Meiosis and Genetic Diversity in the Model Organism, Sordaria.
By: Katie Fiorillo
TA: Lauren Smith
October 30, 2012
In this lab, the organism Sordaria fimicola was used to explore factors contributing to genetic evolution of sexual reproduction. These organisms are found in “Evolution Canyons,” which occur when two mountain slopes with very different climatic conditions congregate with a comparatively small valley between them. The organisms found on the south facing slope (SFS) slope of evolution canyon, which has harsher environmental conditions, have an increased mutation rate. This increased mutation rate may lead to inherited changes in mechanisms that control mutations and recombination.
There is a need to further explore this phenomenon in a controlled study, where different crossover frequencies of the NFS and SFS can be explored, as well as the different crossover frequencies of laboratory strain populations.
There are three types of recombination that can occur. A 4:4, 2:2:2:2, and 2:4:2. A 4:4 ascus occurs if there is no crossing over. However, if a 2:2:2:2 or a 2:4:2 ascus occurs, there was crossing over during the first stage of meiosis.
The overarching research question is “How does exposure to environmental stresses applied under controlled conditions change the crossover frequency in populations of Sordaria fimicola?”
My hypothesis is to observe an equal amount of all the different types of asci. There will be the same amount of recombinant asci as nonrecombinant asci, and the two different types of recombinant asci will have the same amount.
EXPERIMENT PART A
1. Record the two strains that your group is given on two culture plates by your TA. 2. Mark a mating agar plate cover with the date and your group name. On the bottom of the plate, draw a line dividing the plate into quarters. Using T for tan and W for Wild, mark two quadrants with the letter corresponding to each strain. Make sure that two quadrants of the same strain are not next to each other. 3. Set up a mating plate with Sordaria.
a. Wipe off the work surface with a disinfectant wipe
b. Disinfect the scalpel.
c. Cut two small squares of agar, approximately 0.5 cms across, containing fungal hyphae, from one of the culture plates. d. Lift up the lid of the plate on just that side and place the square, hyphae side down, on the agar near the edge of the plate on the quadrants labeled for the strain. e. Repeat for the second culture plate.
f. Disinfect the scalpel when finished.
g. Draw a diagram of the plate in lab notebook.
4. The plate will then be incubated for two weeks at room temperature.
EXPERIMENT PART B: MAKING SQUASHES
1. Draw and label the mating plate after it has been incubated for two weeks. 2. Label the area containing the mature perithecia and indicate the location on the plate where you will collect samples for examination of ascus types. 3. Using an inoculating loop, scrape some perithecia from the center of one of these lines and place them in a drop of water on a microscope slide. 4. Cover the slide with a coverslip. Using a pencil eraser, press down gently, as to release the asci from the perithecia without breaking the sacs and releasing the individual spores. 5. Using a compound microscope, locate the asci using the 100x magnification. Once located, observe under 400x magnification. Look for ascus sacs that contain more than one spore color. These were formed by the fusion of the two strains. 6. Draw a sample perithecium that displays numerous asci suitable for scoring the asci.
EXPERIMENT PART C: SCORING ASCI
1. Use the individual data table to score at least 20 asci in the appropriate columns. 2. Combine this data into the small group data table.
3. Compile class data.
Table 1: Individual Data...