Fungi: Heterokaryosis and Parasexuality (With Diagram) | Mycology

In this article we will discuss about (1) Heterokaryosis (2) Parasexuality

(1) Heterokaryosis

• Heterokaryosis - co-existence of genetically- different nuclei in cytoplasm continuity with one another. 
• Discovered by Hansen and Smith (1932) in Botrytis cinerea.
• Plays major role - variability and sexuality in fungi.

Formation of Heterokaryosis

Heterokaryotic condition arises by- 

• Mutation 
• Anastomosis
• Inclusion of dissimilar nuclei in spores after meiosis, in heterothallic fungi.

Mutation 

A high frequency of mutation is characteristic of fungi - main source of variability.

Anastomosis (fusion of hyphae)- Fusion is mostly intra-specific.  Nuclear migration from the point of fusion to the remainder of the mycelium takes place - heterokaryotic mycelium.  Example: development of heterokaryon in basidiomycota.

Inclusion of dissimilar nuclei in spores after meiosis, in heterothallic fungi : Meiosis results in the production of genetically different nuclei sharing common cytoplasm.  Example: Neurospora tetrasperma, Podospora anserine 

On germination - give rise to a heterokaryotic thallus.  In the asexul phase - occurs frequently in multinucleate spores.

Signification of Heterokaryosis:

• Substitute for heterozygosity and variability 
• Heterokaryosis and pathogenicity- e.g. in rusts and smuts 
• Origin of new race 
• Initial step in Parasexual cycle

(2) Parasexuality

• Discovered in fungi (Aspergillus nidulans) by Pontecorvo and Roper (1952) 
• Parasexuality - genetic recombination is achieved through “ mitotic crossing over” and “ hapoidization”. 
• also known as somatic recombination.
• Sexual reproduction - extremely uniform - fine coordination between recombination, segregation and reduction 
• Parasexual cycle lack such co-ordination 
• The steps of the parasexual - independent of each other and the frequency of each is very low. 
• The karyogamy and haploidization are accidential events not bound by space and time.

Steps of Parasexual Cycle

• Establishment of heterokaryosis 
• Formation of heterozygous diploids 
• Occasional mitotic crossing- over, during multiplication of the diploid nuclei, and 
• Haploidization through aneuploidy

Establishment Of Heterokaryosis

• The presence of haploid nuclei of dissimilar genotypes in the same cytoplasm 
• pre-requisite for recombination. 
• Heterokaryosis is brought about by- 

1. Mutation
2. Anastomosis
3. Inclusion of dissimilar nuclei in spores after meiosis, in heterothallic fungi.

Formation of Heterozygous Diploids

• Nuclear fusion in heterokaryotic somatic cells was first noted by Roper (1952) in Aspergillus nidulans. 
• The nuclear fusion between dissimilar nuclei - the formation of heterozygous diploid nuclei or “zygotes” 
• a rare event, occurring at the rate of one in a million.
• The heterozygous diploid nuclei - fairly stable 
• The diploid colonies are recognized by-

1. higher DNA content of their nuclei 
2. the bigger size of their conidia 
3. certain phenotypic characteristics of their colony 

• The prolonged diploid phase involving repeated nuclear divisions, enhances the chances of “mitotic crossing over”.

Occasional Mitotic Crossing Over During Multiplication Of Diploid Nuclei

• genetic recombination takes place. 
• Penicllium chrysogenum and Aspergillus niger- mitotic crossing over is as frequent as during meiosis.
• In mitotic crossing over - exchange, or chiasmata formation - confined to a single chromosome pair out of the whole component of chromosomes. 
• In meiosis, the crossing – over occurs simultaneously in all the chromosomes. 
• splitting of chromosomes and segregation of strands is same in mitotic crossing over as it occurs in meiosis.

Occassional Haploidization Through Aneuploidy

• The diploid nuclei - haploid nuclei -gradual loss of chromosomes during successive mitotic division - haploidization. 
• Meiosis is not involved.
• The haploidization occurs at a constant rate of 10- 3 per nuclear division - the result of aneuploidy.
• During mitotic division - non-disjunction of the chromatids of one chromosome pair - results in aneuploid nuclei (2N-N) or haploid. 
• The aneuploids – unstable - the loss of chromosomes - favours the development of fully balanced haploid nuclei.

Use Of Parasexual Phenomenon in Antibiotic Industry

• most important antibiotic producing fungi, like Penicillium chrysogenum (penicillin) and Acremonium chrysogenum (cephalosporin) 
• discovery of parasexual phenomenon - suitable techniques to obtain higher antibiotic strains.

Fungi Antibiotic 

Aspergillus nidulans Penicillin G 

Acremonium chrysogenum Cephalosporin C 

Emerocellopsis salmosynnemata Penicillin N 

Penicillium chrysogenum Penicillin G, O,V 

Penicillium patulum Griseofulvin,patulin

Parasexual cycle closely simulates the events of the sexual cycle. It involves everything: plasmogamy, karyogamy and haploidization, but in a modified form, and without any fixed plan with regard to time and space.