Crossover in Sordaria
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Fungi grow as filaments called hyphae. A
mass of hyphae is called a mycelium.
Fungi are either parasites or saprobes,
absorbing the breakdown products from the
action of digestive enzymes they secrete.
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http://www.wisconline.com/objects/ViewObject.aspx?ID=BIO
304
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Fungi live their lives primarily as haploid
organisms, but they form diploid structures
for sexual reproduction in 3 stages:
1. Plasmogamy—the fusing of cells from
two different fungal strains to produce a
single cell with nuclei from both strains.
A pair of haploid nuclei,
one from each strain is
called a dikaryon.
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2. Karyogamy is the fusing of the two
haploid nuclei of a dikaryon to form a single
diploid nuclei.
3. Meiosis of the diploid nucleus restores
the haploid condition.
1.
2.
3.
4.
1. Hyphae from two strains fuse
2. Nuclei from one strain pairs
with nuclei from second
strain
3. The diploid nucleus
undergoes meiosis, producing
4 haploid cells
4. Each of the 4 haploid cells
undergoes mitosis to produce
8 spores.
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During meiosis I, homologous chromosomes
pair and separate.
During meiosis II, each chromosome
separates into two chromatids.
At the end of meiosis, there are 4 daughter
cells, each possessing one chromatid from
each tetrad of homologous chromosomes.
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Each of the 4 daughter cells divides by
mitosis to produce 2 ascospores. The order
of ascospores corresponds to the chromatids.
If no crossing over occurs, then each set of
four adjacent ascospores represents a single
parent strain and will possess the same traits.
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The results of crossing over during meiosis
can be readily visualized under the
microscope in the asci of many ascomycete
fungi.
Asci are hyphae
that contain eight
haploid ascospores.
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In Sordaria fungi, the asci are embedded in
other hyphae that form a fruiting body.
Below: Asci that resulted from
a cross between one black
and one tan strain. Note that
each contains some tan and
some black spores. 4:4
arrangement=no crossing
over.
Asci that resulted
from a cross
between two black
strains. All of the
spores are black.
Asci that resulted
from a cross between
two tan strains. All of
the spores are tan.
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http://www.phschool.com/science/biology_pl
ace/labbench/lab3/intro.html
To the right is an ascus
which has released its
ascospores. Starting at
12 o’clock, count the
number of ascospores
that are recombinants.
Pattern WITHOUT crossing over
Pattern with crossing over
http://histoiresnaturelles.net/vegetaux/microscopie/sordaria_bouquet.JPG
# of crossover asci
TOTAL # of asci
= % Asci showing Crossover
÷ 2 = Gene to centromere distance
(map units)
DRAW THE EVENTS LEADING UP TO A 2:4:2 ascospore
pattern
(HINT: Work backwards from the final spore pattern)
MEIOSIS I
MEIOSIS II
MITOSIS