Parasexuality
Earlier, it was believed that genetic recombination can only be achieved by sexual reproduction, but studies on bacteria have shown that it is also possible through alternative mechanisms. Such an alternative mechanism was observed for the first time by Pontecorvo and Raper in Aspergillus nidulans and they called it as parasexual cycle. In this process, plasmogamy, karyogamy and meiosis do place, but not at specified time or specified points in the life-cycle of the fungus. The phenomenon of parasexuality is of common occurrence in the members of the class Deuteromycetes. Sexual reproduction is absent in these fungi and hence the parasexual cycle is of much significance. Now several fungi are known where both sexual and parasexuality has been studied: Aspergillus nidulans..etc.
Mechanism of Parasexual Cycle
The parasexual cycle is completed in a number of steps as shown by Pontecorvo in Aspergillus nidulans. The sequence of these steps is as follows.
I) Formation of heterokaryotic mycelium
Formation of heterokaryotic mycelium is necessary for genetic recombination. The anastomosis of somatic hyphae of different genetic constitution is the most common method for the formation of heterokaryotic mycelium. But heterokaryotic mycelium may also be formed by the fusion of some nuclei and their subsequent division or by the mutation of one or more nuclei of the homokaryotic mycelium as in some Ascomycetes.
II) Nuclear fusion or formation of heterozygous diploid nucleus
Fusion of haploid nuclei of similar or dissimilar genotypes results in the formation of homozygous 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, two types of homozygous diploid nuclei and one type of heterozygous diploid nucleus.
III) Multiplication of diploid nuclei
The above mentioned five types of nuclei present in the heterokaryotic mycelium presumbly multiply at about the same rate, but the diploid nuclei are present in much smaller number than the haploid nuclei. It has been estimated that the haploid and the heterozygous diploid nuclei are present in the ratio of one is to thousand.
IV) Occassional mitotic crossing over
This is the most important step in the parasexual cycle. Occassional crossing over takes place during multiplication of diploid nuclei and this results in the formation of new gene combinations. These recombinations are dependent on the existence of heterokaryosis and they give the fungus some advantages of sexuality within the parasexual cycle. In Aspergillus nidulans, mitotic crossing over occurs only rarely, with a frequency of 10 the power of 2 per nuclear division. However, in some fungi, it is as frequent as during meosis.
V) Sorting of diploid nuclei
In those fungi which produce uninucleate conidia, sorting of diploid nuclei takes place by their incorporation into conidia. The conidium, which contains diploid nucleus, gives rise to a diploid mycelium on germination. Diploid strains of several imperfect fungi are now known. The conidia of the diploid strains are smaller than those of the haploid strains.
VI) Occasional haploidization of diploid nuclei
Occasionally some hyphae of diploid mycelium form haploid conidia which form haploid mycelia on germination. Formation of haploid conidia by diploid mycelium indicates that haploidization occurs in some diploid nuclei.
VII) Sorting of new haploid strains
New haploid strains are sorted out by the incorporation of haploid nuclei in uninucleate conidia. Some of these haploid strains are genotypically different from their parents because of mitotic recombination.
After the operation of parasexual cycle in the mycelium for sometime, the fallowing types of nuclei may occur i) haploid nuclei in both the mycelia, i,e. the mycelia of the two different genetic constitution which form heterokaryotic mycelium, ii) some haploid nuclei of new genetic constitution by genetic recombinations, iii) several types of haploid homozygous nuclei, and iv) several types of diploid heterozygous nuclei.
Application of Parasexuality
The phenomenon of parasexuality is applied in several industrial processes. Most of the fungi which are significant in industries belong to the imperfect group of fungi and they reproduce by parasexual cycle. The better strains of these fungi are obtained by induced mutation or by making suitable modification in the culture medium. The strains of desirable characters can be developed by mitotic recombination. Parasexuality is also applied in the analysis of genetic and physiological processes of perfect and imperfect fungi. There has been great success in obtaining genetic control of pathogenicity and host range in several species of Fusarium by parasexual cycle.
References:
A text book of botany
