Parasexuality - genetic analysis of nuclear genes without meiosis BIO341

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Parasexuality - genetic analysis of nuclear genes
without meiosisBIO341

• Meiotic analysis limited to those organisms that
  can be crossed sexually.
• Organism of interest may not reproduce sexually.
• Organisms of interest may be too distantly
  related to cross.
• Your concern may be with somatic cells.

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Stages of the parasexual cycle in Aspergillus

• 1. heterokaryosis
• 2. nuclear fusion - diploidy (1/106 to 1/107)
• 3. propagation of diploid
• 4. mitotic crossing over ( 2/103)
• 5. haploidization (1/103)

Note steps 2, 4, and 5 occur at low
frequency. Also, steps 4 and 5 are completely
independent of one another and occur by
completely different mechanisms.
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Example in Aspergillus nidulansw pro x y ad
paba
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5
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Example in Aspergillusw pro x y ad paba

• Pair strains on complex medium.
• Transfer from confluent zone of colonies to
  minimal medium.
• Yellow and white clusters of conidia mixed
  together are characteristic of heterkaryons.
• Each cluster is either yellow or white
  (indicating what?).
• Harvest conidia, plate on MM to find diploids.
  Conidia are uninucleate. Why is this important
  in this scheme?
• Diploids have green conidia.
• During growth colored sectors may be produced.
  The ploidy of these sectors can be inferred from
  conidial size.
• Segregants in following selected by color (From
  Pontecorvo and Kafer).

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Diploid formation
A
B
a
b
C
c
A
B
a
b
c
C
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Diploids (post chromatid replication)
A
B
a
b
c
C
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Diploid - metaphase plate
c
A
B
C
a
b
a
b
c
A
B
C
13
Normal chromosome segregation to form daughter
diploids
a
c
A
B
C
b
A
B
C
a
b
c
14
Abnormal chromosome segregation to form an
aneuploid
a
c
A
B
C
b
a
b
A
B
C
c
15
Aneuploid - metaphase plate
c
A
B
C
c
A
B
C
16
Another abnormal chromosome segregation to form a
haploid
C
c
A
B
C
A
B
c
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Summary of haploidization
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Mitotic crossing over
A
B
A
B
1
b
A
B
2
b
B
a
a
3
a
a
b
4
b
50 1,3 and 2,4 heterozygosity at A,
homozygosity at B 50 1,4 and 2,3
heterozygosity at both A and B
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Mapping chromosomes by mitotic crossing over
between a selectable marker and the
centromere. Plate on MM containing histidine,
proline, biotin and arginine, but no
adenine. Recover ade-suppressed, assay other
markers.
arg
pro
bio
Adesegregants 50 Prototrophic 200 Arg- 100
Arg-Bio- 150 Arg-Bio-Pro- What is map for Arg,
Bio, Pro, and the centromere?
ade
his
su
ade





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Parasexual cross w pro x y ad pb Below
are the phenotypes and ploidy of segregants from
this parasexual cross. Diploid 32 Prototrophic,
Yellow 78 Paba-, Yellow 56 Prototrophic,
White Haploid 19 Paba-, Ad-, Yellow 25 Pro-,
White 25 Paba-, Ad-, White