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Reasons
Sex Persists in Fungi
1.
To bring together in the same nucleus
genetic information from different
individuals
2.
To redistribute it in different
combinations to different progeny
The redistribution is the result of:
a)
Independent assortment of chromosomes
(during first division of meiosis-meiosis I)
b)
Crossing over of alleles during
synapsis in metaphase 1*
*
Gives rise to Mendelian segregation patterns
known as:
1)
1st division segregation
2)
2nd division segregation
384
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Nonsexual
Mechanisms for Generating Genetic
Variability or Stability
1.
Heteroplasmonosis (Extrachromosomal)
2.
Heterokaryosis
3.
Parasexuality
388
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Heteroplasmonosis
(extrachromosomal or nonnuclear gene inheritance)
The
co-existance in the same cytoplasm of
nuclear and nonnuclear entities that affect
differently the phenotype of the fungus.
Examples - RD systems in S.
cerevisiae
- 2m DNA inheritance in S.
cerevisiae
- poky in N.
crassa
- Killer in Ustilago
- senescence in Podospora
- sexuality vs nonsexuality in Aspergillus
nidulans
- mycoviruses
389
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General
Properties of S.
cerevisiae r (petite) mutants.
(non-nuclear
or haploid or homozygous diploids)*
1.
Cannot respire aerobically (obligately
fermentative)
2.
Cannot form ascospores
3.
Exhibit absence of certain functional
membrane-bound mitochondrial cytochromes
\ respiratory deficient (RD) mutants (r)*
- nuclear mutations -> tetrads
with 1 wt: 1 RD
- nonnuclear mutations ->
non Mendelian ratios
390
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ro (neutral) petite (nr-)
N
2N
1.
anr- X anr- ---> aa
nr-nr- (presumed
genotype of diploid)
(homozygous cross)
1.
Respiratory deficient
2.
--> Petite colonies
3.
--> No ascospores
2.
anr+ X anr-
--->
aa nr-nr+ (presumed
genotype of diploid)
(heterozygous cross)
1.
Respiratory sufficient
2.
Normal colonies
3.
Ascospores
â
tetrad analysis
â
--->
2a nr+
& 2a nr+
(lost r character) \ not nuclear gene
391
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Segregational
(sr-)
petites*
1.
asr-
x
asr+
---->
aasr-sr+ (normal
respiratory sufficient diploid)
ascosporogenesis & tetrad
analysis
1.
2 asr-
&
2 asr+
mating type gene & sr-
gene
or
2.
2 asr+
&
2 asr-
on different chromosomes*
or
3.
infrequently** 1asr-,
1asr-,
1asr+,
1asr+
**(when
pet
18
on chrom III with mating locus)
*general
Mendelian segregation patterns for 2
unlinked pairs of nuclear genes or for 2
pairs of linked genes.
-
petite phenotype recessive because not
expressed (masked) in diploid
392
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Seg x neutral petite
cross
1.
anr- X
asr-
---->
aanr-sr- (presumed
diploid genotype)
1. all respiratory
sufficient
2. \ all
can form ascospores
Why
respiratory sufficient?
anr-
strain has sr+
nuclear gene to complement asr- defect in
diploid.
asr- strain has nr+
mitochondrial gene to complement anr-
gene defect in diploid.
393
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Seg r-
X
Neutral r- =
asr-nr+
X
asr+nr-
---->
aasr-sr+nr-nr+
â
growth &
â
sporulation
2asr-
&
2asr+
or
2asr+
&
2asr-
1:1
pattern =
2RD+ (petite strains)
2RD-
(normal wt)
again
lost the Nr-
phenotype character because nonchromosomal
gene.
Suppressive
r- -> non-Mendelian ratios different than that of
nr-, because have mitochondrial DNA with only one of
a few mutations, mitochondrial (mt) ribosome
function and antibiotic resistance.
e.g. – chloramphenical resistance
394/251
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Inheritance
of nonnuclear trait for sexuality in Aspergillus
nidulans using nonsexual cross
Some
strains sexual
Some
strains nonsexual
Question: are these nuclear genes?
Nonsexual
cross (by anastomosis)
G = green conidia (nuclear gene
marker)
g = yellow conidia (nuclear gene
marker)
Homokaryons
yellow (g) sexual X green (G) nonsexual
NXN
â
Heterokaryon
green (Gg) sexual
N+N
isolated & analyzed
conidia w/ single nuclei
yellow
(g) sexual
green (G) sexual
395/252a
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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)
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. 249 of M-L for mechanisms
that might produce new phenotypes)
401/256
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