general description

1. In some fungi, true sexual cycle
comprising of nuclear fusion and
meiosis is absent. These fungi
derive the benefits of sexuality
through a cycle known as
Parasexual Cycle.

3. .
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.

4. As in meiosis , parasexual recombination may occur either by
segregation of whole chromosome during haplodization, or by
mitotic crossing over. The main diference from sexual process
are that steps of parasexual cycle are independent of one
another and the frequency of occurrence of each is much
lower.
Since its discovery in A nidulans, parasexual cycle has been
demonstred in eight other sps pf Aspergillus.
The parasexul cycle has been reviewed by Rpper 1966 and in
phytopathogenic fungi by Timline and McNeil 1969

5. Parsexual cycle has its impotance
1 for mapping markers in sexual and asexul
fungi in lab
2 For strain improvement in biotechnology
3 for creating Biodiversity in nature.

7. 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.

8. i) Formation of heterokaryotic mycelium:
Heterokaryotic mycelium is formed in several ways.
The most common is by the anastomosis of somatic hyphae of
different genetic combinations.
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 heterokaryoticby the
fusion of some of the nuclei and their subsequent
multiplication and spread among the haploid nuclei. In this
type of heterokaryotic mycelium a fixture of haploid and
diploid nuclei occur.

9. The nucelar fusion may be of two types:
(a) fusion between like nuclei and
(b) fusion between unlike nuclei.
The nuclear fusion results in the formation of homozygous or
heterozygous diploid nucleus.
If two types of genotype (unlike nuclei) present in the
heterokaryotic mycelium is A and B, then five types of nuclei can be
formed
ii) Fusion between two nuclei (Karyogamy):
B B BABA AA

10. (iii) Multiplication of diploid nuclei:
( one diploid heterozygous nucleus to 1000 haploid nuclei.)
B
B
B
B
A
AB
A
A
AB
AA
A
A
A
AA
A
Multiplication of Diploid nuclei

11. v) Occasional mitotic crossing over:
During multiplication of diploid nuclei,
mitotic crossing over may take place.
results in the formation of new gene combinations.
give the fungus some of the advantages of sexuality
recombinations is 500 times lesser

12. (v) Sorting out of Diploid nuclei:
diploid conidia which germinate to produce
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.

13. vi) 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 confirms haploidisation in some diploid
nuclei.

14. vii) Sorting of new haploid strains:
Some diploid nuclei undergo haploidisation in
the mycelium and noticed in the uninucleate
conidia.
Some of these haploid strains are genotypically
different from their parents because of their
mitotic recombinations.