Parasexual Cycle in Fungi | Parasexuality in Fungi (With Simple Diagram)

In this article we will discuss about Parasexual Cycle in fungi

Parasexual Cycle in Fungi

• Until 1944, sexual reproduction was the main method for fungi to exchange genetic material.
• The parasexual cycle includes plasmogamy, karyogamy and mitosis (haploidization), was recognized as a distinct process occurring at various stages in the life cycle of certain fungi.
• Parasexuality was first discovered in 1952 by Pontecorvo and Roper at the University of Glasgow while they were studying Aspergillus nidulans.
• In Deuteromycetes, lacking a sexual cycle and they typically undergo a parasexual cycle.
• Parasexual cycle has been discovered not only in Deuteromycetes but also in fungi belonging to Ascomycetes and Basidiomycetes.

Steps in Parasexual Cycle in Fungi:

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

1. Formation of heterokaryotic mycelium
2. Fusion between two nuclei (Karyogamy)
3. Multiplication of diploid nuclei
4. Occasional Mitotic crossing over.
5. Sorting out of diploid nuclei
6. Occasional haploidisation of diploid nuclei, and
7. Sorting of new haploid strains.

1. Formation of heterokaryotic mycelium

• Heterokaryosis is a condition where a fungal cell (or a group of cells) contains two or more genetically different nuclei.
• Heterokaryotic mycelium is formed in several ways:
  • Anastomosis: Most common way, fusion of hyphae from different individuals leading to genetically distinct nuclei within the same cell.
  • Mutation: Leading to the presence of genetically distinct nuclei within the same cell. E.g., Ascomycetes

2. Fusion between two nuclei (Karyogamy)

• The nuclei in heterokaryotic mycelium (A and B) can fuse, leading to formation of five different types of nuclei:
  • No fusion of A nuclei results in haploid A nuclei.
  • No fusion of B nuclei results in haploid B nuclei.
  • Fusion of two A nuclei produces homozygous diploid nuclei (AA).
  • Fusion of two B nuclei produces homozygous diploid nuclei (BB).
  • Fusion of A and B nuclei produces heterozygous diploid nuclei (AB).

3. Multiplication of diploid nuclei

• The five types of nuclei multiply but the diploid nuclei are present in much smaller number than the haploid nuclei.
• Portecarvo (1958) estimates a proportion of one diploid heterozygous nucleus to 1000 haploid nuclei.

4. Occasional mitotic crossing over

• During multiplication of diploid nuclei, mitotic crossing over may take place.
• Mitotic crossing over is the most important, or ‘key’ event in the parasexual cycle.
• This results in the formation of new gene combinations.
• These recombinations, which are 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 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.
• 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 the diploid nuclei

• Occasionally, some hyphae of diploid mycelium form haploid conidia which form 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

Significance of Parasexual Cycle in Fungi

• Parasexual cycle is of importance in industrial processes. E.g., Some of the most important antibiotic producing fungi, (penicillin) and Acremonium chrysogenum (cephalosporin), lack sexual reproduction. The discovery of parasexual phenomenon in them has provided suitable techniques to obtain higher antibiotic strains.
• New and better strains of these fungi are obtained by mutation through parasexual cycle.
• The strains of desirable characters can be developed through mitotic recombination.

Reference

S, K. (2016b, November 28). Parasexual cycle in Fungi (With diagram) | Botany. Biology Discussion. https://www.biologydiscussion.com/fungi/parasexual-cycle-in-fungi-with-diagram-botany/63738

Yadav, R. (2017, January 14). Parasexuality in fungi [Slide show]. SlideShare. https://www.slideshare.net/slideshow/parasexuality-in-fungi/71017283