Drosophila Evolution: changes in allelic frequency
“Two
years work wasted, I have been breeding those flies for all
that time and I've got nothing out of it.”
-Thomas Hunt Morgan
Objectives
After completing this exercise you should be able to:
1.
2.
3.
Track and display changes in allelic frequencies over time.
Determine relative fitness and explain how it impacts allelic frequency in populations.
Understand and describe Drosophila lifecycle and morphology.
Summary
This lab is based on an exercise published by Mark Salata. Darwin’s third postulate states
that: individuals with favorable variation (traits) are better equipped to survive and reproduce. By
definition then those with favorable traits are more fit. Those individuals with favorable traits
produce more progeny, and in turn those progeny produce yet more progeny than those with
unfavorable traits, which over time leads to the elimination of the unfavorable trait. In short we have
the ‘survival of the fittest’. This exercise will demonstrate how the frequency of an allele of lower
fitness decreases over time until it is finally eliminated from the gene pool.
Materials
Drosophila melanogaster - wildtype
Drosophila melanogaster - ebony
growth chambers
15 ml disposable conical tubes
Drosophila melanogaster food
Dry active yeast
2% calcium proprionate
Index cards
Paint brushes
Fly-nap
Stereoscope
Sharpie
Procedure
Label ports on growth chamber as: A1, A2, B1, B2, C1 and C2. Place conical tubes on ports B1C2. Label tubes for ports A1 and A2 with today’s date. Fill tubes A1 and A2 to approximately the 2 ml
mark with food, add 4-10 grains of yeast, and add water to the 2.5 ml mark. Attach tubes A1 and A2 to
their respective ports. Finally, add 21 ebony fruit flies and 9 wildtype fruit flies to the container.
On every Monday and Thursday after starting the population growth chamber a fresh food tube is
exchanged for an empty tube or a sample tube. The order of replacement is as follows: B1, B2, C1, C2,
A1, A2, etc.. This allows nineteen days for the original food tubes at A1 and A2 to give rise to a second
generation of flies.
On day nineteen the first sample tube (A1) will be removed and the adult flies discarded. The flies
that emerge from the sample tube over the next few weeks are identified by sex and phenotype. The flies
are discarded after each count. The same procedure is followed with each subsequent sample tube. Good
record keeping is essential during this process.
During week two you will determine the relative fitness of the wildtype and ebony fruit flies. Each
group will start cultures of all 4 permutations of wildtype and ebony flies. Use two virgin flies of each sex
to do the following crosses:
2 female wildtype x 2 male wildtype
2 female ebony x 2 male ebony
2 female ebony x 2 male wildtype
2 female wildtype x 2 male ebony
Discard the parents after one week. Determine the number, sex and phenotype (wt or ebony) of all
progeny during the following weeks. Fill in the accompanying chart in the Data Analysis section below.
Data Analysis
Determine the relative fitness of the ebony versus wildtype phenotype.
Each tube represents a three day period starting on day fifteen with tube A1. Determine the frequency of
each phenotype and plot it versus time. Using the relative fitness calculate the expected frequency of each
phenotype for each time period. Plot the expected frequency on the same graph as the observed
frequency.
To determine relative fitness fill in the chart below:
Females
Males
ebony
wildtype
ebony
wildtype
total offspring
Relative fitness is the ratio total offspring from ebony females (column 1) to total offspring from
wildtype females (column 2).
1.
2.
3.
Answer the following questions. (Think of better questions)
What is your starting frequency of phenotypes?
What are the expected phenotypes in the F1 and F2 generations (assuming the F1 could be
separated from the F2)?
What is the relative fitness of the ebony phenotype versus the wildtype phenotype?
Drosophila Fitness DataSheet:
A1 (9/3)
males wt:
Male ebony:
Female wt:
Female wt:
A2 (9/3)
males wt:
Male ebony:
Female wt:
Female wt:
B1 (9/8)
males wt:
Male ebony:
Female wt:
Female wt:
B2 (9/11)
males wt:
Male ebony:
Female wt:
Female wt:
C1 (9/15)
males wt:
Male ebony:
Female wt:
Female wt:
C2 (9/18)
males wt:
Male ebony:
Female wt:
Female wt:
A1 (9/22)
males wt:
Male ebony:
Female wt:
Female wt:
A1 (9/22)
males wt:
Male ebony:
Female wt:
Female wt:
A2 (9/25)
males wt:
Male ebony:
Female wt:
Female wt:
B1 (9/29)
males wt:
Male ebony:
Female wt:
Female wt:
B2 (10/2)
males wt:
Male ebony:
Female wt:
Female wt:
C1 (10/6)
males wt:
Male ebony:
Female wt:
Female wt:
C2 (10/9)
males wt:
Male ebony:
Female wt:
Female wt:
A1 (10/13)
males wt:
Male ebony:
Female wt:
Female wt:
A2 (10/16)
males wt:
Male ebony:
Female wt:
Female wt:
B1 (10/20)
males wt:
Male ebony:
Female wt:
Female wt:
B2 (10/23)
males wt:
Male ebony:
Female wt:
Female wt:
C1 (10/27)
males wt:
Male ebony:
Female wt:
Female wt:
C2 (10/30)
males wt:
Male ebony:
Female wt:
Female wt:
A1 (11/3)
males wt:
Male ebony:
Female wt:
Female wt:
A2 (11/6)
males wt:
Male ebony:
Female wt:
Female wt:
B1 (11/10)
males wt:
Male ebony:
Female wt:
Female wt:
B2 (11/13)
males wt:
Male ebony:
Female wt:
Female wt:
Notes:
count the dead flies the next day to determine initial frequency.
Use polypropylene containers with flat bottoms.