Introductory Mycology Alexopoulos Pdf File. Fungus - Wikipedia. A fungus (; plural: fungi. UNIVERSITY OF MADRAS. 4 25 75 II Core Systematic Medical Bacteriology 4 25 75 II Core Mycology and Parasitology 4 25 75. Introductory practical. The English word fungus is directly adopted from the Latin fungus (mushroom), used in the writings. The Basidiomycota contains about 30,000 described species, which is approximately 37% of true Fungi. They include the rusts, smuts, mushrooms, polypores, bracket fungi, puffballs, stinkhorns, etc. Rust (Puccinia graminis) and smut (Ustilago spp.) cause disease of crop plants and are responsible for serious.

1. Introductory Mycology Pdf

Dothideales Cercospora beticola damage Kingdom: Division: Subdivision: Class: Subclass: Order: Dothideales Lindau (1897) Families Dothideales are an of bitunicate fungi consisting mainly of saprobic or plant parasitic species. Description Taxa in this order are characterized by the absence of a hamathecium (defined as or other tissues between ) in a, and formation of ovoid to cylindrical fisstunicate asci (asci that have two wall layers that split at maturity in a -like fashion), usually in bundles or cluster called fascicles. During development, the asci push through the stromatic tissue, creating the locules. Classification Until 2001, this order was thought to contain five families:, and the. Several molecular phylogenetic studies since that time have resulted in an organization restructuring of classification. In the most recent revision of Ascomycota classification (December 31, 2007) the Dothideales contains two families, the Dothidiaceae and the Dothioraceae.

Hi, please a need this book Introductory Mycology of autor: Alexopoulos, If anyone has its copy then please send it to me on edwingraf@hotmail.com, thanks a lot!!!!! 4th Apr, 2014 Irfan Khan.

The Botryosphaeriaceae (order ) and the Planistromellaceae have been moved to order and family, respectively, pending the acquisition and analysis of additional molecular data. References.

Introduction Ascomycota or sac fungi is the largest group of fungi includes 65,000 species (Kirk et al 2008) belonging to over 3400 genera. The name sac fungi is derived from the Greek word ‘ askos’ ( a leather bottle, bag or bladder) and ‘ mykes’ (a fungi). The characteristic feature of this group is the presence of sexually produced spores- the ascospores in a sac called the ascus (pl. Each ascus contains eight ascospores (formed from meiosis) that are released or ejected by a squirt mechanism. Figure: The sac fungi includes diverse organisms such as A.

Baker’s yeast ( Saccharomyces cerevisiae), B. Cup fungi ( Peziza), C. Dead man’s finger or Candle snuff fungus ( Xylaria), D.

Morels ( Morchella), E. Truffles ( Tuber) and F. Ascolichens such as Cladonia. Pathogenic fungi such as G.

Venturia (apple scab), H. Claviceps purpurea (Ergot of rye), I. Dibotryon morbosum causing black knot of plums and cherries and J. Erysiphe graminis causing powdery mildew of wheat.

Source:, The group ascomycetes also includes the fungi that do not reproduce sexually (anamorphic species) but are placed in this group due to their morphological and physiological features as well as the molecular data. These include Saccharomyces cerevisae, Peziza sp., Xylaria sp.

Earlier this group was placed by taxonomists in the artificial phylum the Deuteromycota also called “ fungi imperfecti”. But the molecular evidences have identified a close relationship with the Ascomycota and have therefore been placed in this group in spite of absence of ascus. The sexual (Teleomorph) and the asexual (Anamorph) isolate of the same species bear different names for example: Aspergillus nidulans is an asexual isolate of Emericella nidulans which is the sexual isolate. Holomorph of a fungus refers to all the possible forms of a fungus either latent or expressed.

In other words: Holomorph = Anamorph + Teleomorph Ascomycota versus Ascomycetes Before the recognition of the fungal kingdom, the sac fungi were considered to be a class, not a phylum. The original collective term for these taxa was “Ascomycetes”, which was first coined in the 1800s for a ankles nonlichenized taxon that possessed asci. The names Ascomycota, Ascomycetes, and others with the same root are based upon the term “ascus”.

“Ascomycetes” was soon used to include lichenized taxa, and became the standard term, at the class level, for all ascus-bearing species, just as the term “Basidiomycetes” became used for their basidium-bearing counterparts. Elevation of the taxonomic rank of the Ascomycetes resulted in the names Ascomycetae, Ascomycotina, and finally Ascomycota. Together, the Ascomycota and the Basidiomycota form the subkingdom Dikarya. The more familiar term, Ascomycetes, is still loosely used, e.g. At fungal forays it is often said of a fungus, such as Peziza, “It is an ascomycete, not a basidiomycete” in reference to their sexual reproductive mode. The terms are further abbreviated to “ascos” and “basidos”, which are not officially sanctioned technical names. Source: Ascomycetes and Basidiomycetes share and also differ in a number of characters The shared characteristics include:.

Compartmentalized/septate mycelium. A dikaryotic stage in the life cycle. Plectenchymatous structures associate with spore production. Conidia and complex dispersal systems. Difference: The meiosporangia of ascomycetes are asci (singular, ascus). They are cylindrical or sac-like and at maturity usually contain eight haploid spores (ascospores) which are expelled into the air through the top of the ascus. The meiosporangia of basidiomycetes are basidia (singular, basidium): they usually have four tiny projections called sterigmata, each bearing a haploid spore (basidiospore) which is shot away individually at maturity.

The formation of asci or basidia marks the end of the dikaryophase: the paired nuclei have fused and the resulting zygote has undergone meiosis (and a mitosis in ascomycetes) to produce 8 haploid ascospores or 4 haploid basidiospores. General Characteristics These are the fungi which possess an ascus, containing ascospores formed after karyogamy and meiosis. They have a septate vegetative mycelium which is profusely branched. The cell wall is made of chitin.

The vegetative hyphae are often organized into somatic tissues like sclerotia, stroma and mycelial strands. The septa contain a septal pore or perforate septum. Source:Author.

Sclerotia are firm aggregations of modified hyphae which serve as resting bodies to overcome adverse conditions. Mycelial strands are linear hyphal aggragates capable of unlimited growth in one directions. Stromata are compact somatic stuctures which bear spores or the fructification. These fungi reproduce sexually as well as asexually.

Asexual reproduction takes place by budding, fragmentation, chlamydospores and conidia. The conidia are borne on conidiophores which may be scattered on the mycelium or formed in special structures –the fruit bodied like pycnidium, acervuli and sporodochia.

Pycnidium: is an ostiolate, spherical or flask shaped fruit body whose inner wall is also lined with short conidiophores. The acervulus is a disc shaped flat stromatic mass of hyphae. Sporodochium consists of cushion shaped stroma bearing conidia externally. Sometimes the conidiphores aggregate and fuse up to varying lengths to form structures called synnemata( sing. Synnema) and coremia( sing. In synnema the conidiophores are joined at the base and free near the tip, while in coremium the conidiophores are only loosely aggregated.

Sexual reproduction is initiated by the formation an ascogonia (sing. Ascogonium; the female sex organ). The ascogonia are globular structures and bear a hair like out growth called as the trichogyne. The trichogyne receives the male nuclei from the antheridium. The male nuclei move into the ascogonium where they do not fuse.

The ascogonium now gives rise to the so called ascogenous hyphae, each of whose cells contains one male and one female nucleus (dikaryotic hyphae). The nuclei divide simultaneously.

Fusion of the dikaryon is initiated by a specific cell division, ‘ hook formation’. The hyphal apex curves in the form of a hook, the nuclear pair divides conjugatively. The upper pair of nuclei are separated by a septum both form the stem cell and form the hook. The hook then fuses with the stem cell, thus producing another dinucleate cell. The upper hook cell becomes the ascus in which the fusion of the two nuclei occurs. The resulting primary ascus nucleus then undergoes two divisions of which one is meiotic.

The eight daughter nuclei then form the eight ascospores. Sometimes the number of divisions can be smaller (four spores) or very large ( 1000 spores). Both the ascospores and the mycelium are thus haploid. With few exceptions when the sexual organs reach maturity the hyphal network gives the characteristic shape to the fruiting body. Three different forms are distinguished:. the completely closed fruiting body called the cleistothecium which is the characteristic of Plectomycetes. a usually flask shaped fruiting body called a perithecium which is typical for the Pyrenomycetes.

an open, bowl shaped fruiting body called an apothecium which is typical for Discomycetes. The asci are formed in fruiting bodies ( ascocarps). In addition, there are some fungi that bear naked asci (protoascomycetes). In the truffles (Tuberales) the ascocarps remain closed. The ascus wall may be unitunicate or bitunicate and this is an important character in the taxonomy of Ascomycotina.

The unitunicate asci have two thin walls which appear as a single wall. The bitunicate asci have an outer rigid and an inner extensible wall. The inner wall in maturity comes out of the breakage of the outer wall. The extensible inner wall has an apical pore through which the spores are forcibly ejected. In unitunicate asci, the ascospores are ejected through several ways, a pore, a slit, an operculum (lid) or by breaking of the apical wall. In a large number of Asomycotina the ascospores are released forcibly. Reproduction The reproductive life cycle of Ascomycetes in general consist of two stages – Sexual and Asexual.

There is complete absence of flagellate cells in class Ascomycetes. Asexual reproduction Asexual reproduction in Ascomycetes is the dominant form of propagation and is responsible for multiplication and expansion of these fungi into previously uncolonised areas. The enormous number of spores and their increased resistance favors the survival of the fungus during unfavorable conditions. The fungi earlier classified in the artificial group Deuteromycotina or Fungi Imperfecti are now known to be the conidial forms or anamorphs of Ascomycota and Basidiomycota. The evidences are based on morphological similarities like mycelial structure, hyphal wall structure, characteristics of nuclear divisions as well as the conidial structure and development.

The data from DNA sequence comparisons have also confirmed their placement. However some species reproduce only by asexual means having lost the ability to form ascocarps during the course of evolution. Aspergillus and Penicillium are known anamorphs of several genera of Ascomycota. Similarly Fusarium is the anamorph of Gibberella and Nectria.

Asexual reproduction takes place by budding, fission, fragmentation, chlamydospores and conidia. Some Ascomycetes under normal conditions reproduce by vegetative methods of fragmentation, fission and budding.

Fragmentation. In this method there is separation of mycelium or segments of hyphae from the thallus called the fragments. The fragments may consist of one or more living cells. By repeated cell division of apical cell, each fragment produces a new mycelium. Fragmentation thus results in as many new individuals as there are fragments. The Ascomycetes with a unicellular thallus such as fission yeasts usually multiply by this method.

The mother cell elongates. The nucleus divides into two. The daughter nuclei move apart. Meanwhile a ring-like ingrowth of the wall material appears at the wall, in the middle of the cell.

The division is transverse. It grows inwards covered in front by the plasma membrane.

Finally it meets in the center stretching across the cell forming a complete partition called the septum. The septum thickens and then splits into two layers, one for each daughter cell before they separate. Saccharomyces (baker’s yeast) under normal conditions reproduces by this method only. At the time of budding, the cell protoplast covered by the cell wall bulges out in the form of a small protuberance at one end of the yeast cell. The protuberance gradually increases in size and is called the bud.

Meanwhile the nucleus of the parent cell along with the vacuole divides. One of the daughter nuclei migrates into the enlarging bud. The bud grows and becomes constricted at the base.

Eventually the opening between the bud and yeast cell closes, double wall forms and the two cells become physiologically distinct. They subsequently separate from each other leading independent life. Conidia Reproduction takes place by means of exogenously produced non-motile spores called conidia (sing. These conidia are produced terminally on special reproductive hyphae called as conidiophores. The cell from which conidium develops is known as conidiogenous cell. The conidiogenous cell maybe similar to the somatic hyphae (micronematous) or they may be different morphologically (macronematous). The conidiogenous cell may be one to few in number and borne on a stalk called the conidiophore.

The conidiogenous cells give rise to several conidia of various shapes, colors and sizes. These conidiophores may be scattered on the mycelium or formed in special structures- the fruit bodies e.g. Pycnidia, acervuli and sporodochia. In Hemiascomycetes (as yeasts) asci are naked whereas in most Ascomycetes asci are contained in a fruit body, ascocarp. The conidiophores maybe branched or unbranched or unicellular or multicellular arise singly or are aggregated. Coremia or Synnemata (Sing.

Conidiophores which are aggregated into parallel bundles are termed Coremia (Gk. Korema = brush) or Synnemata. Synnema consists of a group of conidiophores often united at the base and part of the way up their length.

The conidia maybe formed along the length of Synnema or only at the apex. In some Synnemata the stalk like portion is longer in comparison to the branch top e.g. Stemonitis and Penicillium claviforma. Figure: Penicillium sp.- Culture on Malt Extract Agar at 5 days and 25 oC. Note conspicuous coremia Source:,. Stromata (Sing. Stroma=bed, cushion) Sclerotia germinate to produce a number of pin shaped stromata.

A stroma in sclerotia have compact, hard masses of pseudoparenchymatous hyphal cells, with or without tissue of host or substrate. They are like a cushiony mattress on which or in which fruiting bodies usually are formed. They are irregular in shape and produce sexual or asexual spores after over wintering. Just below the stromata a large number of cylindrical ascogonia and club shaped antheridia are formed, later undergo plasmogamy e.g. Claviceps purpurea, Ergot of rye; Xylaria a wood rotting fungus. Hypoxylon, Figure: Xylaria hypoxylon – conidia are borne at the tips of branches. Sporodochium (pl.

Sporodochia; Gr. = spore, seed; = container, receptacle). The conidiophores are packed together closely into cushion-like pseudoparenchymatous conidiomata.

At the tip the conidiophore give rise to conidia e.g. Nectaria cinnabarina. Nectria asiatica. Perithecia and short stipitate sporodochia in the natural environment.

Perithecia on nature. Median section of perithecium. Median section of perithecial wall. 0–1 septates ascospores.

Short stipitate sporodochium in the natural environment. Median section of short stipitate sporodochium. Edge of short stipitate sprodochium. Acropleurogenous conidiophores in the natural environment. Conidia in the natural environment. Aerial conidiophores and conidial mass on SNA. Lateral phialidic pegs and conidia on SNA.

Short aerial conidiophores and conidia on SNA. Densely blanched aerial conidiophores and conidia on SNA. Mature conidia and young conidia on SNA. Budding mature conidia on SNA. Budding and germinating mature conidia (arrow) that were streaked onto SNA.

Source: (CC-BY-SA). Acervulus (pl. Acervuli; Lat. Acervulus – a little heap). It is a saucer shaped fructification consisting of a mat of hyphae giving rise to a flat layer of relatively short conidiophores developing from a pseudoparenchymatous stroma and producing mass of spores. It may develop inside the tissue of a host plant, subcuticular, lying under the outer layer of the host cuticle or it may develop under the epidermis- subepidermal or deeper inside the host. When conidia mature they split open the epidermis due to increasing pressure allowing the conidia to disperse.

The conidia are held together in droplets of mucilage and they are dispersed by rain splash e.g. Causing anthracnosis of beans. The telomorph of Colletotrichum is genus Glomerella. Figure: Colletotrichum sp. An infected leaf of Sorghum; B-C. Magnified of the acervuli Source: (CC), (CC).

Pycnidia (Sing. Pycnidium Gr. Pyknon = concentrated, dense, packed). They are rounded or globose or flask shaped structure. These structures may be superficial or embedded in host tissue. From the inner wall of pycnidium conidiogenous cells are formed giving rise to conidia at the tip. They are held together in slimy masses which ooze out through a small circular opening at the apex called ostiole.

They are dispersed by rain splash or observed in water films. Pycnidia may have a long neck or provided by a small papilla having an opening called the ostiole. They greatly vary in size, shape and color. In some cases pycnidia instead of producing conidia (asexual in function) may produce spermatia (sexually involved in fertilization) e.g. Leptosphaeria acuta with Phoma anamorph. Source:, For images of Pleospora visit: Conidium ontogeny The various steps in the formation and release of conidia:. Formation of conidia- conidiogenesis.

Maturation. Delimitation. Secession. Proliferation of conidiogenous cell or conidiophore. Conidia can be produced from conidiogenous cells by two methods: I.

Blastic - In blastic conidiogenesis the conidia is visible before the septum to separate it is formed. The conidium originates from a narrow point on the conidiogenous cell and elongates and swells before being cut off by a septum. Figure: Blastic conidiogenesis A specialized type of blastic development called occurs when the conidia originate from a broad rather than a narrow base. Further variations in the type of blastic conidiogenesis: a) Blastic solitary- borne singly b) Blastic sympodial- The apex of the conidiogenous cell extends sympodially and each new apex becomes the blastic conidium. The next apex then grows out on one side of it. As more and more conidia are produced the longer the condiogenous cell becomes.

Examples- Leptographium the anamorph of Ophiostoma. C) Blastic acropetal- Conidia youngest is at the tip of the chain e.g. Cladosporium d) Blastic synchronous - Many conidia produced synchronously from a single cell for e.g. Gonatobotryum, Botrytis. E) Tretic- In some genera even after the conidia have developed holoblastically a narrow channel persists in the cell wall. This development is termed as tretic or porogenous. F) Basauxic- The conidia arise as series of gradually maturing conidia with the oldest at the tip and youngest at the base.

The swollen basal cell adds new material from the base. Example: Oidium anamorph of Erysiphe graminis. G) Phialidic- A basipetal (youngest at the base) succession of conidia develops from the conidiogenous cell called the Phialide is usually shaped like a bottle with a narrow neck. The conidia (phialospore/ phialoconidia) are formed either singly or in clusters at the tip of conidiophore.

Examples- Penicillium, Aspergillus, Fusarium etc. H) Percurrent – In Spilocaea ( anamorph of Venturia inaequalis) as the conidia develop and release they leave behind a ring like scar- an annellation around the conidiogenous cell. The next conidium grows through the annular scars( Percurrent development). Other examples are Scopulariopsis brevicaulis (anamorph of Microascus). I) Retrogressive- Successive conidia develop one after the other and are delimited by a cross wall. As the chain elongates the conidiogenous hyphae becomes shorter.

Examples- Baipetospora, Trichothecium. Source: The cell wall of conidia and conidiogenous cells have two wall layers that may or may not be continuous. Depending on the condition the wall layers the blastic ontogeny: Holoblastic - When both layers are continuous and contribute to the wall of the new conidia. Enteroblastic - If the outer wall is rigid and breaks open as the conidia is pushed through. The conidium initial is surrounded by a new wall. Two types of enteroblastic development can be distinguished:.

Enteroblastic Phialidic- a basipetal succession of conidia develop from a specialized conidiogenous cell called the phialide often shaped like a bottle with a narrow neck. Source:.

Enteroblastic Annellidic- development resembles the phialidic conidiogenesis however the difference is that new wall material produced is secreted beyond the neck. As the new conidia develop the wall material is left at the neck as a ring or collars (annellation). Figure: Scopulariopsis brevicaulis Source: II. Thallic In thallic ontogeny the conidium is demarcated by a septum before the swelling occurs. Several types are distinguished: Solitary: Example- Microsporum anamorphs of Nannizzia have large thallic phragmospores at the end of the hyphae. Arthric - The thallic conidia are formed in chains and separate easily.

Figure: A common soil mold Oidiodendron has arthric conidia borne on a tree like conidiophore. The conidia are separate leaving behind the stalk like trunk. Source: Alternate-In some genus like Coremiella some of the hyphal cells degenerate and release the alternate cells as conidia. Similar to the blastic development depending upon the condition of the wall layer the ontogeny of the conidia in thallic ontogeny may be - holothallic or enterothallic. Sexual reproduction There are three stages in sexual reproduction:. Plasmogamy: union of two protoplasts resulting in a cell with two nuclei (heterokaryon). Karyogamy: fusion of the two nuclei producing diploid cells.

Meiosis: results in formation of haploid spores Sexual reproduction in Ascomycetes begins by union of two compatible nuclei. These nuclei are brought together in the same cell i.e. By plasmogamy between cells of two mating type, by the following methods –. Gametangial copulation. Two morphologically similar gametangia come close to each other by their tips or coil around each other. The intervening walls disappear in the region of contact, the two nuclei unite and the fusion cell is diploid and enlarges to become an ascus. In this method, plasmogamy is immediately followed by karyogamy and there is no dikaryotic phase e.g.

Eremascus (Hemiascomycetidae). Hologamy:. iii.

Gametangial contact. In same Ascomycetes, morphologically differentiated uninucleate or multinucleate gametangia are formed. The female organ is called ascogonium which is commonly a coiled, multinucleate cell with its tip having a receptive appendage or outgrowth called trichogyne. The male organs or antheridia are multinucleate and clavate in shape. Source: The Figure: A. Illustration of male and female gametangia; B. Scanning electron micrograph of the gametangia of Pyronema domesticum.

The ascogonia are ballon like structures and each poduces a tubular trichogyne that is in contact with club-shaped antheridium. Courtesy K.L. O’Donnell The male nucleus passes from the antheridium into the ascogonium either through the trichogyne or through a pore formed at the point of contact between the two gametangia.

In some Ascomycetes, the antheridia, although present are non-functional, whereas, in others no antheridia are formed. In such species, ascogonia receive nuclei from their trichogynes which pair with the ascogonial nuclei or the ascogonial nuclei themselves form functional pairs. Neurospora iv.

In some of the Ascomycetes that do not form antheridia, male nucleus reaches the ascogonium through specialized male cells called spermatia. These are minute, spherical or rod shaped, uninucleate, male sex cells which are produced from short, erect structures known as spermatiophores. Spermatia are disseminated by insects, wind and water to the receptive organs, such as the trichogyne or receptive hyphae.

A pore is developed at the point of contact through which the spermatial nucleus is eventually emptied into the female sex organ. This kind of sexual fusion is called spermatisation e.g. Neurospora sitophila v. In some members of Ascomycetes, fusion takes place between two undifferentiated somatic cells of the same or different hyphae and this process is called as somatogamy. The nucleus from one cell (the male) migrates into the other (female cell) through septal perforations resulting into the pairing of genetically unlike nuclei to form dikaryons. Morchella vi.

In some Ascomycetes the antheridia are either absent or they are non functional e.g. Talaromyces vermiculatus syn.

Penicillium vermiculatum. The tubular antheridum touches the clavate multi nucleate ascogonium. The intervening wall dissolves at the point of contact and the two sex organs come in contact but no migration of male nucei into the ascogonium has been observed i.e. No plasmogamy has been observed (Dangeard,1907).The ascogonium nuclei then arrange themselves in functional pairs or dikaryons (64 in number). Once the plasmogamy or fertilization has occurred and the two nuclei do not fuse immediately and remain separate forming the dikaryophase of the life cycle. The nuclei divide repeatedly to produce a large number of paired nuclei. The paired nuclei later fuse to form diploid cells.

However the timing of the fusion varies in different groups. The diploid cell develops into an ascus. The diploid nuclei in the ascus later undergoes meiosis to form eight nuclei. The nuclei get enclosed in membranes to form ascospores that are aligned in the ascus like peas in pod. Development of Asci. Direct development.

In lower Ascomycetes as yeasts and related fungi, the two compatible haploid nuclei brought together in a common cell by plasmogamy, fuse (karyogamy) immediately to form a diploid cell, the zygote. This zygote enlarges directly to form the ascus. The diploid cell or zygote in this way behaves as an ascus mother cell. Diploid nucleus undergoes meiosis which is followed by mitosis to form eight haploid nuclei. Cytoplasm gathers around each nucleus, and each protoplast secretes a wall around it to become an ascospore. Some asci are 4-spored. Thus ascospores are formed by cell free formation and not by cleavage.

The asci are not enclosed in any enveloping protective sheath of sterile hyphae and, therefore, no definite fruit body or ascocarp is formed. Asci are thus naked and present in scattered and unprotected condition among the vegetative cells.

Indirect development. In higher Ascomycetes there is a long interval between plasmogamy and karyogamy. After the transfer of male nuclei into the ascogonium, the male and the female nuclei arrange themselves in pairs.

The tip or end cell which is to form the ascus has a dikaryon. It curls over to form a hook or crozier (shepherd’s crook). Within the ascogenous hyphae, both the nuclei in the hook divide simultaneously in such a way that the threads of their mitotic spindles run parallel and thus two pairs of genetically different daughter nuclei arise.

Thus, two nuclei lie in the bend or arch of the crozier (hook) and two nuclei near the basal cell of the crozier. Two septa are laid down to form three cells to cut out a terminal or ultimate uninucleate hook cell, a binucleate arch cell which is also called the penultimate cell (as it is last but one in position) and a uninucleate basal cell or antepenultimate cell or stalk cell. These three cells compose the characteristic hook or crozier of higher Ascomycetes. The penultimate cell acts as ascus initial.

The basal and the apical cells later fuse and nucleus migrate into the basal cell, through a pore formed at the point of contact. The basal cell now becomes binucleate and this event repeats to form a second ascus initial cell. Repeated division of the tip of crozier result in a cluster of asci. In the ascus mother cell, the two nuclei fuse and diploid nucleus undergoes meiosis to form four haploid daughter nuclei. These nuclei then undergo a mitotic division to form eight haploid nuclei.

Cytoplasm is cleaved out around each nucleus to form an ascospore. The eight nuclei may divide further mitotically, so that each ascospore is binucleate and if more mitosis follows, the ascospore becomes multinucleate. Figure: Development of ascospores Source: Adapted from Mehrotra and Aneja, 1990. Types of asci Three types of asci are seen in ascomycetes:. Prototunicate asci- with thin delicate walls that dissolves at maturity to release the ascospores. These are found in cleistothecial and sometimes in perithecial forms.

Pictorial depiction and B.TEM of a single ascus of Eleutherascus Source:. Unitunicate operculate asci- have single walls and a built in lid or opeculum which at maturity pops open to liberate the spores. Figure: Unituniacte operculate asci (note the blue color stained operculum) Source:. Unitunicate inoperculate asci- These unitunicate asci do not have operculum but have a special elastic ring mechanism in their tip that stretches or turns inside out to let the spores shoot through.

Unitunicate inoperculate asci. The transmission electron micrograph (left - courtesy of A. Beckett) shows a section directly through an ascus tip of Xylaria.

The apical apparatus is actually shaped like a ring, a doughnut or a torus, depending on your background. Source:. Bitunicate asci: these asci have double wall.

The outer wall is thin and inextensible while the inner wall is thick and elastic. At maturity the inner wall absorbs water swells up and expands upwards carrying the spores through the neck of the ascoma and releasing them.

Figure: Bitunicate asci Source. Types of ascocarps (fruiting bodies) The asci may be naked as in Hemiascomycetes but in others they are intermingled with mycelium (centrum) and form ascocarps or fruiting bodies of different types. The term centrum was coined by Wehmeyer 1926. Centrum is the totality of the structures surrounded by hypha to form an ascocarp (Gr askos = sac; karpos = fruit) or ascoma. The taxonomy of Ascomycetes is based on the centrum and it can be classified according to the way they bear asci. Cleistothecium (Pl. Cleistothecia; Gr.

Kleistos=closed; theke=case) A Cleistothecium is globose in shape, completely enclosed and has continuous definite pseudoparenchymatous wall called peridium which lacks an opening called ostiole. Asci are arranged in a hymenial layer or scattered throught the interior of the ascocarp. Asci and ascospores are liberated by disintegration of the wall. Along with asci sterile hyphae are present. They are monokaryotic as they arise from mycelium which are not product of plasmogamy e.g. Aspergillus and Penicillium.

Figure: Cleistothecium of Erysiphe. Gymnothecium (Gr. Gymnos = naked) Consists of a loose open network of peridial hyphae.The wall over the asci consists of a thin, weft of hyphae forming a reticulate cage- like structure known as reticuloperidium. Naked asci are produced only in rare instances. Peridial hyphae extend as hooked hairs as seen in Plectomycetes e.g. Gymnoascus, Myxotrichum and Ctenomyces. Figure: Myxotrichum - gymnothecium.

Source:. Chasmothecium (Gr. Chasma – an open mouth) A modified cleistothecium capable of cracking open along a line of weakness e.g. Figure: Powdery mildew- Golovinomyces cichoracearum chasmothecium Source:. Apothecium (Pl.

Apothecia; Gr Apo = away from, separate) It is a cup or saucer shaped ascocarp and at maturity the tips of asci are freely exposed with a mass of differentiated non-fertile hyphae called paraphyses. Internal structure shows outermost, hymenium made up of elongated cells at right angle to the surface which are asci and paraphyses. The middle layer is thin, consists of light colored hyphae running parallel to the surface is called hypothecium. The third layer forms the basal larger part of apothecium called excipulum e.g. Cup fungi Pezizales and Helotiales. Section through the apothecium of an ascomycete Physcia B.

Magnified view of apothecia in Peziza Source:, 5. Perithecia (Gr peri = around) Flask shaped fruiting bodies that is more or less closed but at maturity opens by a pore or ostiole through which each ascus discharges separately. It has a short neck lined with short hairs called periphyses. Each mature perithecium bears elongated cylindrical asci containing eight ascospores.

These ascocarp are enclosed but at maturity an ostiole is formed through which each ascus discharge its spores separately. The perithecia are borne or embedded in a mass of fungal tissue forming the stroma. The perithecial wall is formed from sterile cells derived from hyphae, which surrounds the ascogonium during development. Sometimes single as in Sordaria and Neurospora but in Pyrenomycetes the perithecia are embedded in or seated on a mass of tissue forming a perithecial stroma e.g. Sphaeriales and Hypocreales. Pseudothecia Perithecia is different from pseudothecia in structure and development. The asci are contained in one or several cavities (locules formed within a pre-existing ascostroma (Gr.

Stroma = mattress, bed). The ascostroma is made up of somatic hyphae modified into pseudoparenchyma e.g. Heptosphaeria and Sporormiella. Source: (CC) Figure: Life Cycle: Ascomycetes can reproduce asexually via conidia. A sexual cycle begins when conidia of different mating types fuse (plasmogamy) to form dikaryotic, multicellular hyphae. Fusion of the nuclei (karyogamy) and meiosis followed by mitosis forms 8 ascospores contained in asci of ascocarps. Non-sexual variations Ascomycetes conserve their genetic diversity by the following two methods: Heterokaryosis Heterokaryosis (Gr.

Hetero: different, karyos: kernel or nucleus ) occurs in many Ascomycetes and was discovered by Hansen and Smith (1932) in Botrytis cinerea. The nuclei of same or different genotypes may coexist side by side in the same mycelium and in the same cell of a hypha. All cells may not have the same number of nuclei or the same kind of nuclei or the same proportion of each kind in a mixture of different nuclei. This phenomenon of the existence of different kinds of nuclei in the same individual is known as heterokaryosis and the individual as heterokaryon.

In such an individual, each nucleus is independent of all other nuclei but the structure and behavior of the individual appear to be governed by the kinds of genes it contains and the proportion of each kind of nuclei. Heterokaryosis may originate in the following ways:. By the germination of a heterokaryotic spore. By the introduction of genetically different nuclei into a homokaryon (a soma in which all nuclei are genetically similar). It may occur by somatogamy. By mutation in a multinucleate, homokaryotic structure and subsequent survival, multiplication and spread of diploid nuclei among the wild-type nuclei.

By fusion of two haploid nuclei (diplodization) in a homokaryon and subsequent survival, multiplication and spread of the diploid nuclei among the haploid. Thus, different nuclei in a heterokaryotic cell do not divide at the same rate and are randomly distributed varying greatly in their relative numbers.

Heterokaryons has an important feature that the nuclei complement each other, in so far as their genes outwardly show the same type of dominant/recessive relationships as are found in diploid cells. On one hand, heterokaryosis enables haploid fungi to shield recessive genes from selection pressure.

Whilst on the other hand, the fungi continuously change their genetic constitution in response to environmental selection pressure. Heterokaryosis may breakdown in two ways: i. Arising of a homokaryotic branch near the colony margin, it forms further branches, giving rise to a homokaryotic sector (sectors differ in growth rate, color, density of sporulation etc.). Automatic breakdown of heterokaryons during sporulation, provided that the spores are uninucleate. Figure: Diagrams illustrating the breakdown of a heterokaryon to a homokaryon during (a:i) Production of spores in Aspergillus (a:ii) hyphal branching; (b) conidia development in Fusarium (c) formation of conidia in Neurospora. Source: Courtesy of Jim Deacon, The University of Edinburgh Significance of Heterokaryosis i.

Heterokaryons are better adapted in nature than homokaryons. Heterokaryons make better growth than their component homokaryons. Heterokaryosis involves the recombination during mitotic crossing over.

Thus it plays an important role in generating variation. It also enables a growing colony to alter nuclear (gene) ratio in response to the prevailing conditions as it grows. As a substitute for heterozygosity, heterokaryosis is important for maintaining variability by the occurrence of genetically dissimilar nuclei and is altered by environmental conditions. It is a first step of parasexual cycle. Heterokaryosis is known to play an important role in pathogenicity of rusts and smuts.

Parasexuality Pontecorvo and Roper first discovered parasexuality in 1952 in Aspergillus (Emericella) nidulans, the imperfect stage of nidulans. Some fungi do not go through a true sexual cycle but go through parasexuality.

In parasexual cycle, genetic recombination occurs through nuclear fusion and crossing-over during mitosis instead of the meiosis. Haploidization occurs by successive loss of chromosomes during mitosis.

Fusarium oxysporum, Cephalosporium mycophilum, Verticillium albo-atrum, Penicillium chrysogenum, Cochliobolus sativus, Ustilago maydis, Coparinus cinereus, C. Radiates, Schizophyllum commune and probably Puccinia graminis tritici show this cycle. Since then parasexual phenomena have been observed in several imperfect fungi which possess no sexual stage, as well as in Basidiomycetes and Ascomycetes.

Pontecorvo (1956, 1958) gave details of the sequence of events in a complete parasexual cycle in the following ways:. Formation of heterokaryotic mycelium i. Takes place by the anastomosis of somatic hyphae of different genetic constitutions.

By the fusion of some of the nuclei and their subsequent spread among the haploid nuclei. Fusion between two nuclei. Multiplication of diploid nuclei side by side with the haploid nuclei i.e.

Karyogamy and multiplication of diploid nuclei. Thus at this stage the mycelium may contain at least five types of nuclei: two type of haploid, two type of homozygous diploid, and heterozygous diploid nuclei. All these nuclei presumably multiply at about the same rate, but the diploid ones are present in much smaller number than the haploid. Pontecorvo (1958) estimated a proportion of one diploid heterozygous nucleus to 1000 haploid nuclei.

During the multiplication of diploid nuclei occasional mitotic crossing-over takes place. It occurs within the heterozygous diploid nucleus.

The result is new combinations and new linkages, which is the most important outcome of the parasexual cycle. These recombinations that are dependent on the existence of heterokaryosis gives the fungus some of the advantages of sexuality within the parasexual cycle. Pontecorvo calculated the amount of recombinations that may be expected to occur in an ascomycete through its parasexual mechanism. It is 500 times smaller than through its sexual mechanism. Thus in Penicillium chrysogenum and Aspergillus niger, neither of which is known to reproduce sexually, diploidization and mitotic crossing-over occur much more frequently, so that the importance of parasexual cycle is great from the perspective of evolution of the species. Sorting out of diploid nuclei. In fungi which produce uninucleate conidia, sorting out of the diploid nuclei occurs by their incorporation into conidia which then germinate and produce diploid mycelia.

Diploid strains of several imperfect fungi have been isolated. The first such record was by Roper in 1952 who synthesized and isolated diploid strains of Aspergillus nidulans. Occasional haploidization of the diploid nuclei- This is poorly understood event. It seems that chromosomes are progressively lost from the diploid nucleus by non-disjunction in successive divisions. Thus, initially a single diploid nucleus would give rise to nuclei with 2n + 1 and 2n -1 chromosomes. These aneuploid nuclei (i.e. Nuclei with incomplete multiples of the haploid chromosome number) are unstable and tend to revert to euploid nuclei by repeated loss of chromosomes in successive divisions: the 2n + 1 nucleus reverts to a normal diploid, whereas 2n -1 nucleus eventually reverts to a haploid one.

In Aspergillus nidulans (n = 8) nuclei have been detected with 17 (2n + 1), 16, 15, 12, 11, 10, 9 and 8 chromosomes, so this lends strong support to the proposed mechanism of haploidisation. Sorting out of new haploid strains. Some diploid nuclei undergo haploidization in the mycelium and are sorted out. Some of the haploid strains are genotypically different from either parent because of mitotic recombinations producing new linkage groups, which are sorted out in the haploid conidia. Figure: Steps in parasexuality Source: Figure: The fungal mycelium of A. Nidulans is a web of branched filaments (hyphae) of connected compartments or cells, which each contain several nuclei (see centre figure). This mycelium, or homokaryon, which develops from a single haploid spore, differentiates many identical asexual spores known as conidia or conidiospores (see the asexual cycle in the figure).

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Nidulans is homothallic, which means that it is self-fertile, but crosses can be initiated by hyphal fusions between homokaryons with genetically different nuclei (shown by white and dark green nuclei). The resulting heterokaryons are not stable, but can be forced to maintain a balanced ratio of the component nuclei by including complementing auxotrophic mutations in the parental nuclei and forcing growth without the corresponding supplements. Nidulans can also reproduce sexually.

In the fruiting body, which produces the sexual spores, a pair of nuclei that is destined for meiosis divides in synchrony to form a mass of cells known as the ascogenous hypha. These hyphae are highly branched and each tip cell becomes an ascus (a specialized cell) in which the two haploid nuclei fuse. The diploid nucleus undergoes meiosis followed by a post-meiotic mitosis, which results in the formation of eight haploid ascospores. The fruiting body, called the cleistothecium, can hold tens of thousands of ascospores, which are released into the environment when the cleistothecium bursts open.

In addition to an asexual cycle and sexual cycle, a parasexual cycle offers the genetic benefits of meiosis achieved through a mitotic route. The parasexual cycle is initiated when haploid nuclei fuse in the vegetative cells of a heterokaryon and continue to divide mitotically. Crossing over might occur between homologues and random chromosome loss restores the haploid chromosome number, which is eight in the case of A. These events can be used to map gene orders and assign new genes to the eight linkage groups.

Source: (displayed with permission) TABLE: Comparison of the Para sexual and sexual cycles in Aspergillus nidulans (From Roper, 1966) Sexual cycle Para sexual cycle Heterokaryosis Heterokaryosis Nuclear fusions in specialized structures to yield “selfed” and “hybrid” zygotes. Rare nuclear fusion in vegetative cells. Zygote persists for only one nuclear generation. “Zygote” may persist through many mitotic divisions. Recombination at meiosis: crossing-over, at 4-strand stage, in all chromosome pairs, random assortment of members of each chromosome pair, and reduction to haploid state. Recombination by rate “accidents” of mitosis (a) mitotic crossing-over, at 4-strand stage, usually only one exchange in a single chromosome arm: (b) haploidization probably via aneuploidy. Independent of crossing-over, random assortment of members of each chromosome pair.

Products of meiosis readily recognized and isolated. Recombinants occur among vegetative cells. Recognized by use of suitable markers.