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konkurenceschopnosti
studentů oboru botanika
a učitelství biologie
CZ.1.07/2.2.00/15.0316
© 2011 B. Mieslerová, A. Lebeda, (KB PřF UP v Olomouci)
GENETICS AND GENETIC VARIABILITY OF FUNGI
STRUCTURE AND ORGANIZATION OF THE
FUNGAL GENOME
The fungal genome has four components - nuclear DNA, mitochondrial
DNA, plasmids and fungal virus genes
NUCLEAR DNA
Forming chromosomes – in different numbers and ploidy (the number
of single sets of chromosomes)
The eukaryotic plants and animals are basically diploid.
The vegetative hyphae of the majority of fungi are haploid (all
Zygomycotina, Ascomycotina and most of Deuteromycotina have a
prolonged and dominant vegetative haplophase).
In the Oomycota diploidy and polyploidy is common.
Some yeasts are predominantly diploid.
A few fungi can alternate between haploid and diploid somatic phases
(Chytridiomycota)
Part of life cycle of some fungi (Ascomycotina, Basidiomycotina) is
dikaryotic – two compatible nuclei form a pair – dikaryon. It means
plasmogamy is not immediatelly followed by karyogamy
IMPORTANCE OF HAPLOID PHASE:
Mutation arise are easy to induce and are immediatelly
expressed. The natural selection are more rigorous and more
immediate than in most diploid organisms.
IMPORTANCE OF DIKARYOTIC PHASE:
Dikaryon may have selective advantage over the diploid in
terms of the establishment of dikaryotic mosaic. This involves
the direct exchange of one nucleus of the pair between
adjacent dikaryons after hyphal fusion to establish a new
dikaryon of different genotype - this provides immediate
variation in genotype.
Life cycle of
Podosphaera
leucotricha
Ascomycotina
The nuclear genome size of fungi is very small in comparison with
other eukaryotes (x 104 kbp – kilobase pairs).
Haploid chromosome number in fungi and fungi-like organisms
Group and species
Chromosome number
Slime moulds
Dictyostelium discoideum
7
Physarum polycephalum
40
Ascomycotina
Neurospora crassa
7
Aspergillus nidulans
8
Saccharomyces cerevisiae
16
Basidiomycotina
Schizophyllum commune
6
Ustilago maydis
20
MITOCHONDRIAL DNA
Mitochondria contain a small circular molecule of DNA. The
size of mitochondrial genome is 19-121 kbp.
All mitochondrial DNAs code for the same things: some
components of the electron transport chain (ATPase
subunits), some structural RNAs and mitochondrial transfer
RNAs.
Mitochondrial DNA
PLASMIDS AND TRANSPORTABLE ELEMENTS
Plasmids usually are closed-circular molecules of DNA with
the ability to replicate autonomously in a cell.
Plasmids or plasmid-like DNAs have been found in several
fungi. It has no known function, but it has major practical
applications in the construction of vectors for gene cloning in
yeast.
Most other plasmids of fungi are found in the mitochondria.
Incubate these two pieces of DNA together
VIRUSES AND VIRAL GENES
Fungal viruses (virus-like particles) were discovered in some fungi,
including representatives of all the major fungal groups
Viruses consist of double-stranded RNA and a capsid composed of
one major polypeptide
The presence of viruses was not associated with any obvious
disorder, so most fungal viruses seem to be symptomless.
Fungal viruses
OYSTER MUSHROOM
•
•
oyster mushroom virus I
oyster mushroom virus II
Zdravá
Healthy
Infected
Virózní
Oyster mushroom on cultivated media
Healthy
Infected
BUTTON MUSHROOM
La France isometric virus
• Mushroom baciliform virus
• Vesicle virus
Healthy
Infected
GENETIC VARIATION IN FUNGI
Sources of Genetic Variability
NON-SEXUAL VARIATION: MUTATION
Main source of variability is mutation, which is spontaneous heritable
changes in the genetic information of an organism (additions,
deletions, substitutions, inversions..)
The haploid organisms cannot accumulate mutations that are not of
immediate value: they cannot store variation.
Any mutation can lead to increase of the fitness or loss of fitness.
Diploid organisms: mutations are often recessive to the wild type
and so they are not immediatelly expressed, instead they accumulate
and can be recombined in various ways during sexual crossing
 Selection and mutation pressure has a role on stability of mutations
NON-SEXUAL VARIATION: HETEROKARYOSIS
HETEROKARYOSIS
 The hyphae of closely related fungi can fuse with one
another (anastomose) locally during normal assimilative
growth, exchanging nuclei and thereby producing
heterokaryons (mycelia containing genetically different
nuclei).
 The heterokaryotic condition confers great flexibility on
many conidial fungi, helping them to cope with different
substrates and conditions. The degree of heterokaryosis in
natural environments is largely unknown
 There is a major difference between dikaryon and
heterokaryon: In the dikaryon the nuclei eventually fuse and
undergo meiosis, as part of the sexual process. In
heterokaryons, this is not usually so; there heterokaryon
breaks down.
Heterokaryosis can arise in two ways:
1/ When hyphae of any two strains fuse at points of
contact (anastomosis) so that their nuclei are
present in the common cytoplasm.
2/ When mutation occurs in one of the nuclei in a
hypha and a mutated nucleus proliferates along
with the wild-type of nuclei.
In both cases the nuclei would need to proliferate in the
apical cells to form a stable heterokaryon.
NON-SEXUAL VARIATION: PARASEXUALITY
PARASEXUALITY: alternation of sexual recombination without benefit
of sex. Typical for some of Deuteromycotina
The parasexual cycle has four stages:
1/ Fusion (anastomosis) of adjacent somatic hyphae, and exchange of
nuclei, establishing a heterokaryon.
2/ Fusion of different nuclei in the vegetative hyphae, to form somatic
diploids.
3/ Somatic recombination (mitotic crossing-over). Result is a creation
of chromosomes that are hybrids of the parental chromosomes.
4/ Non-meiotic reduction of the altered nuclei via aneuploidy (loss of
individual chromosomes) to the haploid condition.
THE WHOLE PROCESS IS VERY RARE
THIS IS RESULT OF THE RANDOM CHANGES
This sequence of events is rare, happening in fewer than one conidium
in a million, but the number of conidia produced by most conidial
anamorphs is astronomical, so parasexuality is a practical means for
producing genetic variation.
Stages of the parasexual cycle are numbered as follows
(1) Hyphal conjugation (plasmogamy).
(2) Heterokaryosis.
(3) Nuclear fusion (karyogamy).
(4) Mitotic recombination and nondisjunction.
(5) Haploidization and nuclear segregation leading to homokaryosis.
SIGNIFICANCE OF PARASEXUALITY
 Significance in nature is largerly unknown
 It is not clear why Deuteromycotina abandoned a efficient
sexual mechanism of genetic recombination in favour of a
more random and seemingly less efficient process.
 The answer may be that the parasexual events can occur at
any time during normal somatic growth and with no
preconditions like those needed for the production of sexual
stages.
 Each of this events is relatively rare and they do not
constitute a regulated cycle like sexual cycle involving
meiosis
COMPARING SEXUALITY AND PARASEXUALITY
Sexual reproduction
Parasexuality
Highly organized, often precisely
timed process, which is genetically
programmed
Involves a sequence of uncommon
events which seems to operate by
chance, rather than by design
Nuclear fusion is often mediated by
genetic factors, expressed as 'mating
types,' happens in highly specific
structures, and often involves many
pairs of compatible nuclei
Nuclear fusion is an isolated event, not
mediated by mating-type factors, not
found in specialized structures, and
involving only individual nuclei
During meiosis, crossing-over
probably takes place in every
homologous pair of chromosomes,
and multiple crossovers are common
During somatic recombination,
crossing-over commonly involves only
one or a few chromosomes, and never
happens as often as during meiosis
In meiosis, segregation happens in a
highly organized way during two
specialized nuclear divisions
Somatic haploidization probably
occurs as a result of successive
chromosome losses from an
aneuploid nucleus (2n-1) over several
mitotic divisions until the stable
haploid is reached
SEXUAL VARIATION
 Sex is the major mechanism for producing
recombinants
genetic
 The pairing of parental chromosomes in meiosis leads to
multiple crossing over (chiasma formation).
 The independent assortment of homologous chromosomes
(segregation) from the two parents will mean that the individual
daughter haploid nuclei might have different chromosomes
from different parent.
 HOMOTHALLIC - describes fungi in which a single strain can
undertake sexual reproduction; self-compatible
 HETEROTHALLIC - describes fungi in which two genetically
distinct but compatible mycelia must meet before sexual
reproduction can take place
CROSSING-OVER
Each chromosome is composed of two parallel strands or
chromatids.
In the simplest crossover, shown in (b), a break occurs at the same
place in one of the 'white' chromatids and one of the 'black'
chromatids. The ends rejoin, but in a new arrangement: the part of
the 'black' chromatid carrying the dark ascospore gene is now joined
to part of a 'white' chromatid and vice versa. Crossing-over is one of
the main mechanisms involved in providing the pool of variability on
which natural selection acts.
Crossing-over: dark-spored
'wild-type' strain is crossed
with a pale-spored mutant.
MATING-TYPES OF BASIDIOMYCOTINA
If the alleles occur at a single locus
(A1,A2), the offspring of a single
basidioma will be of TWO
DIFFERENT MATING TYPES; the
mating system of the fungus is
called BIPOLAR (most smuts, some
Gasteromycetes, Coprinus comatus)
If the alleles are at two loci (A1, A2,
B1, B2), offspring of a single
basidioma will be of FOUR MATING
TYPES; the mating system is called
TETRAPOLAR (most
Aphyllophorales, Agaricales and
Gasteromycetes) A1B1; A1B2; A2B1;
A2 B 2
GENETICS AND PLANT PATHOLOGY
PLANT
NON-HOST RESISTANCE – plant could not by any means
support the propagation of an otherwise pathogenic microbe.
(unsuitable conditions for survival and propagation of most of
fungi and bacteria). Plants are completely immune to infection.
HOST-RESISTANCE - plant could support the propagation of
pathogenic microbe
VERTICAL RESISTANCE – RACE-SPECIFIC RESISTANCE- can be
conferred by a single resistance gene, which is often controlled by
„yes“ or „no“ response. Plants showed very high levels of
resistance to a particular physiological race of pathogen. Often
showed HR (hypersensitive response)
HORIZONTAL RESISTANCE – RACE-NONSPECIFIC RESISTANCE is usually polygenic resulting from cumulative effect of many
genes. Equal resistance is shown to all pathogen races; but these
resistance may not totally protect plants from infection
Horizontal and vertical resistance of potato to Phytophthora
infestans
PATHOGEN
PATHOGENICITY – term referring to the potentiality of particular
strains of a microorganism to induce disease in certain individuals of
plant species. Genetic determinants of pathogenicity are pathogenicity
genes.
VIRULENCE - is a qualitative trait referring to the degree of
pathogenicity – contolled by single gene
AGGRESSIVENESS - is a quantitative trait referring to the degree of
pathogenicity controlled by polygenes
Whithin a fungal species some individuals may be pathogenic on
particular host species. Such fungi are often grouped as FORMAE
SPECIALIS
 Whithin such FORMAE SPECIALIS those individuals which are
specifically pathogenic on a particular host variety or cultivar are
grouped in to RACES.
 Term RACE is used to designate genetically distinct mating groups or
to describe different genotypes within a species.
RACES orr FORMAE SPECIALIS can be recognized by
testing each fungal strain against standard set of host
varieties.
Interactions of races of Cladosporium fulvum with 3 tomato varieties
Tomato with resistance gene
Cladosporium races
0
1
2
3
1+2
1+3
1
R
S
R
R
S
S
2
R
R
S
R
S
R
3
R
R
R
S
R
S
Examples of L. serriola responses to B. lactucae (race BL18)
Resistant
Moderately
susceptible
Susceptible
GENE-FOR-GENE HYPOTHESIS (Flor, 1971)
This suggests that the evolutionary paths of host and pathogen
have been so closely linked for so long, that for every gene in the
host that is capable of mutating to give resistance, there is a
corresponding gene in the pathogen which can mutate to overcome
that resistance.
Breeding experiments usually shows host resistance to be dominant
(R ) and susceptibility recessive (r ). The genes for avirulence is hence
classifiable as dominant (Av) and for virulence as recessive (av).
Three combinations: R-a, r-A and r-a give rise to compatible reactions
and infections are successful. One combination, R-A results in a
incompatible reaction and no infection occurs (resistant)
It is though that a resistance gene acts by controlling the production
of protein receptor molecules at the plant cell surface capable of
interacting with glycoprotein molecules (elicitors) located at the fungal
surface.
Basic interaction of genes
avirulence/virulence and
resistance/susceptibility
Product of resistance
gene is RECEPTOR on
surface of plant leaf
Product of avirulence
gene is ELICITOR on
surface of fungus