PARASEXUAL CYCLE (Note FOR UG/PG STUDENTS)

Parasexual cycle 

                                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 mechanism .Such an alternative mechanism was observed for the first time by Pontecorvo & Raper (1952) seen in Aapergillus nidulans . They observed that in this specialized reproductive mechanism, plasmogamy , karyogamy and meiosis do take place but not at specified time or specified points in the life cycle of the fungus .They considered the group as Deuteromycetes or fungi imperfecti and the specialized cycle of reproduction as “Parasexual Cycle “

Mechanism of parasexual cycle: According to pontecorvo (1958) parasexual cycle in Aspergillus nidulans involves the following step -----

(a)Formation of heterokryotic mycelium: heterokryotic mycelium is obligatory for parasexual cycle. In Deuteromycetes , heterokaryotic mycelium is formed in several ways. The most common is Anastomosis of somatic hyphac. In this case , the foreign nucleus or nuclei introduced into a mycelium multiplys and its progeny spreads through the mycelium rendering it heterokaryotic. Mutation in one or more nuclei of heterokaryotic mycelium also makes it heterokaryotic.

(b)Nuclear fusion or formation of heterozygous diploid nucleus: Fusion of haploid nuclei of similar or de-similar genotype results in the formation of homozygous or heterozygous diploid nucleus respectively. It the genotype of unlike nuclei present in the heterokaryotic my celium is A&B , than 5 types of nuclei can be formed by their fushion – 2 types of haploid nuclei (A&B) , 2 types of homozygous diploid nuclei (A,A & B,B ) and one types of heterozygous diploid nucleus (AB).

(c) Multiplication of diploid nuclei: The above mention 5 types of nuclei present in the heterokaryotic mycelium presumabiy multiply at about the same rate. But the diploid nuclei are present in much smaller number then the diploid nuclei. It has been estimated that the haploid and the heterozygous diploid nuclei are present in the ratio of 1:1000.

(d)Occational mitotic crossing over : This is the most important step in the parasexual step . Occational crassing over takes place during multiplication of diploid nuclei and this result , in the formation of new gene combination. These recombination are dependent of the exhistens heterokaryosis and they give the fungus some advantages of sexuality with the parasexuality.

                                       According to pontecorvo’s (1958) estimates the amount of recombinations which may be expected to occur in an Ascomycetes through its parasexual cycle is 500 times smaller than through its sexual cycles.

(e)Sorting of diploid nuclei: In those fungi which produce uninucleate conidia shorting of diploid nuclei takes place by their incorporation into conidia. The conidium which contains diploid nucleus , give rise to a mycelium or germination. The conidia of the dipoid strain are smaller than those of haploid strains.

(f)Occational haplodization : Occationally some hyphae of diploid mycelium form a haploid conidia which ultimately form haploid mycelia on germination. It was clearly concluded by the Mycologists that haplodition occura in some diploid nuclei.

(g)Sorting of new haploid strain: New haploid strains are sorted out by the incorporation of haploid nuclei in uninucleate conidia .Some of the haploid strains are genotypically different from their parents because of mitotic recombination.