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Heterokaryosis
- co-existence in the same cytoplasm of 2 or
more genetically different nuclei*
*
a dikaryon is a specialized
heterokaryon, which has different
mating-type genes (idiomorphs).
397/254
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How do heterokaryons come into
existence?
1.
Mutation
2.
Anastomosis
3.
Spore formation*
4.
Diploidization of homozygous or
heterozygous nuclei
*
really an extension of 1 or 2 which
may be more significant because can yield
more obvious variant strain.(changes nuclear
ratios)
Note: Hyphal fungi are
products of all their nuclei (genetics of
hyphal fungi can sometimes be thought of in
terms of population genetics)
398/255a
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Parasexuality
Operational definition -
1.
Heterokaryon formation
2.
Fusion in the heterokaryon of unlike
somatic (non-dikaryotic) nuclei
3.
Mitotic crossing over (recombination)
4.
Haploidization*
*Expression or discovery only
apparent after spores --> colony.
(see pg. 229 of M-L for mechanisms
that might produce new phenotypes)
401/256
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Sexual
Mating-type
Regulation
1.
Homothallism = self fertility
Homothallic = Homomictic
a.
Self-fertile species found among all
divisions of fungi and fungal-like protists.
b.
Homothallic species co-exist with
heterothallic species among many genera.
Achlya,
Saprolegnia, Zygorhynchus, etc.
404/257
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2.
Heterothallism = self-sterility
Heterothallic = Heteromictic
a.
Dimictic = controlled by a single
pair of nonhomologous mating-type gene
sequences (idiomorphs) that reside at a
single chromosomal locus:
a/a, A1/A2,
A/a, +/-
b.
Diphoromictic = controlled by
multiple pairs of nonhomologous mating-type
gene sequences (idiomorphs) that reside at
either one or two chromosomal loci:
A1/A2/A3/A4/etc....
(Only in some (many) Basidiomycota
Models:
S. cerevisiae
}
N. crassa
}
heterothallic
S. commune
}
405/258
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*Diaphoromictic Systems
a.
Bipolarity = multiple nonhomologous
pairs of mating-type gene sequences (idiomorphs)
at one chromosomal locus
tetrad analysis =
2A12A2*
2a, 2a, etc.**
b.
Tetrapolarity = multiple
nonhomologous pairs of mating-type gene
sequences (idiomorphs) at 2 independently
assorting loci
A1
A2
A3
A4 A5 A6
....***
B1
B2
B3
B4
B5
B6....
* "most" basidiomycetes?
** a & a
= a1,2,3,
a1,2,3,
in some Ustilaginales
*** as per S. commune
406/259
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Tetrad Analysis of Tetrapolar
Species*
2A1B1
or
2A1B2
2A2B2
2A2B1
or
1A1B1,
1A1B2,
1A2B1,
1A2B2*
*involves a
single reciprocal crossover of 1 chromatid
(double crossover --> 2 types again)
Analysis of 4 basidiospores
from one basidium
407/260
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Homodimixis &
Homodiphoromixis
Homodimictic systems =
dimictic species that, as a consequence of
spore formation, appears homothallic*
Homodiphoromictic systems =
diphoromictic systems that, as a consequence
of spore formation, appear homothallic
* Strains derived from
uninucleate cells (e.g. often conidia) will
exhibit heterothallic condition.
414/262
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REVIEW
1.
Homothallic/Homomictic =
self-fertility
2.
Heterothallic/Heteromictic =
self-sterility
a.
Dimictic = self-sterility controlled
by two and only two idiomorphs
(a/a, A1/A2,
A/a, +/-)
b.
Diphoromictic = self-sterility
controlled by multiple idiomorphic
series.
(A1/A2/A3/A4/etc)
1.
bipolar = multiple idiomorphic series
at 1 locus
(tetrad analysis 2A1
2A2)
2.
tetrapolar = multiple idiomorphic
series at two unlinked loci (tetrad analysis
2A1B1
or
2A1B2
2A2B2
2A2B1
usual
1A1B1,
1A1B2,
1A2B1,
1A2B2*
infrequent
Also –
Homodimictic
Homodiphoromictic
417
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Plasmogamy
Terminology
1.
Planogametic copulation-mating
fusions involving at least one flagellated
cell*
a.
Isogamy - copulation between two
morphologically indistinguishable
flagellated cells
b.
Anisogamy - copulation
between two morphologically
distinguishable flagellated cells
c.
Oogamy - copulation between a
flagellated cell and a nonflagellated cell
(true oogamy in sense of animal biology)
Flagellation structure
suggests taxonomy and trend toward more
advanced forms is from isogamy to anisogamy
to oogamy (as per Chytridiomycota)
418
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Fungal
Plasmogamy Terminology (cont)
2.
Gametangial Contact - differentiated
or undifferentiated gametangia exchange or
donate and received nuclei without fusion of
gametangia. e.g. Oomycota
3.
Gametangial Copulation -
differentiated or undifferentiated
gametangia exchange or donate and receive
nuclei with fusion of the gametangia.
e.g. widespread in Euascomycotina,
Hemiascomycotina, Zygomycota,
Chytridiomycota
4.
Spermatization - spores act as donor
cells and provide mating nucleus to
receptive hypha or vegetative hypha. e.g.
Euascomycotina, Heterobasidiomycotina
5.
Somatogamy - copulation of
undifferentiated vegetative cells. e.g. Hemiascomycotina, Homo- and Hetero- basidiomycotina.
gametes = protoplasts of
various cells
422
REVIEW
1.
Homothallic/Homomictic =
self-fertility
2.
Heterothallic/Heteromictic =
self-sterility
a.
Dimictic = self-sterility controlled
by two and only two idiomorphs
(a/a, A1/A2,
A/a, +/-)
b.
Diphoromictic = self-sterility
controlled by multiple idiomorphic
series.
(A1/A2/A3/A4/etc)
1.
bipolar = multiple idiomorphic series
at 1 locus
(tetrad analysis 2A1
2A2)
2.
tetrapolar = multiple idiomorphic
series at two unlinked loci (tetrad analysis
2A1B1
or
2A1B2
2A2B2
2A2B1
usual
1A1B1,
1A1B2,
1A2B1,
1A2B2*
infrequent
Also –
Homodimictic
Homodiphoromictic
417
Plasmogamy
Terminology
1.
Planogametic copulation-mating
fusions involving at least one flagellated
cell*
a.
Isogamy - copulation between two
morphologically indistinguishable
flagellated cells
b.
Anisogamy - copulation
between two morphologically
distinguishable flagellated cells
c.
Oogamy - copulation between a
flagellated cell and a nonflagellated cell
(true oogamy in sense of animal biology)
Flagellation structure
suggests taxonomy and trend toward more
advanced forms is from isogamy to anisogamy
to oogamy (as per Chytridiomycota)
418
Fungal
Plasmogamy Terminology (cont)
2.
Gametangial Contact - differentiated
or undifferentiated gametangia exchange or
donate and received nuclei without fusion of
gametangia. e.g. Oomycota
3.
Gametangial Copulation -
differentiated or undifferentiated
gametangia exchange or donate and receive
nuclei with fusion of the gametangia.
e.g. widespread in Euascomycotina,
Hemiascomycotina, Zygomycota,
Chytridiomycota
4.
Spermatization - spores act as donor
cells and provide mating nucleus to
receptive hypha or vegetative hypha. e.g.
Euascomycotina, Heterobasidiomycotina
5.
Somatogamy - copulation of
undifferentiated vegetative cells. e.g. Hemiascomycotina, Homo- and Hetero- basidiomycotina.
gametes = protoplasts of
various cells
422
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REVIEW
1.
Homothallic/homomictic = self-fertility
2.
Heterothallic/heteromictic = self-sterility
A.
Dimictic = self-sterility controlled by two and
only two idiomorphs
(a/a, a1/a2, a/a, +/-)
B.
Diaphoromictic = self-sterility controlled by
multiple idiomorphic series.
(a1/a2/a3/a4/etc)
1)
Bipolar = multiple idiomorphic series at 1 locus
(tetrad analysis 2a1
2a2)
2)
Tetrapolar = multiple idiomorphic series at two
unlinked loci (tetrad analysis)
2a1b1
or
2a1b2
2a2b2
2a2b1
usual
1a1b1,
1a1b2, 1a2b1,
1a2b2*
infrequent
also
- homodimictic
homodiaphoromictic
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Value
of tetrapolarity vs bipolarity
Dimictic
and bipolar diaphoromictic species can mate
with themselves 50% of the time, because
half of all spores are of one mating type
and half are of the other.
Therefore, such species are less
efficient outbreeders.
Diaphoromictic
tetrapolar individuals of a species can mate
with themselves, in contrast, only 25% of
the time, because there are four, not two,
mating types represented in progeny of
meiosis.
Therefore, these individuals have
greater outbreeding potential.
Diaphoromixis
increases outbreeding potential even
greater.
The larger the number of mating-type
alleles in the world-wide population of a
species, the greater the outbreeding
potential: [1/n x (n-1)] x 100%.
For
S.
commune, with 9, 32, 9 and 9 different
specificities for aa,
ab, ba,
and bb respectively, there are estimated to be 28,000
world-wide mating types.
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