L.13-G.Biology
Mycology
D.Ebtihal Muiz
Parasexual Cycle in Deuteromycota
There are many species of Deuteromycota in which a sexual stage is
not known. Of these, there are, undoubtedly, species in which sexual
reproduction occurs only in a restricted set of environmental conditions
so that the occurrence of the sexual stage is infrequent. However, it is
also apparent that some species have lost the ability to reproduce,
sexually. Yet, many of the Deuteromycota are highly successful in their
environment. Since sexual reproduction is the means by which genetic
diversity is maintained in eukaryotic organisms, and diversity is the the
key to survival in species, how would a species that has apparently lost
the ability to reproduce, sexually, survive? A possible mechanism that
provides an answer to this question is the parasexual cycle. This is a
process in which plasmogamy, karyogamy and haploidization takes place,
but not in any particular place in the thallus nor at any specific period
during its lifecycle.
Parasexuality was first discovered by Pontecorvo and Roper (1952) in
Aspergillus nidulans. During the parasexual cycle, the following events
take place:
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Formation of heterokaryotic mycelium (Fig. 1).
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Occasional karyogamy between two nuclei to form diploid nuclei
(Fig.2).
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Mitosis of 2N and 1N nuclei.
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Mitotic crossing over during mitosis of some diploid nuclei
(Fig. 3).
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Haploidization (not meiosis) of some diploid nuclei (Fig. 4).
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Sorting out of new haploid strains.
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Figure 1. Heterokaryon formation refers
to the condition by which genetically
different nuclei are associated in a
common cytoplasm. The most common
way in which this can occur is by
anastomosis (fusion) of genetically
different hyphae (see Fig. 1a on left).
Another means by which genetically
different nuclei may enter a common
protoplasm is by mutation of one or
several nuclei. We will refer to the former
in this description of parasexuality.
Following initial fusion of hyphal cells, to
form a genetically different cell, mitotic
division perpetuates the cell and
mycelium that is made up of genetically,
different nuclei is formed.
Figure 2. Karyogamy and mitotic
division of diploid nuclei: Following
heterokaryon formation, fusion of some
haploid nuclei that are genetically the
same will fuse as well as those that are
genetically different. The latter will result
in heterozygous diploid nuclei. It is
estimated
that there is one heterozygous diploid nucleus will occur per one
million haploid nuclei (Pontecorvo, 1958).
Figure 3. Mitotic Crossing Over: Figs.
3a-b. During prophase of mitosis, mitotic
crossing over can occur between
chromatids of homolous chromosomes
and may produce a unique genetic
recombinant. Fig 3c. Recombinant
chromosomes separate, during anaphase,
and give rise to nuclei that are genetically
different from existing nuclei in
protoplasm. This is also a rare event,
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occurring in diploid nuclei, once, in 500
mitoses.
Figure 4. Haploidization (not meiosis) of
diploid nuclei. During mitosis, errors are
common. Diploid nuclei often form one
nucleus with three copies of one
chromosome
(2N+1) and the other with one copy of one chromosome (2N-1).
In the latter nucleus, the continual, sequential loss of
chromosomes with two copies can occur to eventeually give rise
to a haploid nucleus. When haploidization occurs in heterozygous
diploids, the resulting haploid will result in a new genetic
combination.
While the parasexual cycle appears to be a viable mechanism by which
genetic recombination occurs, many mycologist believe that it does not
play a role in maintaining genetic diversity in fungi that have lost their
ability to reproduce, sexually. Instead, this has been looked upon as a
laboratory phenomenon and that heterokaryon formation, in nature, is not
a common event. Thus, the parasexual cycle must also be a rare event.
The Deuteromycota (Greek for "second fungi") were once considered a
formal phylum of the kingdom Fungi. The term is now used only
informally, to denote species of fungi that are asexually reproducing
members of the fungal phyla Ascomycota and Basidiomycota.
There are about 25,000 species that have been classified in the
deuteromycota. Fungi producing the antibiotic penicillin and those that
cause athlete's foot and yeast infections are imperfect fungi. In addition,
there are a number of edible imperfect fungi, including the ones that
provide the distinctive characteristics of Roquefort and Camembert
cheese.
Emericella nidulans.
Phylogeny and taxonomy
Phylogenetic classification of asexually reproducing fungi now
commonly uses molecular systematics. Phylogenetic trees constructed
from comparative analyses of DNA sequences, such as RNA, or
multigene phylogenies may be used to infer relationships between
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asexually reproducing fungi and their sexually reproducing counterparts.
With these methods, many asexually reproducing fungi have now been
placed in the tree of life] However, because phylogenetic methods require
sufficient quantities of biological materials (spores or fresh specimens)
that are from pure (i.e., uncontaminated) fungal cultures, for many
asexual species their exact relationship with other fungal species has yet
to be determined. Under the current system of fungal nomenclature,
teleomorph names cannot be applied to fungi that lack sexual structures.
Classifying and naming asexually reproducing fungi is the subject of
ongoing debate in the mycological community.
Common species
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Tolypocladium inflatum → from which we obtain the
immunosuppressant ciclosporin;
Penicillium chrysogenum
Penicillium griseofulvum
Penicillium roqueforti
Penicillium camemberti
Other species of Penicillium are used to improve both the taste and
the texture of cheeses
Aspergillus oryzae[4]
Aspergillus sojae[5]
Aspergillus niger[6]
Cladosporium resinae
Lecanicillium sp. → these produce conidia which may control
certain species of insect pests
Other entomopathogenic fungi, including Metarhizium and
Beauveria spp.
Pochonia spp. are under development for control of Nematode
pests.
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