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Educational
Communication
Module on
Fungi: Degeneration of Sex
By
Dr. Mudassir Ahmad
Lecturer in Botany
Mobile No.: +91 – 9419490026
Email: [email protected]
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Text
Fungi are eukaryotic, spore-bearing achlorophyllous
organisms that generally reproduce asexually and sexually,
and whose usually filamentous branched somatic structures
are typically surrounded by cell walls containing chitin and/or
cellulose. The lower fungi or Phycomycetes (‘phycos’ meaning
alga, and ‘mycetes’ meaning mushrooms) comprise the simplest
and most primitive group of fungi, represented by about 1500
species. Sexual process ranges from isogamy to oogamy through
anisogamy. Bringing together male and female protoplasts
or nuclei, that is plasmogamy is achieved by planogametic
copulation (in Chytridiomycetes, Plasmodiophoromycetes),
gametangial contact (Oomycetes) and gametangial copulation
(Mucorales).
Planogametic copulation: This type of sexual reproduction
involves the fusion of two naked gametes where one or both of
them are motile. The motile gametes are known as planogametes.
The most primitive fungi produce isogamous planogametes, e.g.
Synchytrium, Olpidium, Plasmodiophora, etc. The anisogamous
planogametes are only found in the genus Allomyces of order
Blastocladiales. Monoblepharis illustrates the most advanced
type of planogametic copulation. It produces small opisthocont
male gametes (sperms) in large numbers in the sex organ called
antheridium. The oogonium produces a single large immobile
female gamete, the egg or ovum. The male gamete enters the
oogonium and fertilizes the egg.
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Fig 1: Planogametic copulation in Phycomycetes
Gametangial contact: A group of aplanogametic lower fungi
(Oomycetes) produce nonmotile gametes (aplanogametes)
that are never released. The male gametangium is called
the antheridium and the female oogonium. No sperms are
organised. The gametangia do not actually fuse and do not
lose their identity. The antheridium puts out a slender, tubular
outgrowth – the fertilization tube at the point of contact with the
oogonium. Meiosis is gametangial. Examples are Saprolegnia,
Achlya, Pythium, Phytophthora, etc.
Fig 2: Gametangial contact in Phycomycetes
Gametangial copulation: In another group of aplanogametic
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lower fungi (terrestrial Mucorales – Mucor, Rhizopus,
Entomophthora, etc), gametic union is brought about by the
fusion between the gametangia. The uniting gametangia lose
their identity in the sexual act. The intervening walls between
the two gametangia dissolve, forming a common fusion cell.
Plasmogamy is immediately followed by karyogamy. The
diploid contents of the zygospore or the diploid nuclei undergo
zygotic meiosis, the meiospores or meiozoospores on liberation
germinate to produce a new haplomycelium.
It is obvious that sexual reproduction arose several times
in evolutionary development of lower fungi. There is no linear
series to suggest that all the lower forms are isogamous and
higher forms oogamous. Sexual reproduction becomes less
frequent than asexual and is resorted to towards the end of
the growing season when conditions unfavourable for growth
are to set in. Zygospores help to tide over unfavourable period.
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Fig 3: Gametangial copulation in Zygomycetes
The gradual transition from the aquatic forms to purely
terrestrial species noticeable in the phycomycetes culminates
in Ascomycetes which are fungi of drier terrestrial habitats
completely lacking motile cells in their life cycle. From the
primitive to the more advanced zygomycetous phycomycetes,
there is a tendency for the sporangia to be transformed into
sporangioles and these into conidia – the typical asexual
spores of Ascomycetes that have played significant role in their
spread. In fact, in some species of Ascomycetes conidia alone
appear to be sufficient for survival.
Degeneration of Sex in Ascomycetes
The ascomycetes are characterized by the presence of an
ascus. The ascus is the product of sexual act and the ascospores
produced inside it are thus sexual spores. The young ascus is
always binucleate (n + n). These two nuclei fuse (Karyogamy)
to form the diploid zygote nucleus (2n). This is followed by
3 successive nuclear divisions, the first division is normally
reductional, the others equational forming a definite no of
ascospores (usually 8) by free cell formation. Some important
features of the sexual apparatus of the ascomycetes are:
(i)
The presence of receptive hyphae, the trichogyne.
(ii)
Formation of dikaryophase
(iii)
Maintenance of dikaryophase through conjugate
divisions of nuclei.
(iv)
Formation of ascogenous cells and hyphae.
(v)
Karyogamy in the ascus mother cell.
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(vi)
Formation of ascospores by free cell formation.
These are the basic processes found in the group.
The differences, however, arise in the form and the type of
sexual apparatus, the manner and the extent of formation of
ascogenous hyphae and the type and form of fructifications
produced. Moreover, the various methods and mechanisms that
bring the compatible nuclei together in the progressive process
of reduction in sexuality vary widely in different groups of fungi
and are discussed below:
A) Gametangial copulation: This process may be regarded
similar to that found in the zygomycotina. This persists among the
primitive ascomycetes (i.e. Hemiascomycetes). In this method
two morphologically similar gametangia copulate in the same
manner as those of the zygomycetes. Plasmogamy takes place, a
zygote is immediately formed at the fusion bridge and the zygote
gets converted into an ascus. No dikaryotic phase is developed
because plasmogamy is immediately followed by karyogamy.
This is exemplified by members of the family Dipodascaceae
(Endomycetales). But interestingly enough, the hyphal cells or
gametangia may be multinucleate as in Dipodascopis albidus
or uninucleate as in Dipodascus aggregatus and Dipodascopis
uninucleatus The fusion of the multinucleate gametangia of D.
albidus is reminiscent of gametangial copulation in some of the
zygosmycetes and the multispored ascus has been compared
to the germ sporangium (Alexopoulos and Mims, 1979).
The life cycle of Dipodascus aggregatus and Dipodascopsis
uninucleatus are a step ahead of that of Dipodascus albidus which
have become simplified by the elimination of the nonfunctional
supernumerary nuclei from the very beginning.
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In the family Saccharomycetaceae, however, modification
of the above process can be seen as complete suppression of
the specialized gametangia and copulation takes place between
vegetative cells, a process known as somatogamy. The gradual
suppression of the sexual phenomenon is characteristic of yeast
fungi. This shows an increasing trend in the degeneration of
the process.
The formation of ascospores in yeasts is contingent upon
a sexual process. Of course no sex organs like antheridia and
oogonia are produced. Instead the sexual process is extremely
simplified. It consists of three phenomena characteristic of
the sexual process, namely plasmogamy (sexual fusion),
karyogamy (fusion of nuclei) and meiosis.
In the culture of brewer’s or baker’s yeast (Saccharomyces
cerevisiae) there occur intermixed two kinds of somatic cells,
namely small dwarf strain and large strain cells. Under normal
conditions, but absence of cells of the opposite mating type,
these multiply by budding. Under normal conditions and
presence of small haploid cells of the opposite mating strain,
they function as gametangia and resort to sexual or gametangial
conjugation. During conjugation + and – strains of haploid
dwarf yeast cells agglutinize to form clusters. In this condition
each mating type secretes a sex hormone (Tkacz and Mackay
1977) which induces the haploid spherical cells A and B of the
opposite strains to elongate to a pear shaped form (A2 & B2)
and also causes alterations in the cell walls of the newly formed
regions. These surface alterations facilitate fusion. The zygotes
formed as a result of sexual fusions between two haploid yeast
cells of opposite mating types are, in fact the large strain yeast
cells. During unfavourable conditions, these diploid large strain
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yeast cells resort to ascospore formation.
In Saccharomycodes ludwigii, the ascospores directly
function as gametangia and copulate. Sexual fusion takes
place within the ascus between the two adjacent ascospores of
opposite mating types A and a or + and – strains. Consequently
two diploid cells or zygotes are formed within the wall of the
ascus. Each zygote germinates in situ to form a small germ-tube
which grows and emerges through the ascal wall and functions
as a sprout mycelium. The cells of the sprout mycelium bud
off diploid yeast cells. The latter separate from the sprout
mycelium not by a constriction but by the formation of a
septum at the base. Soon the diploid sprout cell gets severed
from the parent cell. This detached diploid somatic or sprout
cell functions as ascus mother cell. Each of these becomes an
ascus after its diploid nucleus undergoes meiosis to produce
ascospores. It is obvious that the haplophase (gametophyte) in
Saccharomycodes ludwigii is extremely reduced, represented
only by ascospores.
In fission yeast, Schizosaccharomyces octosporous, each
haploid somatic cell is a potential gametangium. At the time of
sexual reproduction two somatic cells come to lie side by side
and each sends out a short narrow beak like process, the two
meet, intervening walls dissolve to form a conjugation tube
later functioning directly as ascus mother cell.
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Fig 4: Sexuality in Schizosaccharomyces
octosporus
Further modification of the sexual process is seen
in the Taphrinales, where any (dikaryotic) vegetative cell
becomes an ascus. Moreover the ascospores are capable of
budding; copulation occurs between blastospores giving rise
to binucleate mycelium in the host. Thus the intervention
of the dikaryophase ultimately leads to karyogamy in the
ascus mother cell followed by meiosis and the production of
a definite number of ascospores in the ascus. The type of
sexuality exemplified by Taphrinales has several interesting
phases, both of degenerative and evolutionary character, such
as suppression of specialized gametangia, copulation between
blastospores, development of dikaryophase, formation of
ascogenous cells, delayed karyogamy, alternation of haplophase
with dikaryophase and production of many asci and ascopores
with a single plasmogamous fusion.
B). Gametangial contact: This process though similar to the
oogamous type in its initial stages, has many evolutionary and
distinct features. In this method well-developed uninucleate or
multinucleate gametangia develop. The males are designated
antheridia and the females’ ascogonia, which are provided with
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a specialized receptive hypha, the trichogyne, not formed in the
oomycetes. The male nucleus or nuclei migrate into the female
via the trichogyne, do not fuse but remain associated, thus
undergoing dikaryophase. Then one or more ascogenous hyphae
arise from the ascogonium, and ultimately asci are formed by
the crozier or hook method, characteristic of the ascomycetes.
This type of sexuality occurs commonly in taxa belonging to
several groups such as Plectomycetes, Pyrenomycetes and
Discomycetes.
Fig 5: Gametangial contact in Ascomycetes
•Class Plectomycetes includes the Ascomycotina in which
the asci are irregularly arranged within the ascocarp. Many
members show degeneration of sex organs especially of male
gametangium.
Let us illustrate sexuality in Aspergillus (from Plectomycetes)
and see how it throws light on the degeneration of sex in
Ascomycetes: The sexual or perfect stage is rather rare. Possibly
the species lacking it have lost it during the course of evolution.
This view is supported by the fact that even in species that
form asci there is evidence of sexual degeneration. Different
species of this genus show variation in their sexual behaviour.
These different species of Aspergillus illustrate the way in which
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elimination of male sex organ (antheridium) in the Ascomycetes
has taken place. These species can be arranged in series. The
series indicates the stages or steps in which the progressive
degeneration of the male sex organ (antheridium) has been
accomplished. These steps are:
i) In some there is a degeneration of male nuclei,
whereas in others the antheridium is rudimentary,
non-functional or even absent.
ii) In Aspergillus herbariorum the pollinodium cuts off a
terminal antheridium. The tip of the antheridium
fuses with that of trichogyne but there is no
migration of contents of the antheridium towards
the trichogyne.
iii) In some species the antheridium may develop
late. By that time the ascogenous hyphae have
already been developed from the ascogonium.
e.g. A. nidulans
iv) In a few species, the male nuclei in the antheridium
are reported to degenerate before the antheridium
has reached maturity.
v) The antheridium in some species, such as A. flavus,
A. fisheri and A. fumigatus does not develop at
all. Only the ascogonia are seen to be developing.
In Eurotium, the antheridia and ascogonia are produced
close to each other on the aerial somatic hyphae. The antheridium
may or may not be functional but pairing of the nuclei takes
place in ascogonium. When the antheridium is functional, its
nuclei enter the ascogonium and pair with its nuclei. However,
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when the antheridium is non-functional, the ascogonial nuclei
form pairs.
No sex organs have been found in Elaphomyces, but
Bessey (1950) and Gaumann, (1952) reported a multinucleate
ascogonium coiled around an antheridium in the closely related
Ascoscleroderma. He believed, however that the antheridium
does not function. The ascogonium produces ascogenous
hyphae the tips of which elongate after karyogamy and form
asci directly without crosiers. It appears that in Elaphomyces,
however, crosiers are formed.
Over 100 species of Penicillium are known and in most
of them ascocarp formation is not known. Like Aspergillus,
Penicillium is also a form genus based on conidial morphology.
Only a few species are known to reproduce sexually. The sexual
or perfect state of Penicillium has been assigned to different
genera, such as Talaromyces, Eupenicillium and Carpenteles
(Webster, 1980) A few species that show sexual reproduction
are Penicillium vermiculatum (=Talaromyces vermiculatus)
and P. stipitatum. Here the mycelium contains uninucleate
cells from which develops a uninucleate ascogonium. The
ascogonium elongates and its nucleus divides several times to
produce as many as 64 nuclei. Simultaneously, a uninucleate
antheridium also develops and coils spirally around the
multinucleate ascogonium. The antheridial nucleus does not
migrate into the ascogonium. It remains very much there in
the antheridium, even after the development of ascogenous
hyphae. This indicates that antheridia are formed but remain
non-functional.
In Sartorya, the ascoma originates exclusively from the
ascogonium and the antheridium is not formed at all. Emericella
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with 15 species bears no sex organs-neither ascogonia nor
antheridia and the ascoma appears to originate from a loose
hyphal knot.
• Pyrenomycetes is a class of sub division Ascomycotina
(Ainsworth, 1971) and includes the fungi in which the
ascocarp is usually of perithecium type and the asci are
inoperculate with an apical pore or slit. Here again the
actual process of nuclear fusion and fertilization are still
not clearly known in most of the species of Erysiphe and
others, even the actual process of ascosporogenesis is still
not clearly known in most of the species. In Sphaeriales,
sexual reproduction takes place by spermatia uniting with
the trichogyne of ascogonia. Many species bear antheridia
but are generally non functional. In Neurospora crassa the
spermatia or conidia act as male cells, female sex organ is
represented by protoperithecium or bulbil (Webster, 1980)
• Discomycetes are the ascomycetes with apothecium type
of fruiting bodies which are cup, saucer or disc-shaped.
Members show progressive reduction in their sexuality. The
antheridia and ascogonia are both functional in Pyronema.
In Lachnea stercorea, the antheridia are non-functional and
in L. cretea the antheridia are totally absent. Along with
the total absence of antheridia, Humaria granulata shows
the disappearance of trichogyne from the ascogonium.
Morchella shows the total absence of both antheridia and
ascogonia. The sex organs which constitute the accessory
parts of sexual process are completely suppressed in
Morchella. Consequently, the sexual process is extremely
simplified. It involves two distinct processes, namely
Plasmogamy and Karyogamy. The later is immediately
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followed by meiosis.
a)
Plasmogamy: It consists in the union of cytoplasmic
contents of two cells without nuclear fusion, which is
delayed. This results in a sequence of binucleate cell
generations, constituting the Dikaryophase. Plasmogamy
occurs at a very late stage during the development of
the ascocarp in the subhymenial layer. It takes place just
before the formation of asci. Plasmogamy in Morchella
takes place by the following two methods:
i)
Somatogamous Copulation: Two vegetative
hyphae of the subhymenium region of the pileus
come in contact. The intervening walls between the
copulating cells dissolve at the point of contact. The
two multinucleate protoplasts intermingle in the
fusion cell. Two functional nuclei, one from each
copulating cell form a dikaryon. All other nuclei in
the fusion cell disappear. Later ascogenous hyphae
arise from the fusion cell containing the dikaryon.
Each young ascogenous hypha receives a pair of
nuclei. These are the derivatives of the parent
dikaryon. The ascogenous hyphae afterwards
become septate. The terminal cells of these
hyphae function as ascus mother cells. They are
binucleate. The nuclei are the derivatives of the
parent dikaryon.
ii) Autogamous pairing: It has been reported in
Morchella elata. In this species, any vegetative
cell of the sub-hymenium (Hypothecium) may
establish a dikaryon. It is accomplished by
pairing of two of its own vegetative nuclei. It is
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called autogamous pairing. The remaining nuclei
in the cell degenerate. The cells with dikaryons
(dikaryotic cells) develop ascogenous hyphae as
in somatogamous copulation.
b)
Karyogamy: The two nuclei in the ascus mother cell
fuse. The fusion cell with a diploid nucleus is called the
young ascus. It represents the transitory diplophase in
the life cycle. The asci in Morchella thus develop from the
tip cells of ascogenous hyphae without the formation of
crosiers.
c)Meiosis: The young ascus cell elongates. The synkaryon
in the ascus undergoes two successive divisions.
These constitute meiosis. During meiosis the number
of chromosomes in the resultant four daughter nuclei
is reduced to half the diploid number. The four haploid
nuclei undergo the third division. It is mitotic. In this way
eight haploid daughter nuclei are formed in each ascus
cell. Each of these is fashioned into an ascospore. With
the formation of ascospores, the haploid or gametophyte
phase is again initiated in the life cycle of Morchella.
It is speculated that the over wintering stage in the life cycle
of Morchella is the sclerotium.
The sexual apparatus is wholly lacking in Peziza vesiculosa.
This does not prevent the development of fructification or the
apothecium. The sexual process does take place, however it is
extremely simplified and consists in the association of two purely
vegetative nuclei in a pair to form a dikaryon by somatogamous
copulation of adjacent hyphae or by autogamous pairing.
Sexual Reproduction in Coprobia (syn: Humaria) varies
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widely. In C. granulatus only the female sex organ is developed
and is called ascogonium. The antheridium is lacking. It is only
when the fusion between two vegetative hyphae of opposite
strains of mycelia takes place that the ascogonium develops
further. Coprobia rutilans develops no sex organs. The entire
sexual apparatus is absent. The sexual process is extremely
simplified. Sexual fusion takes place between any two vegetative
cells of the hyphae of opposite strains.
In Ascobolus furfuraceus, the antheridium is absent and
the function of the male component is performed by oidia
produced in chains. The ascogonium develops only when the
oidium comes in contact with the mycelium of opposite strain.
In Ascobolus citrinus both the antheridia and oidia are absent.
The development of apothecium from the female branch is
parthenogenetic with copulation.
C). Autogamy: This is a stage in the retrogressive evolution
of sexuality, where the female sex organ is retained and
sexual development proceeds without any direct fertilization
(plasmogamy) from an antheridium, (antheridium is nonfunctional as there is no migration of the male nuclei). The
contact stimulus of the male gametangium is enough to
initiate further development and the dikaryon stage is attained
by pairing between the ascogonial nuclei only. This type of
development is seen in certain aspergillii and pencillia, such as
species of Eurotium like E. amestelodam, E. repens (Aspergillus
repens) and E. glaucus (Aspergillus glaucus); Talaromyces
vermiculatus (Pencillium vermiculatum), in powdery mildews,
such as Phyllactinia corylea, Sphaerotheca mors-uvae, in many
Sphaeriales and Pezizales (e.g. Lachnea stercorea).
D) Parthenogenesis: This is further modification of the above
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process (autogamy) where there is a complete elimination of
the male gametangium. Dikaryotization and the production of
the ascogenous hyphae occur in one of the following ways:
i) Humaria granulata (Discomycetes): This is a
heterothallic fungus. The ascogonia are produced in the
normal way. The compatible nuclei of the opposite strain
enter one of the nearby (connected with ascogonium)
hypha by means of mycelial fusion. This enables the
opposite strain to migrate into the ascogonium, which
then gives rise to ascogenous hyphae in a normal way. If
contact with the mycelium of the opposite strain is denied,
the ascogonia die without developing any further.
ii) Ascobolus citrinus (Discomycetes) In this fungus
ascogonia are produced normally. No outside nuclei are
introduced from other hyphae but nuclei from one of the
adjoining cells of the ascogonium (usually the next cell
of the stalk) enter the ascogonium by the dissolution
of the partition wall. The ascogonium then gives rise to
ascogenous hyphae and proceeds to develop asci.
iii) Sartorya (Plectomycetes): In Sartorya which is the
perfect state of a number of aspergillii e.g. A. funigatus,
the ascocarp is exclusively initiated by a coiled ascogonium
and no antheridium is produced.
E) Spermatization of trichogynes: This is a highly
specialized type of sexual process in which no antheridia are
produced and the ascogonia provided with trichogyne are
fertilized by means of spermatia, microconidia, or conidia.
It is found in many higher ascomycetes, such as in the
members of Sphaeriales, Pseudosphaerales (Pyrenomycetes),
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Laboulbeniales (Laboulbeniomycetes), Sclerotiniaceae and
Pezizales (Discomycetes), Dothideaceae (Laculoascomycetes)
and Lecanorales (Disclichens).
Fig 6: Spermatization of Trichogynes
This can be illustrated by the following examples:
i) Mycosphaerella (Loculoascomycetes): Spermatia
are formed in special ostiolate spermogonia. These are
produced internally in basal spermogonial cells or sperm
mother cells and are pushed out through a sterigma-like
phialide of the parent cell. In perithecial primordia, coiled
ascogonia are formed each with a basal ascogonial cell
and a long trichogyne. After fertilization, the ascogonium
throws off short ascogenous hyphae which develop asci
by hook or crozier method.
ii) Collema (=Collemodes) (Lecanorales): The ascogonium
consists of one to three coils of cells which terminate in a
filament (the trichogyne). The male organs are branched
hyphae that project into a conceptacle-like spermogonium.
Minute, uninucleate non-motile sperms bud off the
branches externally. Fertilization is accomplished and asci
are formed by hook method.
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iii) Neurospora sitophila- This is another step towards
degeneration of sexuality. The ascogonia are more
degenerate and consists only of a coiled hypha which give
out a number of hyphae, each acting as a trichogyne. No
special spermatia are produced here and the spermatial
function is either performed by compatible (since it is
a heterothallic fungus) conidia and/or microconidia, or
a germ-tube from the conidium supplies nuclei to the
receptive trichogynes. Fusion between the trichogyne and
the fertilising cell is followed by migration of one or more
nuclei down the trichogyne into the ascogonium. After
fertilization, ascogenous hyphae emerge and asci are
formed by the crozier method.
From the above, it is clear that various modifications of the
sexual processes take place and that the sexual apparatus has
degenerated by various degrees.
F) Somatogamy: The reduced type of sexuality is the final stage
in the elimination of both the antheridium and ascogonium. The
sexual process becomes extremely simplified. It takes place by
the copulation between the cells of two compatible hyphae at
any point along the mycelium through anastomosis. Two strains
of the opposite sex, however, are necessary. After dikaryon
formation, ascogenous hyphae and finally asci and ascospores
are formed normally in the ascocarp. This type of reproduction
occurs in Gelasinospora tetrasperma, Humaria rutilans, some
species of Penicillium [e.g. P. brefeldianum (Carpenteles
brefeldianum), P. egyptiacum (Carpenteles egyptiacum)] and
Aspergillus that have their perfect state in Emericella, e.g.
Emericella nidulans (Aspergillus nidulans).
G) Even this impulse towards sexuality finally disappears.
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The two vegetative nuclei of adjacent cells of the same
homothallic mycelium come together to establish a dikaryon.
The two nuclei of the dikaryon fuse in the ascus mother cell to
form a synkaryon. The latter by meiosis, followed by mitosis,
gives rise to four or eight haploid ascospores within the ascus.
Hence the final stage in the development of the sexual apparatus
in the life cycle is totally abandoned.
H) Total absence of sexuality: In the cases discussed
above, though various modifications have taken place, the
pairing of nuclei is still essential. Total apomixis (absence of
sex) has been reported in some cases. In Ascobolus equinus
although solitary ascogonia are produced, karyogamy is totally
absent. Only haploid nuclei are found everywhere. And since
karyogamy does not occur, neither does meiosis, only a single
haploid phase occurs in the life cycle, in which all nuclei are
alike. This is indeed a rare and exceptional phenomenon.
Thus it becomes clear that among the lower ascomycetes,
the sexual apparatus is well-developed. Both the sex organs,
antheridia and oogonia or ascogonia are well-marked and
functional. They are usually developed on the same mycelium.
In few cases as in the family Laboulbeniaceae, they occur on
distinct individuals. There is however, a gradual and progressive
degeneration of the sex organs as we proceed from the lower to
the higher ascomycetes. In the most advanced ascomycetes,
there is complete suppression of the sex organs. In them the
sexual apparatus is wholly lacking. The progressive reduction
on sexuality can be well illustrated by a comparison of species.
They can be arranged in a series illustrating the gradual reduction
and gradual elimination of the sexual apparatus. Firstly the
antheridium ceases to function. The different species of
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Aspergillus illustrate the way in which elimination of male sex
organ (antheridium) in the Ascomyceltes has taken place.
These species can be arranged in series. The series indicates
the stages or steps in which the progressive degeneration of
the male sex organ (antheridium) has been accomplished.
These steps are:
1.
In some, there is a degeneration of male nuclei,
whereas in others the antheridium is rudimentary,
non-functional or even absent.
2.
In Aspergillus herbariorum the pollinodium cuts off
a terminal antheridium. The tip of the antheridium
fuses with that of trichogyne but there is no
migration of contents of the antheridium towards
the trichogyne.
3.
In some species the antheridium may develop late.
By that time the ascogenous hyphae have already
been developed from the ascogonium. E.g, A.
nidulans
4.
In a few species the male nuclei in the antheridium
are reported to degenerate before the antheridium
has reached maturity.
5.
The antheridium in some species such as A. flavus,
A. fisheri and A. fumigatus does not develop at all.
Only the ascogonia are seen to be developing.
In some species of Aspergillus, both the sex organs are
well-developed. Sexual fusion takes place. Same is the case
in Lachnea stercorea. There is, however, no migration of
contents of the antheridium into the oogonium in this species.
It shows that the antheridium has become non-functional.
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There is another species of the same genus, which goes a step
further. It is Lachnea cretea. In this species the antheridium
is absent. The ascogonium is well developed and functional.
Coprobia (Humaria) granulatus illustrates another step towards
degeneration of sexuality. It is characterized by the absence
of trichogyne from the ascogonium. The antheridium is also
lacking. Coprobia (Humaria) rutilans illustrates the final stage
in the elimination of sex organs. The antheridia are absent and
so are the ascogonia or the oogonia. The sexual process in these
advanced forms becomes extremely simplified. It takes place
by the copulation between the cells of two vegetative hyphae.
Finally even this impulse towards sexuality disappears. The
two vegetative nuclei of adjacent cells of the same homothallic
mycelium comes together to establish a dikaryon. The two
nuclei of the dikaryon fuse in the ascus mother cell to form a
synkaryon. The latter by meiosis followed by mitosis gives rise
to 8 haploid ascospores within the ascus. Hence the final stage
the development of the sexual apparatus in the life is totally
abandoned.
It is clear that many fungi have no sexual stage or true
sexual nuclear fusion in their life history, and many others
develop parthenogenetically. These must have developed from
types with a normal sexual stage. No evidence exists as to how
and when this partial or complete loss of sexuality occurred.
In culture, however, it is common for fungi to show a
gradual loss in intensity of sexual reproduction over a number
of transplants. This may show itself in a reduction in the number
of fruit bodies formed or in the number of viable spores per
fruit body. In a number of fully investigated examples, such
an apparently gradual loss of sexuality has been shown to be
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due to the unconscious selection of sterile or relatively sterile
mutants during sub-culturing. Mohendra and Mitra (1930)
showed that, with repeated subculturing, a strain of Sphaeropsis
malorum changed from a dark coloured colony with abundant
pycnidia, containing viable spores, to a white one which was
almost sterile. When spores from a fertile type of pycnidia were
plated out they gave rise to both black and white colonies.
Those giving rise to sterile white colonies germinated more
quickly than those producing black colonies and hence with
mass inoculum of spores the white colonies had greater chance
of developing.
Sexuality in Basidiomycetes
Basidiomycetes are the most advanced of all fungal
classes. The development of basidium is a unifying character
common to all members of the class. The somatic phase
consists of a well-developed septate filamentous mycelium
which passes chiefly through two important phases – primary
mycelium and the secondary mycelium. The primary mycelium
is monokaryotic, bears neither sex organs and is short lived.
The monokaryotic mycelium originates with the germination
of a basidiospore and is often called primary mycelium. The
secondary mycelium (which is dikaryotic) originates variously
by fusion between two monokaryotic structures (hyphae,
oidia, sporedia etc) and is binucleate. The binucleate condition
begins when the protoplasts of two uninucleate cells fuse i.e.
plasmogamy without karyogamy. Obvious manifestations of
sexuality, as specially differentiated gametes and gametangia,
are usually absent in the basidiomycetes (excepting rusts). But
there are other ways of coming together of the two compatible
nuclei. These may be:
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1) Somatogamy or mycelial fusions
2) Spermatization or oidization i.e. fusion between oidia
and hyphal cells and
3) Fusion between two oidia which are small, hyaline one
celled and uninucleate spores formed on oidiophore tip. Thus
sexuality is said to be reduced in Basidiomycetes. The primary
manifestations of sexuality i.e. the union of cells and nuclei
and the association of chromosomal and gene complements
are obviously present in the basidiomycetes, in spite of the
absence of such secondary criteria as specialized gametes and
gametangia. The compatibility mechanism in fact, is much more
highly developed in basids than the other fungi. In view of the
diversified nature of the processes formed in different groups
of these fungi we would discuss some important representative
examples to illustrate the range in variation of sexual mechanism.
Sexuality in Ustilago
No sex organs are developed in Ustilago. The sexual process is
represented by three fundamental phenomena characteristic of
it, namely plasmogamy, karyogamy and meiosis.
a) Plasmogamy: Heterothallism is common in the genus
Ustilago. The mycelia though morphologically alike are
different physiologically. They are physiologically unisexual.
However there is no distinction into male and female mycelia.
They are different only in their sexual behaviour. The difference
of sex is thus very rudimentary. It is denoted by the signs
plus and minus. Such mycelia are said to be heterothallic.
Plasmogamy in such species is brought about by different
methods of diploidization that results in binucleate condition
also called dikaryotic. Plasmogamy thus initiates dikaryophase
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in the life cycle.
b) Karyogamy: With karyogamy, the dikaryophase ends.
Here the two nuclei fuse and this fusion may be regarded
as the culmination of sexual process that started at the
time of diploidization. It is equivalent of fertilization. The
diploid nucleus formed is called a synkaryon. The smut spore
with a synkaryon is the probasidium or hypobasidium that
represents a transitory diplophase in the life cycle of smuts.
c)Meiosis: The diploid smut spore germinates to form
the promycelium or epibasidium. The synkaryon in the
epibasidium undergoes meiosis to form four haploid daughter
nuclei. After walls are formed, the epibasidium converts into
a four celled structure, each cell of which bears a haploid
basidiospore.
In Agaricus and other mushrooms the sexual apparatus in the
form of sex organs is completely lacking. Their function has been
taken over by the somatic hyphae which are heterothallic. The
fusion between two somatic hyphae of + & - strains represents
the first stage. Plasmogamy is accomplished by somatogamy
or somatogamous copulation.
Fungi Imperfecti or Deuteromycetes comprises of a group
of fungi in which only the asexual or imperfect stage is known.
The sexual stage also called the perfect stage is completely
unknown; the sexual spores like sporangiospores, meiospores
(ascospores and basidiospores) are either non existent or have
not been observed or discovered so far. Reproduction takes
place chiefly by the formation of exogenously developed asexual
spores called conidia.
Thus it is clear that sexuality reduces as we proceed from
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primitive fungi i.e. Phycomycetes to Basidiomycetes through
Ascomycetes and the perfect stage is completely lacking in
Deuteromycetes.