NAME
DATE
New Host, New Species? Alleles
Guiding Question
Apples were brought to North America, about 400 years ago. Sometime around 1850, some
Rhagoletis flies moved from living on their native hawthorn fruit to living on apples. Is the population
of hawthorn flies living on the apples becoming a new species?
Background
If flies from the apple and hawthorn populations are freely interbreeding with one another, then they
would be considered the same species. One way we can tell if interbreeding is happening or not
is to see if alleles (different versions of the same genes) in the populations are present in similar or
different frequencies.
Group 1 and 3 are NOT Interbreeding
Group 1 and 2 are Interbreeding
GROUP 1
gene X
Allele D
GROUP 1
GROUP 2
Allele A
Allele C
gene X
Allele D
gene X
Allele A
Allele B
If two groups are interbreeding freely, then
If two groups are interbreeding freely, then alleles from the
alleles from the parents will be mixed together
parents are mixed together in the offspring. We would expect
in the offspring. We would expect to see
to see the same alleles in both populations at about the same
alleles in both populations at about the same
frequency.
frequency.
Experiment 1
Allele A
Allele C
Allele C
Allele B
Allele D
GROUP 3
Allele E
Allele D
gene X
Allele A
Allele C
Allele B
Allele B
If the two groups are different species, then their
If two groups are not interbreeding freely, then their alleles are
alleles should not be mixing. Since natural selection
not mixing in offspring. As natural selection causes reproduccauses species to become more genetically different
tively isolated groups to become more genetically different over
over time, we would expect allele frequencies to be
time, the allele frequencies shift differently in the two groups.
different between the two populations. In some
Often, one group has alleles that another lacks completely.
cases, one group may even have alleles that the
other lacks completely.
Research question: Do fly populations from apple vs. hawthorn fruit have different allele
frequencies for some genes?
Procedure
1. Collect several hundred flies from hawthorn and apple fruit.
2. Isolated the flies’ DNA. For several genes, determine what alleles each individual fly has.
3. For each population, calculated the allele frequencies: For a certain gene, what percentage of the
alleles are allele A vs. allele B, etc.
Results are summarized in the tables on the next page.
© 2016 University of Utah
Updated December 8, 2017 1
B
NAME
DATE
DATE
5 89
D1 5
1Hawthorn
5 5 89
4 10 5 81
A Apple
B C
Allele
D
C
A B
© 2016 University of Utah
© 2016 University of Utah
© 2016 University of Utah
Hawthorn 4 10 5 81
Apple
Allele
62 38
62 38
Apple
Apple
A B
A B
Allele
Allele
Frequencies
Allele
Frequencies
(%):(%):
GENE
GENE
2 2
10 30
Hawthorn
10 50
30 10
50 10
Hawthorn
30 69
B
C 1
C
Hawthorn 20 79 1
Apple
30 69
1 1
20 79
Hawthorn
AppleA
Allele
A B
Allele
Allele Frequencies
(%):
Allele Frequencies (%):
GENE 4
Hawthorn 34 66
GENE 4
Hawthorn Population Flies
D
25 60 10 5
GENE 3
Allele Frequencies (%):
Apple Population Flies
C
25 60 10 5
Apple
Apple
D
Allele
C
A B
A B
Allele
Allele Frequencies (%):
Hawthorn 34 66
Hawthorn Population Flies
EXAMPLE GENE
Allele Frequencies (%):
EXAMPLE GENE
Hawthorn 9 48 35 8
Allele Frequencies (%): Allele
GENE 3
D
9 19 57 15
D
9 19 57 15
Apple
C
C
Allele
A B
Hawthorn 9 48 35 8
Apple
A B
Allele
Apple Population Flies
Apple Population
Hawthorn
Population
Apple Population
Flies FliesHawthorn
Population
Flies Flies
Allele Frequencies
(%):
Allele Frequencies
(%):
GENE 1 GENE 1
Use a different
color forcolor
eachfor
allele.
corresponding
alleleallele
colorcolor
Use a different
eachMark
allele.the
Mark
the corresponding
in the circle
on
the
bottom
of
the
table.
in the circle on the bottom of the table.
Each pie Each
wedge
10%. 10%.
pierepresents
wedge represents
Use the data
Experiment
1 (provided
below)below)
to fill in
Use from
the data
from Experiment
1 (provided
to the
fill inpie
thecharts.
pie charts.
Color in the
allele
frequencies
in
both
fly
populations
for
each
gene.
Color in the allele frequencies in both fly populations for each gene.
Instructions
Instructions
Worksheet
Worksheet
Allele
Frequency
PiePie
Charts
Allele
Frequency
Charts
NAME
(data based on Feder et al, 1990)
Hawthorn Population Flies
Hawthorn Population Flies
Hawthorn Population
Hawthorn
PopulationFlies
Flies
Hawthorn Population Flies
Hawthorn Population Flies
New Host, New Species? Are the Alleles Mixing? 2
New Host, New Species? Alleles 2
(data based on Feder et al, 1990)
New Host, New Species? Are the Alleles Mixing? 2
Apple Population Flies
Apple Population Flies
Apple
PopulationFlies
Flies
Apple
Population
Apple Population Flies
Apple Population Flies
NAME
DATE
Questions
1. Using the data in the tables on the Allele Frequency Pie Charts (page 2), color in the allele
frequencies on the pie charts.
2. In one sentence, summarize the results.
Experiment 2
In order for flies to mate and lay eggs, their life cycle
must be timed so that they become adults when their
host fruit is ripe. If they are too early or too late, they will
miss their window of opportunity.
Apples get ripe about 30 days earlier than hawthorn
fruit, and hawthorn flies vary in their emergence time.
Researchers hypothesized that, after apples were
introduced, hawthorn flies that became adults earlier
were more likely to mate and lay eggs on apples.
Adults mate
Females
lay eggs
Larvae eat
fruit
Adult flies seek
out ripe host fruit.
Fruit falls
Their logic looked like this:
•
The timing of events in a fly’s life cycle (like when
adults emerge from the ground) is influenced by
genes.
Adults emerge in
the spring
Pupae form
underground
•
Variations in these genes—in other words, alleles—
may cause differences in life cycle timing.
•
Alleles associated with an earlier emergence and mating time might have been acted upon by
natural selection in the population of flies that moved from hawthorns to apples.
Rhagoletis life cycle
Research question: Are any alleles in hawthorn flies associated with differences in life cycle
timing?
Procedure
1. Collect fly larvae from hawthorn fruit. Bring them to the lab and wait for them to become pupae.
2. To mimic winter, keep the pupae cold for several months. To mimic spring, warm them.
3. Wait for adults to begin to emerge.
4. Every day, collect the newly emerged adults.
5. Test the flies’ DNA to see what alleles they have for genes 1–4, and calculate allele frequencies
(for a certain gene, the percent of alleles that are A vs. B, etc.)
© 2016 University of Utah
New Host, New Species? Alleles 3
NAME
DATE
Emergence Time vs. Allele Frequency for Gene 2
(Data based on Feder et al, 1993):
Days from
warming to adults
emerging*
Frequency
of allele A
(percent)
50
91
60
64
70
42
80
23
90
15
* Adult flies normally emerge from the ground at different
times.
Questions
3. Use the data from the table to fill in
the bar graph on the right.
Frequency of Allele A in Newly Emerged Adults (%)
Results for Gene 2, Allele A
4. In one sentence, summarize the results.
100
90
80
70
60
50
40
30
20
10
50
60
70
80
90
Number of Days from Warming to Emergence
5. Compare the results above with those for Gene 2 in Experiment 1. Do the data support the
researchers’ hypothesis that hawthorn flies that became adults earlier were more likely to mate
and lay eggs on apples? Explain.
6. Do you think that alleles are freely mixing between apple and hawthorn fly populations? Make a
claim, and support it with evidence and reasoning from experiment 1.
7. Do you think that different heritable traits are being selected for in the apple and hawthorn fly
populations? Make a claim, and support it with evidence and reasoning from experiments 1 & 2.
References
Feder, J.L., Hunt, T.A., & Bush, G. L. (1993). The effects of climate, host phenology and host fidelity on the genetics of apple and hawthorn infesting races
of Rhagoletis pomonella. Entomologia Experimentalis et Applicata, 69(2), 117-135. doi: 10.1111/j.1570-7458.1993.tb01735.x
Feder, J.L., Opp, S.B., Wlazlo, B., Reynolds, K., Go, W. & Spisak, S. (1994). Host fidelity is an effective premating barrier between sympatric races of the
apple maggot fly. Proceedings of the National Academy of Sciences of the United States of America, 91(17), 7990-7994.
Feder, J.L., Chilcote, C.A. & Bush, G.L. (1990). The geographic pattern of genetic differentiation be- tween host associated populations of Rhagoletis
pomonella (Diptera: tephritidae) in the eastern United States and Canada. Evolution, 44(3), 570-594.
© 2016 University of Utah
New Host, New Species? Alleles 4