Transcript Sordaria Lab

MITOSIS AND MEIOSIS LAB
Sordaria / Crossing-Over
Sordaria fimicola is an ascomycete fungus, a division of fungi, that reproduce
by producing haploid spores that are contained in an ascus. It is this property
that makes it excellent material in which to study the results of crossing over.
Strains of Sordaria that produce black and tan spores can be purchased.
The strains can be used to inoculate a Petri plate containing an agar.
At first, the strain grows as a mycelial mat and then it produces fruiting
(reproductive) bodies.
Hyphae growing from the points of the inoculation are seen in the
magnified view of the agar surface to the right.
If a strain producing tan spores is inoculated on one half of the plate and a strain
producing black spores is placed on the other half, hyphae grow from both points
and eventually meet at the center of the plate where they fuse in the equivalent
of mating. Since the hyphae of both strains are haploid, the fusion product is diploid.
The diploid hyphae start to differentiate into a fruiting body called a perithecium as
seen below/right. You can see the perithecia forming in the first picture of the Petri plate;
they are the dark line down the center of the plate.
Perithecia 
Black
Hyphae 
“n”
2n
Tan
 Hyphae
“n”
In the perithecium, diploid cells divide
first by meiosis and then by mitosis to
produce 8 haploid spores.
The spores are contained in a
translucent saclike structure called an
ascus (pl. asci).
To the right , you can see a ruptured
mature perithecium releasing several
asci. Normally the asci would break
open and release haploid spores which
would be air-carried to new
locations where they would germinate
and divide by mitosis to produce new
hyphae.
Sordaria sp. Perithecium
ANALYSIS OF SINGLE MEIOSES
meiosis & post-meiotic mitosis in linear tetrad / octad
To the right is a photo of asci that resulted
from a cross between two black strains.
All of the spores are black.
To the right is a photo of asci that resulted
from a cross between two tan strains.
All of the spores are tan.
Four black ascospores in a row next to four tan ascospores in a row indicates
that crossing over has NOT occurred. Any other arrangement of ascospores
indicates that crossing over has taken place.
Below is a photo of asci that resulted from a cross between black and tan strains.
Look at a single ascus and note that it contains both black and tan spores.
Those on which the pattern of spore distribution in the ascus is 4 tan to 4 black were
produced from cells in which no crossing over occurred. Such asci are called
non-recombinants. Other asci contain black and tan spores that are distributed in 2:4:2
patterns or 2:2:2:2 patterns. These asci only result from cells in which crossing over
has occurred and are called recombinants. Because the recombinant patterns result
only from crossing over, the frequency of occurrence of recombinants is a measure of
how often crossing over occurs.
Possible Arrangements of Ascospores
Ascospore 
4:4
2:2:2:2
2:4:2
Your task is to look at the asci in the following photos and to count the number
of asci that show the recombinant and non-recombinant patterns.
You should count 100 asci and then calculate the percentage of
crossing over that occurs.
Below are asci that have been expelled from a perithecium.
View it as clock face and starting at 12 o'clock, count the asci that
are the non-recombinant or recombinant type.
Photo 1
Below are asci that have been expelled from a perithecium.
View it as clock face and starting at 12 o'clock, count the asci that are the
non-recombinant or recombinant type.
Photo 2
Below are asci that have been expelled from a perithecium.
View it as clock face and starting at 12 o'clock, count the asci that are the
non-recombinant or recombinant type.
Photo 3
Other Slides That Might Help
Establishing Map Distance Between the Centromere and a Linked Locus
Genetic linkage maps are give distances
between loci based on how often recombination
occurs between linked genes.
One map unit (mu or cM) is equal to the
distance between loci that causes 1% of
gametes to have a recombinant
chromosome.
Establishing Map Distance Between the Centromere and a Linked Locus
An example:
You observe 150 octads and discover 30 of
these are recombinant.
Map distance between the centromere
and the locus (spore color locus in our
case) =
mu = (½ x 30) x 100 = 15 x 100 =
10
150
150
From the crossing over data
you gather for Sordaria, you
will be able to calculate the
map distance between the
gene for spore color and the
centromere. To find the
number of map units, you
divide the percent of
crossovers by 2. (In an actual
lab, you should count at least
50 asci before calculating map
units.)
Why divide by two?
Each crossover produces two spores
like the parents and two spores that
are a result of the crossover. Thus, to
determine the number of crossovers,
you must divide the number of asci
counted by two since only half the
spores in each ascus result from
crossing over.
Calculate the Map Units
Number
of 4:4
Asci
Number of
Showing
Crossover
Total
Asci
% Asci
showing
crossing
over, divided
by 2
Gene to
Centromere
Distance (map
units)
• Calculate the percent of crossovers by dividing the
number of crossover asci (2:2:2:2 or 2:4:2) by the
total number of asci x 100.
• Calculate the map distance, divide the percent of
crossover asci by 2.
• % of crossover asci is divided by 2 because only
half of the spores in each ascus are the result of
crossing over.