PARASEXUALITY AND PARASEXUAL LIFE CYCLE IN FUNGI

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In some fungi, true sexual cycle comprising of nuclear fusion and meiosis is absent. These fungi complete the sexuality through a cycle known as parasexual cycle. The parasexual cycle is defined as a cycle in which plasmogamy, karyogamy and meiosis takes place but not at a specified time or at specified points in the life cycle of fungi. Generally parasexuality occurs in those fungi in which true sexual cycle doesnot takes place. The members of class Deuteromycetes in which sexual cycle doesnot occur, exhibit parasexual cycle generally.
Parasexual cycle was discovered by Pontecarvo and Roper (1952) in Aspergillus nidulans, the imperfect stage of Emericella nidulans. Since then, parasexual cycle has been discovered not only in members of Deuteromycetes but also in many members belonging to Basidiomycetes and Ascomycetes. Parasexual cycle, also known as somatic recombinations has been reported in Aspergillus nidulans, A. niger, A. fumigatus, A. oryzae, Penicillium chrysogenum, P. expansum, P. italicum, Fusarium oxysporum, Puccinia graminis tritici, etc.
According to Pontecarvo (1958), parasexual cycle in A. nidulans involves the following steps –
1. Formation of heterocaryotic mycelium:- Heterokaryotic mycelium may be formed by several ways. (a) The most common method is by fusion of protoplast of two somatic hyphae of different genetic recombinations. (b) Mutation of one or more nuclei of monokaryotic mycelium also makes heterokaryotic. (c) The third way is by the fusion of some of the nuclei and their subsequent multiplication and spread among the haploid nuclei.
2. Fusion between two nuclei:- The fusion of nuclei takes place in the mycelium. Fusion of nuclei may be of two types – (a) Fusion between the like nuclei resulting in the formation of homozygous diploid nucleus. (b) Fusion between unlike nuclei resulting in the formation of heterozygous diploid nucleus.
3. Multiplication of diploid nuclei:- The diploid nuclei multiply soon after the fusion. Portecarvo estimated that the ratio between the diploid heterozygous nuclei and homozygous nuclei is 1:1000.
4. Ocassional mitotic crossing over:- During multiplication of the diploid nuclei, mitotic crossing over may take place which results in the 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.
5. Sorting out of diploid nuclei:- In fungi which produce uninucleate conidia, sorting out of diploid nuclei occurs which produce diploid conidia. Such diploid conidia germinate to produce diploid mycelia. The conidia of diploid strains are somewhat larger than those of haploid strains.
6. Occasional haploidisation of the diploid nuclei:- Ocassionally, 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.
IMPORTANCE OF PARASEXUALITY
1. Parasexuality is of importance. Several fungi which are used in various industrial processes belong to fungi imperfecti and in these fungi only parasexual cycle operates.
2. New and better strains of these fungi are obtained by mutation through parasexual cycle.
3. The strains of desirable characters can be developed through mitotic recombinations.
4. Parasexuality can also be applied in the analysis of genetic and physiological processes of perfect and imperfect fungi.
            5. Parasexuality can also be successfully employed in genetic control of pathogenicity and host range in several species of Fusarium.

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