Vegetative Incompatibility and Sex in Fungi Flashcards
What is a prerequisite to anastomosis?
Somatic fusion
Why must somatic fusion be closely regulated?
So that the physiological and genetic advantages of heterokaryosis can be realized without hazard
What are the advantages of somatic fusion?
Functional diploidy
Mitotic recombination via parasexuality (important in clonal species)
Proceed to sexual processes in basidiomycetes
What are the hazards/risks of somatic fusion?
Risk of exposure to contamination with alien genetic information
Infection agents can be transmitted
Mycoviruses
Greedy genomic elements
Senescence plasmids
What is a locus?
A specified location on a chromosome
What is non-self-recognition mediated by?
Heterokaryon incompatibility
What is heterokaryon incompatibility also called?
Vegetative compatibility
What are two specific ways that nonself-recognition is mediated?
Alternate alleles at a single locus (allelic HI)
Alternate alleles at more than one locus (non-allelic HI)
What is a V-C group?
Individuals with matching alleles belong to the same V-C group
How many loci can be involved in a V-C group?
Up to 10 loci, a mismatch at any one can lead to HI
Individuals in the same V-C group can have the same or opposite mating types, how does this affect growth after fusion?
Same mating type = growth after fusion is normal
Opposite mating type = growth is slowed
What do mismatched het genes result in?
Barrages
What is a barrage?
A line or zone of demarcation that develops at the interface where two genetically different fungi meet
What do het genes regulate?
STAND proteins
What are STAND proteins?
Signal transducing ATPases with numerous domains
What do the up or downregulation of STAND proteins lead to?
Massive expressional changes
What do fungal STAND proteins contain?
a PCD switch = a het domain that induces programmed cell death
What does the co-expression of any incompatible het genes lead to?
PCD
How is the PCD phenomenon contained?
by walling off the affected area or closing septal pores
What is the difference between VC/HI and controlled mating?
Vegetative compatibility controlled by self/non-self recognition genes: cytoplasm to mingle must be as similar as possible
Mating controlled by mating-type genes: maximal advantage when nuclei are as different as possible
What does PCD not extend to?
Sexual plasmogamy
In what situation does PCD not occur?
When a trichogyne is united with a spermatium of the opposite mating type (compatible) but incompatible V-C group
What happens when there is no V-C, but matching mating types?
Perithecia in barrage
What is mating controlled by in fungi?
One MAT locus
What is the MAT locus like in ascomycetes?
The alleles are completely different pieces of DNA; they are called idiomorphs
How are mating types in ascomycetes referred to?
mating types A and a or a and alpha
What happens if the idiomorphs at the MAT loci differ and what type of fungi use this system?
Successful mating, heterothallic fungi
When does mating fail in heterothallic fungi?
If strains are of identical mating type
What are homothallic fungi?
Fungi can reproduce sexually without another individual
What is the 2-way arrangement of MAT idiomorphs called?
A bipolar mating system
How many alleles/idiomorphs are possible in ascomycetes?
Two
How many alleles are possible with basidiomycetes with a bipolar mating system?
Many alleles are possible
Many basidiomycetes have what breeding system?
A tetrapolar mating system
Also called bifactorial because two loci are involved
What is non-outcrossing?
Self-fertilization of a haploid homokaryon results in production of identical homokaryotic progeny
What is the result of non-outcrossing?
Clonal propagation - purely asexual reproduction
What is outcrossing?
Conjugation between different parental haploid genotypes following karyogamy and meiosis which results in the production of non-identical haploid homokaryotic progeny
What are two possible types of outcrossing?
Outbreeding and inbreeding
What is outbreeding?
Conjugation between haploids from different spore sources (non-siblings)
What is inbreeding?
Conjugation between haploids from the same spore source (siblings)
What is inbreeding potential?
The probability that a randomly encountered sibling will be compatible
It can also be calculated from the ratio of offspring
What is outbreeding potential?
The probability that a randomly encountered unrelated individual will be compatible
What affects the inbreeding and outbreeding potential?
The number of loci and alleles
What are the inbreeding and outbreeding potentials of ascomycetes?
In: 50%
Out: 50%
What are the inbreeding and outbreeding potentials of unifactorial basidiomycetes?
In: 50%
Out: nearly 100%
Why is the outbreeding potential in basidiomycetes nearly 100%?
Because >2 mating types are available in the population
What are the inbreeding and outbreeding potentials of bifactorial basidiomycetes?
In: 25%
Out: nearly 100%
What makes the outbreeding potential closer to 100%?
The more A and B alleles a population contains
What is homothallism?
In some species, cells are universally compatible, including among clonemates
Does not result in actual recombination
Just an umbrella term
What is primary homothallism?
The presence of both MAT idiomorphs within a single genome allows the production of eight uninucleate ascospores identical to the parental cell
What is pseudohomothallism?
Self-fertility = the result of packing of 2, opposite mating type nuclei in a single cell
Independent sexual reproduction = 4 binucleate ascospores
What is bidirectional mating type switching?
Cells of either mating type undergo mitosis to form two identical cells, one of which is then able to switch to the opposite mating type
This leads to a mixed colony capable of sexual reproduction via functional heterothallism
Why is mating type switching bidirectional?
Since any one of the offspring spores can switch mating type
Is S. cerevisiae homo or heterothallic?
Has been reported to be either
Every individual contains both Mat-1-1 and Mat1-2 idiomorphs
What does the S. cerevisiae Mat1 locus look like?
Flanked by two mating-type-like loci
The first MTL locus (HMLa) contains a MAT1-1 sequence
The second MTL locus (HMRa) contains a MAT1-2 sequence
Both are silent due to tightly wound heterochromatin
What is mating type switching regulated by in S. cerevisiae?
Another gene altogether
An HO endonuclease recognizes MAT1 and excises it; the neighboring locus is used as a template for the construction of a new MAT locus which allows switches in both directions in the lifetime of the colony
How does mating type switching work in S. cerevisiae?
Mating-type genes are present at two additional silent loci on the genome
A ds DNA break at the MAT locus can result in DNA from a silent locus being copied to the MAT locus
What is unidirectional type switching?
Cells of the hyphae of one mating type are able to switch mating type, producing a mixed mating type culture capable of sexual reproduction
The other mating type is unable to switch and thus requires the presence of a second individual to undergo sexual reproduction
What are fertility and spore size linked to?
Mating type
What is the fertility like in large and small spores?
Large = self-fertile
Small = self-sterile
When are individuals from small spores fertile?
Only if cultured together with individuals from large spores
What do individuals produced from self-fertile large spores give rise to?
Half large and half small spores
What spore types must be able to switch?
Large spore type must be able to switch to small spore type but not vice versa
What is unisexual reproduction?
Cells of the same mating type are able to interact and produce sexual spores regardless of the absence of an opposite mating type partner
What do many Neurospora strains exhibit?
Primary homothallism
Which is the only Neurospora species that has only one MAT idiomorph?
N. africana
What are the characteristics of N. africana?
It is self-fertile
Transformation of related heterothallic strains with the N. africana MAT idiomorph does not result in self-fertility
MAT idiomorph is probably not solely responsible for sexual reproduction
What is a benefit gained from homothallism?
Increased number of available mates
What is a benefit lost from homothallism?
Deleterious mutations are not eliminated by recombination
What is a benefit not lost from homothallism?
Processes that kick in during sexual cycle, such as genome editing defences or silencing of unpaired DNA, are kept in place
What is the cost of homothallism?
Machinery and energetic costs of sex are maintained
What is the Buller phenomenon?
Heterothallism by nucleus donation
He + Ho mating = heterokaryon donates nucleus
Also Ho + Ho mating
What is an application of the Buller phenomenon?
Mycoculture, especially for food like button mushrooms
How does the Buller phenomenon in mycoculture work?
Parental strain A goes through fructification and those spores are suspended in a grain spawn of homokaryon
The spores and homokaryon are inoculated in the compost of 4 culture trays
Then there is the fructification of hybrids sporocarps AxB
How is genetic diversity achieved in parasexuality?
Mitotic crossing over
It rarely occurs and comes about through the ability of nuclei to form somatic diploids in fused, heterokaryotic hyphae
Is there true asexuality in fungi?
Some fungi lack a known sexual stage
Cryptic sex?
Some asexual fungi can have functional MAT genes, so maybe other genes are essential for mating that these fungi lack
