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What are you viewing?

Sordaria fimicola is an ascomycete fungus growing in a petri dish
Sordaria grows in its food
How do you think it obtains its nutrients?

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Sordaria fimicola

Sordaria fimicola is an ascomycete fungus
Sordaria is a haploid (n) organism for most of its life cycle.
When the mycelium from two individuals meet, a diploid zygote (2n) is formed.
The diploid zygote then undergoes meiosis to yield 8 haploid ascospores. These ascospores exist in a narrow pod called an ascus (plural, asci). Many asci will grow together forming a reproductive structure called a perithecium.
When ascospores are mature the ascus ruptures, releasing the ascospores. Each ascospore can develop into a new haploid fungus.

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Sordaria fimicola Life Cycle

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Mycelium (whole) & Hyphae (part)

Mycelium is the vegetative part of a fungus, consisting of a mass of branching thread-like hyphae

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Crossing-over

Exchange of genetic material between homologous chromosomes

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Crossing-over in Sordaria sp.

To observe crossing over in Sordaria:
make hybrids between wild-type & mutant strains

Wild-type Sordaria:
black spores (t+)

Mutant strain:
tan spores (t)

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Crossing-over in Sordaria sp.

When mycelia of these two different strains come together and undergo meiosis, the asci that develop will contain four black ascospores and four tan ascospores.

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Crossing-over in Sordaria sp.

The arrangement of the spores directly reflects whether or not crossing over has occurred

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Preparing the crosses

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 to the right. You can see the perithecia forming in the picture of the Petri plate; they are the dark line down the center of the plate.

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Procedure for counting the spores

Use a scalpel to gently scrape the surface of the nutrient medium where the two strains intersect to collect perithecia. At the intersection of the two strains is the region to harvest the perithecia.

Place the perithecia in a drop of water on a slide. Cover with a coverslip and return to your work area. Using the eraser end of a pencil (or a toothpick), press down the coverslip gently so that the perithecia rupture but the ascospores remain in the asci.

View your slide using the 10X objective and locate a group of hybrid asci

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Counting ascospores

Count at least 100 hybrid asci and enter your data in Table 1
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.

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Counting ascospores

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Collect data and record your results

Temperature

# of asci �not showing �crossing over�MI Asci

# of asci �showing �crossing over�MII Asci

Total

%MII Asci

Gene to �Centromere Distance�%MII/2

Group1

Group2

Group3

Group4

Group5

Group6