MITOSIS AND MEIOSIS LAB Sordaria / Crossing-Over

MITOSIS AND MEIOSIS LABSordaria / Crossing-Over Refer to Chapter 31 in the textbook for additional information (Fungus)

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 Tan Hyphae   Hyphae 2n “n” “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 (seen below) which would be air-carried to new locations where they would germinate and divide by mitosis to produce new hyphae. Sordaria sp. Perithecium You can view the asci by taking a perithecium from a culture and placing it in a drop of water on a slide. If you gently press on a coverslip covering the drop, the perithecium will burst open and release the asci.

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.

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.

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

Other Slides That Might Help

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.

Possible Arrangements of Ascospores Ascus (pl. asci)  Ascospore  4:4 2:2:2:2 2:4:2

MITOSIS AND MEIOSIS LAB Sordaria / Crossing-Over