Consortium for Educational Communication
FrequentlyAsked Questions (FAQs)
1. Briefly summarize the progressive degeneration of sex
in Fungi?
The existence of some sort of sexual reproduction in Fungi
has long been suspected, although in earlier instances upon
insufficient grounds; but of late observations have multiplied
and facts accumulated which leave no doubt of its existence.
However, studies on the major groups within the fungal kingdom
have provided significant and wide-ranging clues regarding the
gradual loss in sexuality and sexual identity.
In the flagellate Phycomycetes (Chytridiales, Blastocladiales,
Monoblepharidales, Saprolegniales and Peronosporales) the
sex organs are quite conspicuous, and called the antheridia
(male) and the oogonia (female). The resultants of fusion are
oospores. In the series Aplanatae (aflagellate Phycomycetes,
class Zygomycetes) the gametangial copulation takes place. The
gametangia are represented by + and - signs. The resultants
of gametangial union are zygospores. In Ascomycetes, the sex
organs are not so prominent, and there is a regular phenomenon
of reduction of sexuality. The phenomenon of nuclear fusion is
not completely understood as yet. In some of the Ascomycetes
(e.g., Morchella) no sex organs are produced and the somatic
cells take part in sexual fusion. This process is called the
somatogamy. In the Basidiomycetes there are no sex organs
and only the fusion of the nuclei of a dikaryon represents the
process of sexuality. In the members of Fungi Imperfecti or
Deuteromycetes, the sexual process is altogether absent and
they reproduce asexually by conidia.
Consortium for Educational Communication
It is clear that many fungi have no sexual stage or true sexual
nuclear fusion in their life history, and many others develop
parthenogenetically.
2. Describe Planogametic copulation and Gametangial
contact in Phycomycetes?
The sexuality in the primitive fungi or Phycomycetes ranges
from isogamy through anisogamy to oogamy. Plasmogamy is
achieved through one of the following methods:
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.
Fig 1: Planogametic copulation in Phycomycetes
Consortium for Educational Communication
Gametangial contact: A group of aplanogametic lower fungi
(Oomycetes) produce nonmotile gametes (aplanogametes),
they are never released. The male gametangium is called
the antheridium and the female oogonium. No sperms are
organised. The gametangia donot 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
3. Write a note on Gametangial copulation in Phycomycetes
In 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
Consortium for Educational Communication
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.
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
Consortium for Educational Communication
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.
4. Describe briefly the Sexuality in Basidiomycetes?
In basidiomycetes, manifestations of sexuality, as specially
differentiated gametes and gametangia, are usually absent
except rusts. But there are other ways of coming together of
the two compatible nuclei. These may be:
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 than the other fungi. Some
important representative examples like Ustilago and Agaricus
illustrate the range in variation of sexual mechanism among
Basidomycetes.
In Ustilago No sex organs are developed and 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
Consortium for Educational Communication
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. Plasmogamy in such species
is brought about by different methods of diploidization
that results in binucleate condition also called dikaryotic.
Plasmogamy thus initiates dikaryophase in the life cycle.
b) Karyogamy: 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. This is followed by karyogamy
and meiosis.
Consortium for Educational Communication
5. Explain the degenerative Sexuality in Ascomycetes
with reference to Morchella
In Morchella, both the sex organs i.e. antheridia and ascogonia
are absent. Consequently, the sexual process is extremely
simplified. It involves two distinct processes, namely Plasmogamy
and Karyogamy. The later is immediately 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
Consortium for Educational Communication
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 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.
6. Describe the mode of sexuality in Yeasts?
Consortium for Educational Communication
Of course no sex organs like antheridia and oogonia are
produced in Yeasts. 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
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
Consortium for Educational Communication
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.
Fig 4:
octosporus
Sexuality
in
Schizosaccharomyces
7. Briefly explain Gametangial contact in Ascomycetes
with special reference to progressive degeneration of
Consortium for Educational Communication
sex in Aspergillus?
Gametangial contact in Ascomycetes 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
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
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
Consortium for Educational Communication
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.
8. Give a brief not on the Gametangial copulation in
Ascomycetes?
Gametangial copulation in Ascomycetes may be regarded
similar to that found in the zygomycotina. This persists among
the primitive ascomycetes (i.e. Hemiascomycetes). In this
Consortium for Educational Communication
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. 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.
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.
9. Describe briefly Autogamy?
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,
Consortium for Educational Communication
such as Phyllactinia corylea, Sphaerotheca mors-uvae, in many
Sphaeriales and Pezizales (e.g. Lachnea stercorea).
10. Write a brief note on Parthenogenesis in Ascomycetes?
This is further modification of the above 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.
11.
Describe
Spermatization
of
Trichogynes
in
Consortium for Educational Communication
Ascomycetes?
Spermatization of trichogynes 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),
Laboulbeniales
(Laboulbeniomycetes),
Sclerotiniaceae and Pezizales (Discomycetes), Dothideaceae
(Laculoascomycetes) and Lecanorales(Disclichens).This can be
illustrated by the following examples:
Fig 6: Spermatization of Trichogynes
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.
Consortium for Educational Communication
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.
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.
12. Write a brief note on Sexuality of Deuteromycetes?
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
Consortium for Educational Communication
asexual spores called conidia. They are the most highly evolved
group of fungi which have lost the sexual character during the
progressive process of evolution.
13. Write a short note on sexuality in Ustilago and
Agaricus.
In Ustilago, no sex organs are developed. The sexual process
is represented by three fundamental phenomena characteristic
of it, namely plasmogamy, karyogamy and meiosis.
d) Plasmogamy: In Ustilago, 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 or dikaryotic condition. Plasmogamy
thus initiates dikaryophase in the life cycle.
e) Karyogamy: Here the two nuclei fuse and this fusion may
be regarded as the 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.
f)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
Consortium for Educational Communication
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. This is followed by karyogamy
and meiosis.
14. Differentiate between somatogamous copulation
and autogamous pairing of Ascomycetes.
Answer: In Somatogamous Copulation, two vegetative hyphae
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.
In autogamous pairing, any vegetative cell of the subhymenium (Hypothecium) may establish a dikaryon. It is
accomplished by pairing of two of its own vegetative nuclei.
The remaining nuclei in the cell degenerate. The cells with
dikaryons (dikaryotic cells) develop ascogenous hyphae as in
somatogamous copulation.
15. Write a short note on Somatogamy in Ascomycetes.
Somatogamy is a reduced type of sexuality and represents
the final stage in the elimination of both the antheridium and
ascogonium. The sexual process is extremely simplified. It takes
Consortium for Educational Communication
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).
16. Differentiate between Gametangial Contact of
Phycomycetes and Gametangial Contact
found in
Ascomycetes.
The lower fungi particularly Oomycetes produce nonmotile
gametes which 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.
In Ascomycetes, the 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
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
Consortium for Educational Communication
hyphae arise from the ascogonium, and ultimately asci are
formed by the crozier or hook method. This type of sexuality
occurs commonly in taxa belonging to several groups such as
Plectomycetes, Pyrenomycetes and Discomycetes.