Name:_____________________________________________ Period:________
AP Biology Lab
Meiosis: Sordaria Activity
Today we will be observing crossing over in Sordaria. Sordaria is a type of fungus.
It lives most of its life in its haploid stage and when it comes time to reproduce, it
turns into its diploid stage. We will be looking at the spores within the asci which
were developed from meiosis. The ascus(i) are sacs which contain the spores.
Each spore develops into a new haploid fungus.
To observe crossing over in Sordaria, one must make hybrids between wild-type
and mutant strains of Sordaria. Wild-type (+) Sordaria have black spores.
One mutant strain has tan spores (tn). When the zygote from two different
strains undergo meiosis, the asci that develop will contain four black spores and
four tan spores. The arrangement of the spores directly reflects whether or not
crossing over has occurred. In Figure 2, no crossing over has occurred. Figure 3
shows the results of crossing over between the centromere of the chromosome
and the gene for ascospore color.
Figure 2
Two homologous chromosomes line up at metaphase I of meiosis. The two
chromatids of one chromosome each carry the gene for tan spore color (tn) and
the two chromatids of the other chromosome carry the gene for wild-type spore
color (+).
The meiosis 1 results in two cells each containing just one type of spore color gene
(either tan or wild-type). Therefore, segregation of these genes has occurred at
the first meiotic division.
The meiosis 2 results in four cells, each with the haploid number of chromosomes.
An additional mitotic division simply duplicates these cells, resulting in 8 spores.
They are arranged in the 4:4 pattern. See figure 2 above
In the next example, crossing over has occurred in the region between the gene
for spore color and the centromere. The homologous chromosomes separate
during meiosis 1.
This time, the meiosis 1 results in two cells, each containing both genes (1 tan, 1
wild-type); therefore, the genes for spore color have not yet segregated.
Meiosis 2 results in segregation of the two types of genes for spore color.
An additional mitotic division results in 8 spores arranged in the 2:2:2:2 or 2:4:2
pattern. See figure 3
Figure 3
Any one of these spore arrangements would indicate that crossing over has
occurred between the gene for spore coat color and the centromere.
Procedure:
1. Obtain 5 pictures of sordaria asci.
2. In the first picture, identify all the asci starting at the 12 o’clock position.
Note which have a 4:4 arrangement and which have had crossing over
(2:2:2:2 or 2:4:2) occur.
3. Tally the different asci of that picture and enter in the data table.
4. Repeat steps for the next 4 pictures.
5. To find the % of asci crossing over, take the number of crossovers (2:2:2:2
or 2:4:2) and divide by the number of total asci. Multiply by 100. Enter
results in the data table.
6. Find the average % of crossing over and the map distance for the 5
pictures. Record on last table. To find the map distance: Take the % asci
crossing over and divide by 2. It is divided by 2 because only half the spores
are a result of crossover. (extra mitotic division at end)
Data:
Picture
#
Number of
Asci showing
a 4:4 pattern
Number of
Asci Showing
Crossover
Total Asci
% Asci
Showing
Crossover
Averages for all 5 pictures:
% of Asci showing crossover
Gene to Centromere distance
(map units)
Questions:
1. What meiotic event can account for the difference in the hybrid asci pattern?
2. Published results indicate that the map distance from the centromere to the
gene for spore color in Sordaria fimicola is 26 map units, corresponding to a
crossover frequency of 52%. Calculate your percent error using equation
below. Is your distance accurate?
Actual value – Observed value x 100 = Percent Error
Actual value
3. How do the mechanisms of meiosis affect evolution?
4. What is the difference between somatic cells and gametes?
5. A gamete with a chromosome number of 8 is fertilized. What is its
chromosome number now?
6. A contains the following genes and the distance between those genes:
Gene A to B = 30 map units
Gene C to D = 45 map units
Gene A to D = 15 map units
Based on the map unit info place the genes on the chromosome below:
0 map units
50 map units