the adults. The adults will have laid eggs for 4 days to start the new generation, so keep the new vial at room temperature. After the adults are dead (>1 hour to be certain), pour the flies out onto a sheet of white paper. Count the number of red- and white-eyed flies. ¥Number of red-eyed flies ______ Number of white-eyed flies ______ |
Questions for Generation # 1
1. What is the allele frequency in the vial at the start (in the 10 females; ignore the sperm for now)? Red: f(w+) = p = _________ White: f(w) = q = __________
2. What is the allele frequency in the sample of adults that hatched from your original vial? Red: f(w+) = p = _________ White: f(w) = q = __________
3. Estimate the relative fitnesses of red-eyed and white-eyed flies. (The selection model is different than presented in class since this first generation is not in Hardy-Weinberg. For simplicity, use a ratio of frequencies: p(after) / p(before) to get absolute frequencies, then compute relative frequencies).
4. We defined fitness as Wii = lii ¥ mii. To which component of fitness do you attribute the majority of change in frequency of eye color? ______ Why ? _____________________________________ _____________________________________________________________________
5. Using the variation in frequency among all vials in the class, calculate the effective population size (Ne) for this population (the equation is in the course packet) ---__________.
10/6 Transfer the newly hatched adults to a new vial of food (this will be provided).
10/9 Transfer the adults to an empty vial and put this in a freezer. After they are dead, pour them out onto a sheet of white paper and count the number of red- and white-eye flies. ¥Number of red-eyed flies ______ Number of white-eyed flies _______
NOTE: random mating does not occur until after the new crop of adults has hatched. NOTE: There are two important facts regarding these alleles for eye color at the white locus. 1) red (w+) is dominant to white (w) and 2) the white locus is X-linked (it is on the X-chromosome). What effect will this have? (remember, males are hemizygous at the white locus).
Questions for Generation #2
6. Using the observed numbers of phenotypes, what is the inferred frequency of the w+ and w alleles in the 10/9 sample? Remember that the white locus is X-linked; you may assume that the sex ratio is 1:1. f(w+) = p = _________ f(w) = q = __________
7. What is the expected frequency of the genotypes in the 10/9 sample, assuming random mating? f(w+/w+) = _________ f(w+/w) = __________ f(w/w) = __________
7. What are the expected frequencies of white and red flies in the 10/9 sample, assuming random mating? red = ______ white = ______
10. Using the variation in allele frequency among all vials in the class, calculate the effective population size of the metapopulation.
11. Estimate the relative fitnesses of the different genotypes. Hint: Use the X-Linked Selection model in Populus to reconstruct your initial and final allele frequencies.