L.8- Biotechnology
Mycology
D.Ibtihal Muiz
Introduction to the Ascomycetes
Ascomycota is the largest phylum of fungi
Ascomycota and Basidiomycota share a number of characters:
1-compartmentalized mycelium
2-dikaryotic stage in the life cycle
3-plectenchymatous structures associated with spore production, conidia
the two phyla have diverged from a common ancestor
Primary morphological character that distinguishes members of the Ascomycota are the
ascus a sac-like cell containing the ascospores cleaved from within by free cell formation
after karyogamy and meiosis. Eight ascospores typically are formed within the ascus, but this
number may vary from one to over a thousand according to the species. Mycelial
ascomycetes are characterized by a compartmentalized mycelium with distinctive walls,
septa having simple pores, and the presence of Woronin bodies
-saprobes - biotrophic
-masters of symbioses: mutualists - commensals - parasitic
-terrestrial - aquatic - marine
Ex.:Pneumocystis carinii , Cryphonectria parasitica , Annisogramma
Many ascomycetes are closely associated with insects. Some, such as Ophiostoma,
Ambrosiella, Raffaelea, and Symbiotaphrina, are found in insect mycangia and provide or
detoxify the food of the insects.
Ascomycetes produce most of the known plant growth regulators; disease symptoms
called leaf curls and witches' brooms, seeing the effects of these fungal products that induce
the plants to differentiate in unusual ways. Gibberellin, the growth regulator involved in stem
elongation, was first discovered in rice infected by Gibberella fujikuroi, the cause of
foolish seedling disease. Fungal secondary products may act as pheromones, some of which
provide signals to fungus-associated insects or mammals. E.g., truffles
General Characteristics
Ascomycetes may have two distinct reproductive phases, one sexual involving the
formation of the asci and ascospores mentioned earlier, and the other asexual, with spore
production occurring at different times on the same mycelium.
1
Ascomycetes are delimited and classified by their sexual reproductive structures; vast
numbers of ascomycetes known only by their asexual stages can be difficult, and the practice
has been to place these taxa in the artificial group called Deuteromycota or Fungi Imperfecti.
Somatic Structures Somatic stages of ascomycetes may be single-celled, mycelial or
dimorphic A large proportion of the cell walls of filamentous ascomycetes is chitin In the
yeasts (Saccharomycetales) mannans and ß-1,3 glucans are the principal wall
polysaccharides, and only limited amounts of chitin are present, often restricted to bud scars
In ascomycetes the hyphal wall appears two-layered, with a thick translucent inner layer
and a dense, thin outer layer. Some basidiomycete walls are characterized by several
interspersed dense and translucent layers, but this is not universal. Hyphae of ascomycetes
are divided into compartments by septa that form from the hyphal periphery and advance
toward the center, thus invaginating the plasma membrane. In most ascomycetes a small
circular opening or pore is left near the center of the septum through which the plasma
membrane and cytoplasm extend from one hyphal compartment to the next. In the
filamentous ascomycete septum, the potential exists for cytoplasmic continuity between all
parts of the mycelium; septal pores may be plugged or blocked by various types of
membrane-bound structures
Woronin bodies are spherical, hexagonal, or rectangular membrane bound structures with a
crystalline protein matrix that usually are associated with the septum; elevated levels of
nitrogen, sulfur, and phosphorus in Woronin bodies when they were compared to the
adjacent cell walls of the species they studied. Woronin bodies frequently plug the septal
pores of hyphae and it is believed that they serve to separate aging or damaged hyphae from
the rest of the mycelium, but details of their exact function remain unknown. In addition to
the Woronin bodies that may plug septal pores, a more complex structure occurs in some
filamentous ascomycetes; septal pore organelles, often shaped like pulley wheels, that are
distributed in parts of the mycelium so that structures involved in sexual reproduction
routinely are isolated from other regions of the mycelium. They usually are found at the base
of asci and sterile parts of the hymenium of the ascocarp.
Hyphal compartments often are uninucleate, but mycelia consisting of multinucleate cells
are well known. The perforations in the hyphal septa permit nuclei to migrate from one
compartment of a hypha to another; the ability of nuclei to migrate throughout the mycelium
is important in the phenomenom of heterokaryosis Ascomycete mycelium may be organized
into fungal tissues (plectenchyma); If such a tissue is loosely woven and the mycelial
strands are more or less evident, it is known as prosenchyma. If, however, the hyphae have
lost their individuality and the cells are more or less isodiametric, closely resembling the
parenchyma of plants, it is known as pseudoparenchyma.
2
Sexual Reproduction
Two compatible nuclei are brought together in the same cell by one of several methods:
1. Two morphologically similar gametangia touch at their tips or coil around each other
and fuse. The fusion cell develops into the ascus. No dikaryotic phase is developed in
most species because karyogamy takes place immediately after plasmogamy. In the
yeasts, which are mostly unicellular, the somatic cells themselves act as gametangia
with the zygote becoming transformed directly into the ascus
2. Some species produce morphologically differentiated uninucleate or multinucleate
gametangia. These are called antheridia and ascogonia. Asci develop from
outgrowths of the ascogonium, which is designated the female gametangium. The
male nucleus passes from the antheridium into the ascogonium at the point of fusion
between the two gametangia. The ascogonium bears a specialized hypha, the
trichogyne that receives the male nucleus. This process does not involve formation of
a fusion cell, and a dikaryotic stage may persist for a while before karyogamy occurs.
3. In some species a process called spermatization involves a single detached male cell
that becomes attached to the female receptive organ--whether a trichogyne or somatic
hypha--and empties its nucleus into the receptive cell. The male nucleus or nuclei
migrate to the ascogonium through the septal pores. The male cells are dispersed by
insects, wind, or water to the receptive female organs. The functional male gamete
may be a spermatium, microconidium, or conidium. Spermatia are minute, spherical
or elongated, uninucleate, male sex cells incapable of germination by a germ tube. It
generally is assumed that they are evolutionary reduced from conidia. Spermatia may
be formed on the mycelium or in specialized structures called spermogonia.
Microconidia have been described as minute conidia that behave as spermatia, but
which also are capable of germinating and giving rise to mycelium. Conidia also may
function as spermatia by attaching themselves to the receptive organs and emptying
nuclei into them.
4. Plasmogamy - sometimes referred to as somatogamy - involves the fusion of
unspecialized somatic hyphae of two compatible mycelia with the nuclei migrating to
the ascogonia through the septal perforations. This is a method of plasmogamy that
may not be prevalent in ascomycetes, but is usual in many basidiomycetes.
In the ascomycetes, with yeasts being the major exception, the two nuclei may remain in
close association and undergo successive divisions that result in a number of dikaryotic cells.
Nuclear fusion eventually takes place in the young ascus. Meiosis in the diploid zygote
nucleus occurs almost immediately after fusion, and results in the production of four haploid
nuclei. These four nuclei then divide mitotically, resulting in the formation of eight nuclei
that will become incorporated into the eight ascospores during ascosporogenesis.
Incompatibility Systems
Homogenic incompatibility- is a process that promotes outcrossing in sexual fusions and is
controlled by mating type genes.
3
Heterogenic incompatibility- the process that governs fusion of like somatic or vegetative
hyphae; referred to as somatic or vegetative incompatibility. Sexual reproduction in
heterothallic ascomycetes requires the participation of genetically different strains. The
mating system is controlled by a single genetic locus which specifies one of two alternative
mating types, and is termed unifactorial or bipolar.
Vegetative or somatic incompatibility in ascomycetes prevents the fusion of genetically
different mycelia (heterogenetic incompatibility) and is usually under multigenic control by a
series of bi- or multiallelic genes at vegetative incompatibility loci. (When two mycelia in
different vegetative compatibility) groups meet in a substrate, they may interact with varying
degrees of antagonism, as would be expected of a phenotype under multigenic control. In
some cases barrage reactions may be recognized by a clear zone between the two mycelial
fronts due to the lysis of the interacting cells. Sometimes incomplete interactions occur at the
margins and unstable dikaryons may exist for a short time. In some cases pigments or
enhanced conidium production indicates the meeting of the two mycelia
Life Cycle
Ascomycota are either single-celled (yeasts) or filamentous (hyphal) or both
(dimorphic). Yeasts grow by budding or fission and hyphae grow apically and branch
laterally. Most yeasts and filamentous Ascomycota are haploid, but some species,
Saccharomyces cerevisiae for example, can also be diploid. Mitospores may simply
reproduce the parent, or may also act as gametes to fertilize a compatible partner. Some
Ascomycota must outbreed (heterothallic), others can also self, and some can only self
(homothallic).
4
In many species of filamentous ascomycetes each ascogenous hypha branches and
rebranches in various ways, and produces a cluster of asci. This often is accomplished as
follows: The crook cell elongates into a new hook instead of developing directly into an
ascus, and the tip and basal hook cells fuse and form another hook by the side of the first.
This process may be repeated several times, forming a cluster of hooks, the crook cells of
which finally develop asci.
Croziers and clamp connections:
-Crozier and clamp connections are homologous?
-Ascogonial coils
-Ascus mother cell - basidium
-Endospores vs. exospores
Asci.
-Asci may be spherical to elongated with cylindrical, ovoid, or globose forms
-Asci may be stalked or sessile; they may arise at various levels within the ascocarp or from
a single level.
-A definite layer of asci, whether naked or enclosed in an ascocarp is called a hymenium.
5
-Developmental studies have shown that sometimes asci may develop in a hymenium, but
become rearranged at maturity so that they appear to be scattered.
-Three basic types of asci can be defined from light microscopy studies: prototunicate,
unitunicate, and bitunicate.
-The prototunicate asci have a thin, delicate wall and release their spores by deliquescing.
The wall in both a unitunicate and a bitunicate ascus is said to consist of two layers:
exotunica and endotunica. In the so-called unitunicate ascus these layers adhere closely
throughout the life of the ascus, and the spores are released through a terminal pore, slit, or
hinged cap (operculum).
In the bitunicate ascus the endotunica expands up to twice or more its original length,
separating from the ruptured exotunica at the time of spore release. Spores are released
through a pore in the endotunica. Because of this behavior, the bitunicate ascus has been
called the Jack-in-the-box ascus or the fissitunicate ascus by lichen specialists.
Ascocarp
In general there are five ways that ascomycetes can be separated according to the way they
bear their asci:
1. Those that bear naked asci without any fruiting body
2. Those that produce their asci inside a completely closed ascocarp called a
cleistothecium
3. The perithecium, that is more or less closed, but at maturity is provided with a pore
(ostiole) through which the ascospores escape
4. Those that produce their asci in an open ascocarp, called an apothecium; and
5. Those that form their asci directly in a cavity (locule) within the stroma; the stroma,
which may be likened to a cushion of closely woven somatic hyphae, forms the wall
of the ascocarp. We call such a structure an ascostroma or pseudothecium
Ascocarps may be formed singly or in groups. They may be superficial, erumpent, or
deeply embedded in the substrate. In some cases ascocarps with true walls may form in a
stroma. The stroma may be composed of both fungal and host tissue in some plant
pathogenic species, or a stroma may be made entirely of fungal tissue
Release, Dispersal and Germination of Ascospores.
In some cases the method of release is passive with physical forces or animals breaking the
asci. Many ascomycetes ascospores are released by forcible ejection and the second event,
dispersal, is by another agent. In species that do not form ascocarps, the release of the spores
generally takes place by the breaking or the deliquescence of the asci formed on the
substrate. The spores are then free to be dispersed by wind, water, animals, or other agents.
In some groups the ascocarp is completely closed and the ascospores are liberated only
on the partial or complete disintegration of the ascocarp. The release process may be
6
hastened by ingestion by animals that also may be dispersal agents; e.g. truffles . In some
ascocarps there is a pore through which the ascospores usually are aimed and forcibly
ejected.
Ascocarps with the entire hymenia exposed at maturity also have ascospores that are
discharged forcibly into the air. In the instances when the spores are shot into the air, some
may be carried by air currents for comparatively long dispersal distances, although the
majority fall in the vicinity of the place where they were produced. In certain yeasts and
filamentous ascomycetes, ascospores may multiply by budding or conidium formation
(repetitive or iterative germination) instead of germinating by germ tubes. Depending upon
the environmental conditions, some ascospores have the ability to germinate by either
method.
Somatic structures
-Sclerotia are considered to be resistant or resting structures and their formation may serve
to help a fungus survive conditions that are unfavorable to growth. They often are
characterized by thick walls, and this may be the only criterion applied to assess their
"resistant" function.
-Stromata have a similar function, but rather than initiating mycelial growth directly, they
give rise to conidia or ascocarps
Asexual Reproduction
may be carried out by fission, fragmentation, or formation of chlamydospores or conidia
according to species and environmental conditions it is in the ascomycetes that conidium
development has reached its zenith
-Conidia often are important in propagating and disseminating species throughout the spring
and summer with several generations being produced in a growing season. Conidia may arise
either directly from the somatic hyphae or from specialized conidiogenous cells, often borne
on hyphal branches known as conidiophores.
-Conidiophores vary from short hyphal branches to those that are long and intricately
branched. In some species the conidiophores may be produced free from each other without
any evident organization, while in other species they are joined together to form complex
structures
Dehiscence
-schizolytic - halves of dle septum split apart by breakdown of middle lamella
-rhexolytic - outer wall of cell beneath or between condia breakdown
Conidia:
7
-amerospores - aseptate
-didymospores - single septum
-phragmospores - several horizontal septa
-dictyospores - muriform septation
-helicospores - coiled spores
-staurospores - stellate
-scoleospores - curved, filiform
Conidiophores:
-simple - complex (penicillate) - synnema - sporodochium
-pycnidium - walls of structure are of fungal origin
-acervular - walls of structure are of host origin
Classification. Ascomycetes are taxonomically difficult, and over the last decade
mycologists have concentrated on delimiting monophyletic orders rather than grouping
orders in higher taxa 45 orders unplaced in higher taxa in the Systema Ascomycetum
(Eriksson and Hawksworth, 1993).
Archiascomycetes-Taphrina, Pneumocystis, Schizosaccharomyces
Saccharomycetales are characterized by the loss of simple septal in all except a few taxa,
restriction of chitin primarily to bud scars in the cell walls in most; mannans and ß-1,3
glucans are the primary wall polysaccharides; EMS in most yeasts examined appears to be
derived from membranes that are associated with individual nuclei, rather than as a cylinder
initially enclosing all of the nuclei.
Euascomycetes- the filamentous ascomycetes; mycelium with a simple septal pore, an ascus
vesicle, and forcibly discharged ascospores. Woronin bodies are associated with the septal
pores; production of ascogenous hyphae with many asci resulting from a single mating and
the formation of an ascocarp. This group appears to have diverged rapidly and is marked by
a diversity of mycelial types, ascus structure and function, and ascocarp morphology. In
addition filamentous ascomycetes are notable for their elaboration of conidium structure and
function.
Hymenoascomycetes and Loculoascomycetes : Ascomycetes usually are classified on the
basis of sexual reproduction. Forms that do not reproduce sexually have been placed in an
artificial group, either Deuteromycota or Fungi Imperfecti; the application of molecular
techniques provides a means to incorporate the two.
8