HETEROKARYOSIS AND PARESEXUALITY -Vinod Upadhyay

ID No - 44056

HETEROKARYOSISHeterokaryosis - 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.

FORMATION OF HETEROKARYOSISHeterokaryotic condition arises by-MutationAnastomosis 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.

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.

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.

SIGNIFICANCE OF HETEROKARYOSISSubstitute for heterozygosity and variability

Heterokaryosis and pathogenicity- e.g. in rusts and smuts

Origin of new race

Initial step in Parasexual cycle

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.

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.

STEPS OF PARASEXUAL CYCLEEstablishment of heterokaryosis

Formation of heterozygous diploids

Occasional mitotic crossing- over, during multiplication of the diploid nuclei, and

Haploidization through aneuploidy

ESTABLISHMENT OF HETEROKARYOSIS

The presence of haploid nuclei of dissimilar genotypes in the same cytoplasm

pre-requisite for recombination.

Heterokaryosis is brought about by-MutationAnastomosis Inclusion of dissimilar nuclei in spores after

meiosis, in heterothallic fungi.

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.

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”.

Parasexual cycle in filamentous Ascomycetes

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.

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.

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.

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.

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.

Fungi Antibiotic Aspergillus nidulans Penicillin G Acremonium chrysogenum

Cephalosporin C

Emerocellopsis salmosynnemata Penicillin N Penicillium chrysogenum Penicillin G,

O, V Penicillium patulum

Griseofulvin,patulin

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.

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 ?

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

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