CROSSING OVER IN Sordaria

By Mrs. Jones

MAIN MENU

• Intro to the fungi• Sordaria Life Cycle• Structures of Sordaria• Formation of Asci• Lab

Common Characteristics of Fungi

• eukaryotic• most are multicellular• Reproduce by spores

– Sexual or asexual

• heterotrophic– absorptive nutrition

Structure of a Fungus

• hyphae– mycelium (mass of hyphae)– Septa (cross walls)– chitin cell wall– haploid except during sexual reproduction

mycelium

hyphaesepta

Eubacteria

Archae

Eukaryotes

Viruses ?

Animals

Fungi

Plants

Algae

Protists

Chytridiomycota

Zygomycota

Basidiomycota

Ascomycota

CLASSIFICATION OF THE FUNGI

Ecological Impact of Fungi

• decompose wastes and dead organisms– return nutrients to the ecosystem

• Saprophytic – decomposers in a woodland– One year over a ton of debris falls onto an acre of woodland– return nutrients to the soil.

Pathogenic Fungivalley fever

Valley Fever disease of the

common in the southwestern United States and northwestern Mexico. caused by the fungus Coccidioides immitis,

which grows in soils fungal spores become airborne when the soil is disturbed infection occurs when a spore is inhaled Within the lung, the spore changes into a larger, multicellular structure called a spherule. The spherule grows and bursts, releasing endospores which develop into spherules..

athletes foot

a fungal infection.

lives on the skin and breeds under warm, moist conditions.

more common during hot weather

sweaty footwear is usually the culprit. Adult males suffer most often from this condition.

Pathogenic Fungi Cont.

– Powdery mildews that attack ornamental and food plants

– The chestnut blight, which in a few decades killed almost all of the mature American chestnut trees in the Appalachians of North America.

– The Dutch elm disease, which has killed many of the American elms in the United States.

Food Source• The truffle and the morel are both highly-prized food delicacies.

Truffles establish a symbiotic relationship with the roots of such trees

as oaks.

• Saccharomyces cerevisiae or budding yeast – Ferments sugar to ethanol and carbon dioxide and thus is used to make

alcoholic beverages like beer and wine – to make ethanol for industrial use – in baking (it is often called baker's yeast). Here, it is the carbon dioxide

that is wanted (to make bread and cakes "rise" and have a spongy texture).

Oregon White Truffle Tubermorel

Sordaria Life Cycle

Life Cycle of Sordaria

• The ascospore is haploid.• It divides by mitosis producing haploid

filaments• As it continues to divide by mitosis the

haploid mycelium grow inside the organism the fungus is digesting

Life Cycle Cont.

• The ends of two mycelia fuse.• The nuclei of two haploid cells join

(fertilization) forming a diploid zygote.• The zygote then divide by meiosis

producing 4 haploid cells.• The four cells divide by mitosis producing

8 haploid nuclei.

Life Cycle Cont.

• The 8 nuclei develop into ascospores.• The spores are discharged from the

perithecium and the cycle begins again.

Wild and tan strains being crossed on agar plate. Where the mycelia of the two strains meet fruiting bodies called perithecia develop.

Perithecium containing asci.

When the perithecia are crushed the asci can be seen.

Sordaria ascus (non hybrid)

Fertilization did not take place between different strains of fungi. If genes crossed over we cannot tell because all of the spores are the same color.

Asci containing black and tan ascospores.

Formation of asci

During prophase I of meiosis homologous chromosomes pair and form tetrads through the process of synapsis. While the four chromosomes are close together DNA can be exchanged between homologous chromatids.

Chiasmata (crossing over)

A Closer Look at Chiasmata

The + designate wild type genes (black)tn designates the mutant with tan spores.When no crossing over occurs the tan asci will all be together and the black asci will all be together.

Asci showing no crossing over.

Meiosis with Crossing Over

When crossing over occurs the ascospores will formone of the patterns above .Notice that only half of the chromosomes crossed over .

This means that half of the spores in the ascus are the result of crossover.

Ascospore showing crossover.

Mapping the Genes of Sordaria

• OBJECTIVES– Explain how meiosis and crossing-over result

in different arrangements of ascospores within the asci

– Calculate the map distance between a gene for ascospore color and the centromere of the chromosome on which the gene is found.

The frequency of crossing over appears to be governed largely by the distance between genes, or in this case, between the gene for spore coat color and the centromere. The probability of a crossover occurring between two particular genes on the same chromosome (linked genes) increases as the distance between those genes becomes larger. The frequency of crossover, therefore, appears to be directly proportional to the distance between genes.

Background

A map unit is an arbitrary unit of measure used to describe relative distances between linked genes. The number of map units between two genes or between a gene and and the centromere is equal to the percentage of the recombinants. Customary units cannot be used because we cannot directly visualize genes with the light microscope. However, due to the relationship between distance and crossover frequency, we may use the map unit.

Materials

• perithecia (paper bag) from a black X tan cross

• asci (paper strips) with eight dots representing ascospores

Procedure

• remove the asci form the bag and separate into hybrid and nonhybrid stacks.

• separate and count the MI and MII asci• do NOT count the nonhybrid asci• record your personal data in your lab book

and in the class data in the computer.

Calculate Map Distance (use the class data)

• divide the number of cross-over asci by the total number of asci and multiply by 100 to calculate the percent of cross-over

• divide the percent of cross-over by 2 to calculate the map distance

Questions

• Why are the nonhybrid asci not counted?• Why is the percent of cross-over divided

by 2?• Draw a pair of chromosomes in MI and in

MII, and show how you would get a 2:4:2 arrangement of ascospores by crossing over.

GLOSSARY

Heterotrophic –obtaining nutrients by eating other organisms or their byproducts.

Absorptive nutrition - obtaining nourishment from the surroundings (often having first digested it by secreting enzymes)

Dikaryotic – possess two haploid nuclei

Haploid – single chromosomes NOT pairs

Pathogenic – disease causing

Diploid – the presence of pairs of homologous chromosomes