Gretchen Gurganus
Bio 110 Lab Section
Bradley Carlson
October 19, 11
Anomalous Combinations of Asci in Sordaria fimicola due to Meiosis
Introduction
Sordaria fimicola is a fungi that is a part of the Ascomycota phylum and is an
excellent example of how variations are produced in genetics. Organisms in this phylum
have a defining reproductive characteristic called an ascus. The ascus, contained in the
perithecium, is a sac-like cell containing 4 to 8 haploid spores. These spores are produced
through the process of meiosis to make 4 haploid spores. They then undergo mitosis to
make a complete 8 spore asci. When these spores are ready for dispersal, or when the
perithecium is ruptured, the asci (multiple ascus) are dispersed. The process of meiosis
and crossing over is displayed in the pattern of the spores in the asci. In the asci strains in
S.fimicola, there are two different colors of spores. “When scientists first crossed a wild
type strain of S.fimicola with a black spore coat with another strain with a tan spore coat
they expected to find asci containing 8 spores in just two combinations. They discovered,
in addition four, anomalous combinations” (Hass and Ward 2010). The expected
combinations were WWWWTTTT and TTTTWWWW, while the anomalous
combinations were WWTTWWTT, TTWWTTWW, TTWWWWTT, and WWTTTTWW
(See Figure 1). These spores showed different combinations at different frequencies. To
determine why these spores were in anomalous combinations and how often each of these
combinations occurred, we observed multiple asci that were crossed with wild and tan
strains and recorded the frequencies of each combination. Our class then predicted that
pre-mitotic mixing caused the anomalous combinations and each combination had equal
frequencies in this mixing. We also predicted that crossing over in the prophase stage of
meiosis 1 caused the combination of 4:4 to be more frequent, while the 2:2:2:2
combination and the 2:4:2 combination have equal frequencies. We began this lab by
splitting up a culture plate in four sections and making a cross between tan and wild
Sordaria. After the fungi had crossed (approx. 2 weeks), we used an inoculating loop to
scoop samples from the spots where tan and wild Sordaria had crossed and put them on a
microscope slide lubricated with a drop of water. After pressing down the slide to release
the asci, we observed multiple asci coming out of a single perithecium through a
microscope. Making sure each asci had both tan and black spores, we counted the
frequency of each 2:2:2:2, 2:4:2, and 4:4 combination and recorded the data.
Results
After finding a group of asci, we used 400x magnification to discover the types of
combinations in each ascus. As you can see from Figure 2, the individual data contained
9 asci with the 4:4 combination, 5 asci with the 2:2:2:2 combination, and 6 asci with the
2:4:2 combination. The frequency ratio of anomalous asci out of the total number of asci
is 11:20. Our group data (Figure 3) has 28 asci with the 4:4 combination, 14 asci with the
2:2:2:2 combination, and 22 asci with the 2:4:2 combination. Our frequency ratio was
36:64. As a whole class (Figure 4), we have 188 asci with the 4:4 combination, 120 asci
with the 2:2:2:2 combination, and 139 asci with the 2:4:2 combination. Our class
frequency is 259:447. Our class mapping distance was 28.9%.
Discussion
According to our findings as a class, 42% of asci were 4:4 combinations, 27%
were 2:2:2:2 combinations, and 29% were 2:4:2 combinations. As a result, 4:4
combinations were much more common than 2:2:2:2 combinations and 2:4:2
combinations. The 2:2:2:2 and 2:4:2 combination had about equal frequencies. This data
supported our class hypothesis of how anomalous combinations were formed by crossing
over. For instance, we predicted that the frequency of 4:4 combinations would have
occurred more than the other two, and that the other two would have had equal
frequencies. Our results reinforced our hypothesis by showing matches in our predictions
of frequencies. Our results also contradicted our other hypothesis of pre-mitotic mixing.
In this hypothesis, we predicted equal frequencies in all of the combinations. This wasn’t
true because 4:4 combinations occurred about 12% more frequently than the 2:2:2:2 and
2:4:2 combinations. The map distance of 28.9% also supported our hypothesis of crossing
over. 28.9% describes the percentage recombination between two genes in
correspondence with the centromere. “Frequency of crossover can be affected by location
(crossing over is repressed close to the centromere) and proximity to another crossover”
(“Introduction to Linkage Mapping”). Our 28.9% map distance location shows frequency
in crossing over. Even though we had substantial support for our hypothesis in this
experiment, there are always sources of error. Error could have occurred while the
smaller groups were compiling data into a whole class result. There could have been
miscalculations between the smaller groups that caused incorrect information for our data
as a class. Also, some groups may have been making faulty observations while looking at
asci because the tan and black spores looked very similar in color. In conclusion, this
experiment demonstrated how genetic variations occur in meiosis through crossing over.
It is important as biologists to recognize what genetic recombination are caused by and
how it affects the order of the spores. After performing this experiment, I have a much
better understanding of the effect of crossing over in meiosis in genes. It has lead me to
wonder if in the future we could manipulate the map distance of the centromere and
control the crossing over frequency. If we could do this, could we make an infinite
number of combinations through the process of meiosis? Overall, this experiment
furthered understanding of the effect of crossing over in meiosis. Species need genetic
variation to help them survive natural selection. “Therefore, genetic variation is
"insurance" for organisms against changing conditions because it helps to insure that
some of their offspring will survive if conditions change” (Cyr 2002). This process is
essential to life on earth.
Figures
Figure 1
4:4 Combination(Expected)
WWWWTTTT
TTTTWWWW
2:2:2:2 Combination(Anomalous)
WWTTWWTT
TTWWTTWW
2:4:2 Combination(Anomalous)
WWTTTTWW
TTWWWWTT
Figure 2
Individual Data
# of 4:4
# of
asci
2:2:2:2
asci
# of 2:4:2
asci
Total # of
asci
9
6
20
Group Data
# of 4:4
# of
asci
2:2:2:2
asci
# of 2:4:2
asci
Total # of
asci
28
22
64
5
Total #
Frequency
anomalous of
asci
anomalous
patterns
11
11:20
Figure 3
Figure 4
Class Data
14
Total #
Frequency
anomalous of
asci
anomalous
patterns
36
36:64
# of 4:4
asci
# of
2:2:2:2
asci
# of 2:4:2
asci
Total # of
asci
188
120
139
447
Total #
Frequency
anomalous of
asci
anomalous
patterns
259
259:447
References:
Cyr, R. 2002. Heredity and Life Cycles. In, Biology 110: Basic concepts and
biodiversity course website. Department of Biology, The Pennsylvania State
University. http://www.bio.psu.edu/
Fancher, Lynn 2000. “Introduction to Linkage Mapping.” Division of Natural Sciences,
College of DuPage.
http://www.cod.edu/people/faculty/fancher/genetics/LinkageMapping2.htm
Hass, C and Ward, A 2010. Meiosis and Genetic Diversity in the Model Organism,
Sordaria. Department of Biology, The Pennsylvania State University, University
Park, PA.