This phenomenon is common i deuteromycetes

1. HETEROKARYOSIS
AND
PARESEXUALITY
-Vinod Upadhyay
ID No - 44056

2. HETEROKARYOSIS
 Heterokaryosis - co-existence of genetically-
different nuclei in cytoplasm continuity with one
another.
 Discovered by Hansen and Smith (1932) in
Botrytis cinerea.
 plays major role - variability and sexuality in
fungi.

3. FORMATION OF HETEROKARYOSIS
Heterokaryotic condition arises by-
 Mutation
 Anastomosis
 Inclusion of dissimilar nuclei in spores after
meiosis, in heterothallic fungi.
Mutation- A high frequency of mutation is
characteristic of fungi - main source of
variability.

4. Anastomosis (fusion of hyphae)-
Fusion is mostly intra-specific.
 Nuclear migration from the point of fusion to the
remainder of the mycelium takes place
- heterokaryotic mycelium.
 eg- development of heterokaryon in
basidiomycota.

5. Inclusion of dissimilar nuclei in spores after meiosis,
in heterothallic fungi-
 Meiosis results in the production of genetically
different nuclei sharing common cytoplasm.
 e.g. Neurospora tetrasperma, Podospora anserine
 on germination - give rise to a heterokaryotic thallus.
 In the asexul phase - occurs frequently in
multinucleate spores.

7. SIGNIFICANCE OF HETEROKARYOSIS
 Substitute for heterozygosity and variability
 Heterokaryosis and pathogenicity- e.g. in rusts
and smuts
 Origin of new race
 Initial step in Parasexual cycle

8. PARASEXUALITY
 discovered in fungi (Aspergillus nidulans) by
Pontecorvo and Roper (1952)
 Parasexuality - genetic recombination is achieved
through “ mitotic crossing over” and “
hapoidization”.
 also known as somatic recombination.

9.  Sexual reproduction - extremely uniform - fine
coordination between recombination,
segregation and reduction
 Parasexual cycle lack such co-ordination
 The steps of the parasexual - independent of
each other and the frequency of each is very low.
 The karyogamy and haploidization are
accidential events not bound by space and time.

10. STEPS OF PARASEXUAL CYCLE
 Establishment of heterokaryosis
 Formation of heterozygous diploids
 Occasional mitotic crossing- over, during
multiplication of the diploid nuclei, and
 Haploidization through aneuploidy

11. ESTABLISHMENT OF HETEROKARYOSIS
 The presence of haploid nuclei of dissimilar
genotypes in the same cytoplasm
 pre-requisite for recombination.
 Heterokaryosis is brought about by-
 Mutation
 Anastomosis
 Inclusion of dissimilar nuclei in spores after
meiosis, in heterothallic fungi.

12. FORMATION OF HETEROZYGOUS
DIPLOIDS
 Nuclear fusion in heterokaryotic somatic cells
was first noted by Roper (1952) in Aspergillus
nidulans.
 The nuclear fusion between dissimilar nuclei -
the formation of heterozygous diploid nuclei or
“zygotes”
 a rare event, occurring at the rate of one in a
million.

13.  The heterozygous diploid nuclei - fairly stable
 The diploid colonies are recognized by-
 higher DNA content of their nuclei
 the bigger size of their conidia
 certain phenotypic characteristics of their colony
 The prolonged diploid phase involving repeated
nuclear divisions, enhances the chances of
“mitotic crossing over”.

14. Parasexual cycle in filamentous Ascomycetes

15. OCCASIONAL MITOTIC CROSSING OVER
DURING MULTIPLICATION OF DIPLOID
NUCLEI
 genetic recombination takes place.
 Penicllium chrysogenum and Aspergillus niger-
mitotic crossing over is as frequent as during
meiosis.

16.  In mitotic crossing over - exchange, or
chiasmata formation - confined to a single
chromosome pair out of the whole component
of chromosomes.
 In meiosis, the crossing – over occurs
simultaneously in all the chromosomes.
 splitting of chromosomes and segregation of
strands is same in mitotic crossing over as it
occurs in meiosis.

17. OCCASSIONAL HAPLOIDIZATION
THROUGH ANEUPLOIDY
 The diploid nuclei - haploid nuclei -gradual loss
of chromosomes during successive mitotic
division - haploidization.
 Meiosis is not involved.
 The haploidization occurs at a constant rate of 10-
3 per nuclear division - the result of aneuploidy.

18.  During mitotic division - non-disjunction of the
chromatids of one chromosome pair - results in
aneuploid nuclei (2N-N) or haploid.
 The aneuploids – unstable - the loss of
chromosomes - favours the development of fully
balanced haploid nuclei.

19. USE OF PARASEXUAL PHENOMENON IN
ANTIBIOTIC INDUSTRY
 most important antibiotic producing fungi, like
Penicillium chrysogenum (penicillin) and
Acremonium chrysogenum (cephalosporin)
 discovery of parasexual phenomenon - suitable
techniques to obtain higher antibiotic strains.

20.  Fungi Antibiotic
Aspergillus nidulans Penicillin G
Acremonium chrysogenum Cephalosporin C
Emerocellopsis salmosynnemata Penicillin N
Penicillium chrysogenum Penicillin G, O,V
Penicillium patulum Griseofulvin,patulin

21. Parasexual cycle closely simulates the events of
the sexual cycle. It involves everything:
plasmogamy, karyogamy and haploidization, but
in a modified form, and without any fixed plan
with regard to time and space.

22. INTERESTING
QUESTION?????????????????????
 If a fungus is having 5 haploid chromosome
and entered in the parasexual recombination.
Calculate the total variability in the pathogen
due to heteroploidy and Parasexual
recombination ?

23. Ans: Heteroploidy= 2n-2, here, n=5
variability due to heteroploidy = 25-2= 32-2 = 30
variability due to Parasexual recombination =
2n-2 ×5= 30×5 = 150

24. THANK YOU