sex in fungi Flashcards
Homokaryons and heterokaryons
Homokaryon - cell with many identical nuclei in the same piece of cytoplasm. All nuclei are same genotype.
Heterokaryotic refers to cells where two or more genetically different nuclei share one common cytoplasm.
if heterokaryonic hyphae generates asexual spores, then homokaryonic spores are formed?? Mitotic division from the nucleus in the sporagiophore metula forms spore chains
Homothalli and heterothalli
Homothallic - can self-fertilise (one individual has both mating types)
The opposite sexual functions are performed by different cells of a single mycelium. Both nuclei are derived from the same individual.
Heterothallic - require different mating types.
Outcrossing in which two different individuals contribute nuclei to form a zygote.
Homothalli and heterothalli
Homothallic - having male and female reproductive structures on the same thallus.The opposite sexual functions are performed by different cells of a single mycelium. Self-fertilization or selfing (in homothallic fungi) in which both nuclei are derived from the same individual.
Heterothallic - cells have mating types a and α. … These are outcrossing in which two different individuals contribute nuclei to form a zygote.
Are fungi haploid or diploid?
- Most eukaryotes predominantly diploid, fungi predominantly HAPLOID
- This means in fungi most genes are single copy and mutations are visible – more bacterial-like
- However, many hyphae are coenocytic (have many identical nuclei in the same piece of cytoplasm) – resulting cell is called a homokaryon
- The phenotype can be different in particular sections of hyphae
- Mutation can give local complementation – bit like dominant/recessive in diploids
Consequences of heterokaryons
• Different nuclei provide genetic variation in mycelium
• Mycelial phenotype depends on interactions between all nuclei and the pattern of fusions that have taken place
• SPATIALLY DIFFERENT PHENOTYPES - phenotype of mycelium different in diff part of the mycelium
• Heterokaryon has greater physiological flexibility to react to different nutritional environments, increasing their genetic diversity without the need for sexual reproduction
• Allows acquisition of genetically different mitochondria, plasmids etc (cytoplasmic inheritance!!!)
- However genetically different mitochondria segregate into different regions of the mycelium if incompatible
Conserved features common to both unicellular and multicellular eukaryotes:
- Changes in ploidy level
- Meiosis from diploid to haploid to give recombined novel genotypes
- Mate recognition systems
- Progeny as different genetically from parents
Costs of sex
They’re high…
o Energetically more expensive
o Two fold cost of sex in gendered species (??)
o Breaks apart from successful genomic combinations
→ There’s ongoing debate on why sexual reproduction evolved and is maintained
Why sex is good
Created mutant that couldn’t undergo mitosis – ie . engineered to be asexual
Under starvation, both types can form diploids by fusion (process of haploids fusing called karyogamy!!)
In the WT, recombination generates a wide range of genotypes
In the mutant, much lower variability
WT outcompetes asexual in harsh conditions as it is fitter
Why sex is good
experimental evidence with yeast
Created mutant that couldn’t undergo mitosis – ie . engineered to be asexual
Under starvation, both types can form diploids by fusion (process of haploids fusing called karyogamy!!)
In the WT, recombination generates a wide range of genotypes
In the mutant, much lower variability
WT outcompetes asexual in harsh conditions as it is fitter
Steps in fungal sexual reproduction
Plasmogamy - cytoplasm fuses, dikaryotic (n+n)
Stable heterokaryon
Karyogamy - fusion of nuclei, diploid (2n)
Meiosis - nuclear division, producing haploid spores (n)
Germination - producing mycelium (n)
These events can all be temporally separated (by many yrs even)
Plasmogamy
FUSION OF CYTOPLASM
Plasmogamy is the 1st stage in the sexual reproduction of fungi
Cytoplasm of two parent cells fuses together without the fusion of nuclei, effectively bringing two haploid nuclei close together in the same cell.
Anastomosis
- affect on phenotype
- The fusion of hyphae (anastomosis) is a normal part of colony growth, occurring irrespective of the mating type of interacting hyphae
- Fusions can occur tip-to-tip, tip-to-side or side-to-side
- At point of fusion, nuclei in same cytoplasm remain distinct and separate (heterokaryotic, can persist for years)
- Two auxotrophs (a mutant organism that requires a particular additional nutrient) (lys-) and (ade-) can jointly synthesise both amino acids despite nuclei being unfused!! – the phenotype is local!!
- Some phenotypes (like branching frequency or growth rate) may depend on the ratio of the nuclei
By sharing materials in the form of dissolved ions, hormones, and nucleotides, the fungus maintains bidirectional communication with itself
Karyogamy
Additional requirements for karyogamy
FUSION OF NUCLEI
For two nuclei to fuse, they must have different mating type genes eg. +/- or a/α
Hyphae can be:
Homokaryotic – all nuclei are the same genotype
Heterokaryotic – two or more different nuclei
and also
Homothallic – can self-fertilise (one individual has both mating types)
Heterothallic – require different mating types
What determines whether two hyphae fuse into a stable heterokaryon?
- There is little control you can have over anastomosis (hyphal fusing)
• Vegetative compatibility genes (v-c genes) determine if fusion is maintained or aborted
• If the cells are incompatible (ie. genetic differences at the heterokaryon loci het), apoptosis of fused cells
• There are 8-11 of these loci in many species
Implications for genetic diversity of nuclear fusion (karyogamy)
Both Heterokaryotic+Heterothallic (diff nuclei genotype, require diff mating types)
Independent segregation and Cross-over
Homokaryotic and Heterothallic (nuclei same genotype, require diff mating types)
Cross-over only
Homothallic (can self-fertilise - one individual has both mating types)
Cross-over only
