Parasexual cycle

In some fungi, the sexual life cycle lacks the meiosis process. Those organisms get the benefits of sexuality by a process known as the Parasexual Cycle.

The parasexual cycle is usually found in fungi and single-celled organisms. It is a nonsexual mechanism that involves the transfer of genetic material without meiosis or the formation of sexual structures.

In 1956, an Italian geneticist Guido Pontecorvo first described this process when he was studying Aspergillus nidulans. Since this was discovered, it found not only in members of Deuteromycetes but also in fungi bearing to Ascomycetes and Basidiomycetes.

The Parasexual Cycle is defined as a cycle in which plasmogamy, karyogamy and meiosis (haploidisation) take place but not at a specified time or at specified points in the life-cycle of an organism.

The parasexual life cycle is mainly found in those fungi, that lack the true sexual life cycle, such as members of Deuteromycetes. 

The parasexual cycle begins with the fusion of hyphae (anastomosis). During this step the nuclei and other cytoplasmic elements are occupying the same cell (heterokaryosis and plasmogamy).

Then the fusion is take place between the nuclei of heterokaryon which leads to the formation of a diploid nucleus (karyogamy), which is thought to be weak and can create segregants by recombination including mitotic crossing-over and haploidization. Mitotic crossing-over can lead to the transfer of genes on chromosomes; whereas haploidization apparently requires mitotic nondisjunctions which randomly reassort the chromosomes and rise in the formation of aneuploid and haploid cells. 

Parasexual Cycle Steps

According to Pontecarvo (1958), the parasexual cycle in A. nidulans occurs in the following steps:

1. Formation of heterokaryotic mycelium

• The Heterokaryotic mycelium is mainly formed by the anastomosis of somatic hyphae of different genetic combinations.
• The mating of foreign nucleus or nuclei within the mycelium multiplies and its progeny grows through the mycelium giving it heterokaryotic.
• The mutation of one or more nuclei within homokaryotic mycelium also makes it heterokaryotic.
• This happens in those fungi which are belonging to Ascomycetes.

2. Fusion between two nuclei (Karyogamy)

• The fusion of nuclei is divided into two categories such as; (a) fusion between like nuclei and (b) fusion between unlike nuclei.
• Fusion of nucleus results in the formation of homozygous or heterozygous diploid nucleus respectively.
• If the genotype of unlike nuclei existing in the heterokaryotic mycelium is A and B, then 5 types of nuclei can be developed by their fusion: two types of haploid nuclei (A and B), two types of homozygous diploid nuclei (AA and BB) and one type of heterozygous diploid nucleus (AB).

3. Multiplication of diploid nuclei

• Mentioned 5 types of nuclei can multiply at about the same rate but the diploid nuclei are present in much smaller numbers than the haploid nuclei. 
• Portecarvo (1958) estimates a proportion of one diploid heterozygous nucleus to 1000 haploid nuclei.

4. Occasional Mitotic crossing over.

• When diploid nuclei are multiplied, during this time mitotic crossing over may take place. Which may results in the formation of new gene combinations. These recombinations dependent on the existence of heterokaryosis, give the fungus some of the advantages of sexuality within the parasexual cycle.
• According to Pontecarvo’s (1958) estimates, the amount of recombinations which may be expected to occur in an ascomycete through its parasexual cycle is 500 times smaller than through its sexual cycle.
• However, in Penicillium chrysogenum and Aspergillus niger, diploidisation and mitotic crossing over occur more frequently indicating the importance of parasexual cycle in evolution of new strains.

5. Sorting out of diploid nuclei

• In those fungi which produce uninucleate conidia, sorting out of the diploid nucleus occurs by their incorporation into conidia which germinate to produce diploid mycelia. Diploid strains of several important imperfect fungi have been isolated.
• Roper (1952) first synthesized and isolated diploid strains of Aspergillus nidulans. The conidia of diploid strains are somewhat larger than those of haploid strains.

6. Occasional haploidisation of diploid nuclei, and

• Occasionally, some hyphae of diploid mycelium from haploid conidia which forms haploid mycelia on germination. The formation of haploid conidia by diploid mycelium indicates that haploidisation occurs in some diploid nuclei.

7. Sorting of new haploid strains.

• Some diploid nuclei undergo haploidisation in the mycelium and are sorted out by incorporation of haploid nuclei in the uninucleate conidia. Some of these haploid strains are genotypically different from their parents because of their mitotic recombinations.

After the completion of the parasexual cycle, the mycelium should contain the following types of nuclei:

• Haploid nuclei like those of both the parents.
• Haploid nuclei with various new genetic recombinations.
• Several types of diploid homozygous nuclei.
• Several types of diploid heterozygous nuclei.

Application of Parasexual cycle

• In the industrial process, parasexual cycle has a great impact. Many fungi which are used in different industrial processes belong to fungi imperfecti or Deuteromycetes and in these fungi, only a parasexual cycle operates.
• It is also used to obtain new and better strains of these fungi by mutation. 
• It is also used in analysis of genetic and physiological processes of perfect and imperfect fungi.
• It is used in genetic control of pathogenicity and host-range in several species of Fusarium.

Organism Performing Parasexual Cycle

The following organisms shows parasexual cycle under laboratory condition;

• Fusarium monoliforme
• Penicillium roqueforti (used in making blue cheeses[5])
• Verticillium dahliae
• Verticillium alboatrum
• Pseudocercosporella herpotrichoides,
• Ustilago scabiosae,
• Magnaporthe grisea
• Cladosporium fulvum,
• Candida albicans (human pathogens)
• Candida tropicalis

Difference between Sexual and Parasexual Cycle

Sexual Cycle	Parasexual Cycle
Nuclear fusion takes place in specialised structures.	Nuclear fusion takes place rarely in vegetative cells
Zygotes usually persist one nuclear generation.	Zygote persists through many mitosis.
Recombination by meiosis; crossing over in all chromosomes pairs, of reduction chromosome number, and random assortments of members of each chromosome pair are characteristics of the cycle.	Recombination by rare accidents of mitosis; mitotic crossing over at each event usually confined to one exchange in a single chromosome arm and haploidisation independent of crossing over are characteristics of this cycle.
Products of meiosis can easily be recognised and isolated.	Products can be recognised only by application of suitable genetic markers.

Faq on Parasexual cycle

References

• https://www.biologydiscussion.com/fungi/parasexual-cycle-in-fungi-with-diagram-botany/63738
• https://www.slideshare.net/breaker561994/deuteromycota-presentation
• https://en.wikipedia.org/wiki/Parasexual_cycle
• https://www.researchgate.net/figure/Schematic-Overview-of-the-Parasexual-Cycle-in-the-Filamentous-Fungus-A-nidulans-An_fig1_6363649
• https://www.majordifferences.com/2013/07/difference-between-sexual-cycle-and.html#.YJIL1rUzY_4