Introduction
Parasexual Cycle in Fungi: In certain fungi, the typical sexual life cycle lacks the well-defined meiosis process. However, these organisms have evolved an alternative route to achieve genetic diversity, known as the parasexual cycle.
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.
Generally parasexual cycle occurs in those fungi in which true sexual cycle does not take place. The members of class Deuteromycetes (Deuteromycotina) in which sexual cycle does not occur, exhibit parasexual cycle generally.
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.
Steps Involved in Parasexual Cycle:
According to Pontecarvo (1958), parasexual cycle in A. nidulans involves the following steps:
(i) Formation of heterokaryotic mycelium
(ii) Fusion between two nuclei (Karyogamy)
(a) Fusion between like nuclei
(b) Fusion between unlike nuclei
(iii) Multiplication of diploid nuclei
(iv) Occasional Mitotic crossing over.
(v) Sorting out of diploid nuclei
(v) Occasional haploidisation of diploid nuclei, and
(vii) Sorting of new haploid strains.
A brief account of these steps are being presented below:
(i) Formation of heterokaryotic mycelium:
- Heterokaryotic mycelium is formed through the fusion of somatic hyphae with different genetic compositions. This can occur through anastomosis or even by the introduction of foreign nuclei.
- The foreign nucleus or nuclei introduced into a mycelium multiplies and its progeny spreads through the mycelium rendering it heterokaryotic. Mutation in one or more nuclei of a homokaryotic mycelium also makes 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.
(ii) Fusion between two nuclei (Karyogamy):
- The fusion of nuclei in the mycelium has been demonstrated. The nucelar fusion may be of two types:
- Fusion Between Like Nuclei: This involves the fusion of nuclei with similar genetic content.
- Fusion Between Unlike Nuclei: Here, nuclei with distinct genetic makeup merge. This leads to the formation of homozygous or heterozygous diploid nuclei.
- The nuclear fusion results in the formation of homozygous or heterozygous diploid nucleus respectively.
- If the genotype of unlike nuclei present in the heterokaryotic mycelium is A and B, then five types of nuclei can be formed 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).
(iii) Multiplication of diploid nuclei:
- While various nuclei types multiply at similar rates, diploid nuclei are present in smaller numbers compared to haploid nuclei.
- Portecarvo (1958) estimates a proportion of one diploid heterozygous nucleus to 1000 haploid nuclei.
(iv) Occasional mitotic crossing over:
- During the multiplication of diploid nuclei, mitotic crossing over can occur. This results in new gene combinations.
- These recombinations, which are dependent on the existence of heterokaryosis, contributing to genetic diversity 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.
(v) Sorting out of Diploid nuclei:
- In fungi producing uninucleate conidia, diploid nuclei are incorporated into conidia, leading to the development of 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.
(vi) Occasional haploidisation of the diploid nuclei:
- Some hyphae of diploid mycelium occasionally give rise to haploid conidia, which in turn develop into haploid mycelia upon germination.
- The formation of haploid conidia by diploid mycelium indicates that haploidisation occurs in some diploid nuclei.
(vii) Sorting of new haploid strains:
Diploid nuclei undergoing haploidization are incorporated into uninucleate conidia. This results in the generation of new haploid strains, Some of these haploid strains are genotypically different from their parents because of their mitotic recombinations.
Thus, after the parasexual cycle has operated for some time, the mycelium may contain the following types of nuclei:
(a) Haploid nuclei like those of both the parents,
(b) Haploid nuclei with various new genetic recombinations,
(c) Several types of diploid homozygous nuclei,and
(d) Several types of diploid heterozygous nuclei.
Significance of Parasexual Cycle:
The importance of the parasexual cycle extends to industrial processes. Many fungi used in various industrial applications belong to the fungi imperfecti or Deuteromycetes group. In these fungi, the parasexual cycle becomes a vital tool for generating new and improved strains. The introduction of desired traits through mitotic recombinations enables the development of strains with enhanced characteristics, benefiting sectors like biotechnology and pharmaceuticals.
Parasexuality can also be applied in the analysis of genetic and physiological processes of perfect and imperfect fungi. Parasexual cycle has also been successfully employed in genetic control of pathogenicity and host-range in several species of Fusarium.
FAQs About the Parasexual Cycle in Fungi
- Is the parasexual cycle exclusive to fungi?
- No, while commonly observed in fungi, the parasexual cycle also occurs in single-celled organisms, highlighting its broad significance.
- How does the parasexual cycle contribute to genetic diversity?
- The parasexual cycle involves various steps, including fusion of nuclei and occasional mitotic crossing over. These processes lead to the generation of novel gene combinations, promoting genetic diversity.
- Can the parasexual cycle be harnessed for industrial purposes?
- Absolutely. The parasexual cycle has proven valuable in generating new and improved strains of fungi for industrial applications, such as biotechnology and pharmaceuticals.
- What advantages does the parasexual cycle offer compared to traditional sexual reproduction?
- The parasexual cycle enables genetic diversity and adaptation without strictly following the timeline and constraints of meiosis, providing organisms with flexibility in generating new gene combinations.
- Are there instances of the parasexual cycle in multicellular organisms?
- The parasexual cycle is primarily observed in fungi and single-celled organisms, as these entities can more readily exchange genetic material without the need for complex sexual structures.
