The following points highlight the eight taxonomic groups of fungi. The groups are: 1. Cellular Slime Molds 2. Myxomycetes 3. Chytridiomycetes 4. Oomycetes 5. Zygomycetes 6. Ascomycetes 7. Basidiomycetes 8. Deuteromycetes.
Taxonomic Group # 1. Cellular Slime Molds (Acrasiomycetes):
A representative of this group is Dictyostelium discoideum. The organisms occur in humus-rich soil. Individually they are uninucleate, naked haploid amoebae without cell-wall, showing typical amoeboid movement with the help of pseudopodia. They ingest bacterial cells by phagocytosis and multiply by fission.
At some stage of growth, the independent amoebae start aggregating to form a pseudo plasmodium, but the individuality of separate amoebae is maintained. The aggregation of amoebae which precedes fruit-body formation is signaled by a chemical agent which is called acrasin. In Dictyostelium, the chemical nature of the acrasin has been identified as cyclic AMP (c-AMP).
Fruit-body formation is initiated by building of a macroscopic heap in the pseudo plasmodium. The heap of amoebae is gradually differentiated into a stalked sorocarp having a head containing spores. The sorocarp is produced through a cooperative effort of thousands of amoebae, most of which contribute to the vegetative parts of a sorocarp and comparatively few are transformed into spores.
The stalk of the sorocarp is en-sheathed by a cellulose wall. The amoebae which form the head and the spores, climb up the stalk. The amoebae inside the head are rounded up, enclosed by a cellulose wall and become encysted. On liberation, a spore germinates through a pore in the wall and produces an amoeba which starts a vegetative phase and multiplies by binary fission.
The life-cycle of dictyostelium is shown in Fig. 5.6:
Taxonomic Group # 2. Myxomycetes (Acellular Slime Molds):
Myxomycetes are plasmodial slime molds. They consist of a naked mass of protoplasm containing many nuclei which are diploid. The mass of multinucleate protoplasm is a true Plasmodium which is not differentiated into individual cells. That is why the organisms are called acellular in a loose sense.
The Plasmodium moves in an amoeboid manner over moist surfaces of decaying leaves, logs, barks of trees etc. and on its way engulfs organic debris and microorganisms. These are digested and the Plasmodium grows.
When food becomes scarce, the Plasmodium begins its reproductive phase by formation of fruit-bodies of various shapes, sizes and colours. At the onset of the reproductive phase, the Plasmodium moves to a comparatively dry place.
Interestingly, the organisms in plasmodial phase are negatively phototropic, but when the reproductive phase starts, the phototropic response is reversed. The preparation for fruit-body formation begins by fragmentation of the Plasmodium into several units, each of which gives rise to a separate fruit-body.
The plasmodial fragments undergo dehydration and are differentiated into fruit-bodies having characteristic shape and size. They are often brightly coloured in shades of orange, yellow and red. The outside of the fruit body is covered by a rigid peridium often containing calcium salts. Inside the fruit-body are large numbers of small membrane-bound spores.
The portion of the Plasmodium which is not consumed for spore formation is converted into a net-like structure, known as capillitum. The spores are liberated by breaking open of the peridium and they are spread by air current. The capillitum possibly helps to regulate the rate of spore dispersal.
In contrast to the cellular slime molds, myxomycetes possess sexuality. The diploid nuclei of the Plasmodium undergo reduction division before spore formation in the fruit-body. The haploid spores after release, germinate to produce amoeboid cells which develop into flagellate male and female gametes. Gametic union results in a diploid zygote. The diploid nucleus divides mitotically to produce a multinucleate Plasmodium.
Myxomycete fruit-bodies of a few genera are shown in Fig. 5.7, and the life-cycle of a typical myxomycete in Fig. 5.8:
Taxonomic Group # 3. Chytridiomycetes:
The organisms are unicellular, mostly aquatic and microscopic. They have a cell wall mainly composed of chitin. A representative of this group is Synchytrium endobioticum which causes a disease of potato tuber, known as wart disease.
The pathogen induces hyperplastic growth of epidermal cells of potato tuber resulting in the formation of warts or galls. Another representative of the group is Olpidium brassicae which infects roots of cabbage and also of tobacco seedlings.
Synchytrium endobioticum possesses both a sexual and asexual phase in its life cycle. The haploid zoospores are uniflagellate which may also act as gametes. Fusion of two such gametes leads to formation of a biflagellate zygote with diploid nucleus. Infection of host epidermal cells may take place either by haploid zoospores or by the zygote.
In case of infection by a zoospore, the parasite—after entry into an epidermal cell—enlarges and its nucleus divides many times mitotically to produce a multinucleate structure which fragments into several segments.
Each segment containing many nuclei produces a sporangium and from each sporangium a large number of uninucleate zoospores are differentiated. The uniflagellate zoospores, after release from the host cell, can reinitiate another infection cycle, or may act as isogametes. Gametic union yields zygote (2n) which are also capable of infection.
In case infection takes place by a diploid biflagellate zygote, the pathogen enlarges in size and enters into a resting phase by investing a thick wall (a resting sporangium). Eventually, the diploid nucleus divides meiotically to form a large number of haploid nuclei. Each nucleus with surrounding cytoplasm forms a uniflagellate zoospore. On being released, the zoospores can infect host cells.
The life cycle of S. endobioticum is schematically shown in Fig. 5.9:
Taxonomic Group # 4. Oomycetes (Water Molds):
These fungi grow both in aquatic and terrestrial habitats. The vegetative body consists of a well- developed coenocytic mycelium. Cross-walls develop only to separate reproductive organs. Some aquatic oomycetes, like Saprolegnia parasitica cause disease of fish. Terrestrial members, like Phytophthora, Plasmopara, Peronospora, can parasitize land plants.
The fungi reproduce both asexually and sexually. Asexual reproduction takes place by biflagellate zoospores, while sexual reproduction by gametangial contact between antheridia and oogonia. The product of sexual reproduction is an oospore which germinates by reduction division. Motile male gametes are totally absent. The antheridial nuclei serve as male gametes. The cell wall is made of cellulose.
Some important members of the group are Saprolegnia, Plasmopara, Pythium, Phytophthora, Albugo etc. Among these are some devastating plant pathogens, like Phytophthora infestans causing late blight of potato, Plasmopara viticola causing mildew of grape, Pythium debaryanum causing seedling blight of many plant species, Peronospora tabacina infecting tobacco plants, etc.
Morphological characteristics and life-cycles of an aquatic oomycetes viz. saprolegnia and a terrestrial one, viz. phytophthora are discussed below:
Saprolegnia grows as a cottony mass on decaying plant and animal organisms in water. The fungal body consists of a profusely branched coenocytic mycelium with branched rhizoids growing into the substratum from which the fungus draws nourishment. Asexual reproduction takes place by formation of zoospores in sporangia which are separated from vegetative hyphae by cross-walls.
The entire contents of the sporangia which are cut off from hyphal tips are consumed for formation of biflagellate zoospores. These zoospores are called primary zoospores and they bear two equal tinsel- type flagella at one end of the cell. The primary zoospores, after some time, become encysted. When they germinate, each produces secondary zoospores.
These are elliptical, laterally biflagellate, bearing an anteriorly directed tinsel-type flagellum and another posteriorly directed whiplash-type flagellum. This phenomenon of production of two types of asexual zoospores is called diplanetism. The secondary zoospores germinate to produce haploid somatic mycelium.
Sexual reproduction of Saprolegnia takes place by gametangial contact between antheridia and oogonia which usually develop from the same hypha. The sex organs are separated from the rest of the mycelium by cross-walls and both are multinucleate. The oogonial nuclei are organized into a number of well-developed oospheres (eggs).
The antheridial contents are transferred through a pore in the contact region into the oogonium and they fertilize the oospheres. The oospheres are then transformed into oospores which become thick-walled and pass into a resting phase. Eventually, the oospores are liberated and germinate by meiosis to produce a vegetative coenocytic mycelium.
The life-cycle of Saprolegnia is shown in Fig. 5.10:
Phytophthora infestans is a terrestrial oomycete. It grows saprophytically in soil and can infect potato plants causing late blight. This devastating disease was responsible for the great Irish potato famine. Like other phycomycetes, P. infestans has well branched, non-septate, multinucleate vegetative hyphae. The organism can reproduce asexually by formation of thick-walled lemon-shaped sporangia. They are borne on branched sporangiophores which come out through the stomata of potato leaves.
The sporangia are easily detached and after shedding they can act either as conidia or may produce biflagellate zoospores with laterally placed flagella. When the sporangia act as conidia, they germinate directly to produce vegetative mycelium. The zoospores produced in sporangia have an anterior tinsel- type flagellum and a posteriorly directed whiplash flagellum. The zoospores become encysted and eventually germinate to produce vegetative mycelium.
Sexual reproduction occurs by gametangial contact between an antheridium which encircles the base of the oogonium (amphigynous). Fertilization leads to formation of a single oospore in an oogonium. After release, the oospore germinates to form a sporangium. Its diploid nucleus undergoes reduction division during germination.
The lemon-shaped sporangium produces numerous biflagellate haploid zoospores. The zoospores after liberation become encysted and eventually germinate to form vegetative mycelia. The vegetative hyphae produced either by asexual means or sexual, can grow saprophytically in soil and can infect potato plants when they are available. Another serious disease of potato crops, known as early blight is caused by Alternaria solani, an imperfect fungus.
Asexual and sexual phases of the life-cycle of Phytophthora infestans arc featured in Fig. 5.11:
Taxonomic Group # 5. Zygomycetes:
These fungi are characterized by the formation of zygospores through fusion of two multinucleate gametangia of opposite mating types (+ and -), as well as by absence of any motile spores, sexual or asexual. Vegetative hyphae in young condition are non-septate and multinucleate, but older hyphae are generally septate. Cell wall is made of chitin.
Zygomycetes are mostly terrestrial and they are widely distributed by aerial spores formed within sporangia. The sporangia are borne on a sporangiophore which may arise singly or in groups from a horizontally spreading mycelium. Sporangiophores may be branched as in Mortierella and Blakesleea, or un-branched as in Rhizopus, Mucor etc. In some genera the sporangial head bears sporangioles containing spores.
The different forms of spore-bearing heads are shown in Fig. 5.12:
Sexual reproduction in zygomycetes takes place by fusion of gametangia of opposite mating types, usually designated as (+) and (-). The process is known as gametangial copulation (see Fig. 5.4). The fusion product is a multinucleate zygospore. The zygospores are usually dark coloured and have a highly tuberculate surface.
The two fusing gametangia may be of similar size as in Mucor, Rhizopus etc. or one of them may be larger than the other one, as in Absidia or Zygorhynchus. The fusing gametangia may originate from the hyphae of the same mycelium (homothallic) or from two mycelia of opposite mating types (heterothallic).
Pilobolus is an interesting zygomycete. The sporangium is a small lens-shaped structure borne on the tip of a large sub-sporangial vesicle containing yellow-orange carotenoid pigment. The vesicle is positively phototrophic due to the pigment. A mature sporangium is forcibly ejected from the sporangiophore.
Some members of zygomycetes, like Entomophthora and Empusa include species which can grow parasitically on insects, like wasps and flies. Empusa muscae causes an epidemic of flies.
Mucor is a well-known genus of zygomycetes some species of which can grow also under anaerobic conditions in culture media containing organic substrates, like yeast-extract, peptone and acetate (e.g. Mucor rouxii). Under anaerobic conditions, its growth form changes and the young hyphae produce arthrospores by budding.
Some economically important zygomycetes include Mucor and Rhizopus which are used in preparation of fermented food, such as “tempeh” in Indonesia and a type of cheese, called “sufu”. Rhizopus nigricans is employed for transformation of steroids in synthesis of human hormones. Blakesleea trispora is used commercially for extraction of carotenoids.
The features which distinguish zygomycetes from the other phycomycetes are:
(a) Presence of chitin in the cell wall, a characteristic also shared by some chytrids. Chitin is the characteristic cell wall component of ascomycetes and basidiomycetes.
(b) Absence of any form of motile spores, sexual or asexual.
(c) Formation of a zygospore arising from gametangial copulation, whereby the contents of two mating gametangia are fused.
(d) Asexual reproduction by sporangiospores. The spores are liberated by opening of the sporangia and spread by air current. Thus the zygomycetes are adopted to a land habit.
Taxonomic Group # 6. Ascomycetes:
The fungi belonging to this class are characterized by the presence of well-developed septate hyphae, (though there are also some unicellular forms), asexual reproduction by conidia or arthrospores and sexual reproduction leading to formation of ascospores in sac-like structures, called asci.
Yeasts represent the unicellular ascomycetes. There are a large number of genera and species, some of which are of great economic importance as well as academic interest. Yeasts multiply in two different ways — by budding and by fission. Accordingly, they are known as budding yeast e.g. Saccharomyces spp. and fission-yeasts, e.g. Schizosaccharomyces spp. Saccharomyces produces a protuberance or bud from its cell. This bud gradually increases in size.
The nucleus of the mother cell divides and a daughter nucleus passes into the bud. The bud may get detached leaving a scar on the mother cell, or it may form another bud. Several buds may be produced in succession which remains attached to each other forming a cluster of yeast cells (Fig. 5.13A). In the fission yeasts, a cell divides in the usual way by binary fission to produce two daughter cells which separate (Fig. 5.13B).
Sexual reproduction in budding and fission yeasts occurs in different ways. In the fission-yeast, Schizosaccharomyces octosporus, two haploid yeast cells of opposite sex conjugate with each other to form a diploid zygote. The zygote nucleus (2n) divides meiotically to produce eight haploid nuclei from which eight ascospores develop. The ascospores after being released germinate to produce haploid yeast cells (Fig. 5.14A).
In the budding-yeast, Saccharomyces cerevisiae, sexual reproduction takes place by conjugation of two haploid cells to form a diploid zygote. The zygote nucleus does not undergo meiosis immediately, instead the diploid cell multiplies by budding producing a generation of diploid yeasts.
In some of these yeast cells under appropriate conditions meiosis occurs with formation of four, ascospores. The ascospores after release germinate to yield a generation of haploid yeast cells which also multiply by budding. The life-cycle of budding yeasts has therefore, haplo-diplobiontic phases.
The life cycles of schizosaccharomyces octosporus and Saccharomyces cerevisiae are shown in Fig. 5.14A and 14B:
Some yeast-like fungi, such as Candida, Torulopsis, Rhodotorula etc. are capable of reproducing by budding. They also form rudimentary hyphae, known as pseudo-mycelium. From these hyphae, arthrospores are produced by budding. Ascospore formation in these genera is unknown. In general, ascus formation in specialized structures, called ascocarps is absent in both true yeasts and yeast-like fungi. In contrast, ascocarps are formed in all mycelial ascomycetes.
Ascocarps are well-differentiated fruit-bodies in which asci are formed along with sterile hyphae, called paraphysesJ Ascocarps are morphologically distinguished into three types, — cleistothecia, apothecia and perithecia. A cleistothecium is a more or less spherical closed structure without any opening.
The asci are formed randomly within this structure and the ascospores are liberated by decay of the covering layers of cells. Cleistothecia are formed typically in the perfect stages of the genera, Penicillium and Aspergillus, known as Eurotium. An apothecium is a more or less flat disc-like or sometimes cup-shaped structure bearing asci on their surface. Among the apothecial ascomycetes are genera, like Peziza, Morchella, Ascobolus, Tuber etc.
The third type of ascocarp is a perithecium which is a flask-shaped structure with an opening at the tip. The opening is called an ostiole. Among the well- known perithecial ascomycetes are Claviceps and Neurospora. The three types of ascocarps are formed in separate groups of ascomycetes. The cleistothecial ascomycetes are referred to Plectomycetes, the apothecial members to Discomycetes and perithecial forms to Pyrenomycetes.
The three types of ascocarps are shown in Fig. 5.15:
Among the important genera of cleistothecial ascomycetes (Plectomycetes) are Penicillium, Aspergillus, Erysiphe etc. Penicillium and Aspergillus are imperfect stages of Eurotium. Both genera include large number of soil-inhabiting species which reproduce asexually by conidia.
Several species of both these genera are of commercial importance. Penicillium notatum and P. chrysogenum are used for industrial production of penicillins. Some species are used for making Roquefort and Camembert cheese.
Aspergillus niger is employed for production of citric acid and A. terreus for itaconic acid. A. oryzeae is the source of various therapeutically and commercially used enzymes, specially amylase. Some species of Aspergillus, like A. fumigatus may act as opportunistic human pathogen. A. flavus may infect edible seeds, like ground nut and produce a deadly toxin, called aflatoxin. The compound is also carcinogenic. Erysiphe and related genera are plant pathogenic and cause mildew disease of many plants of economic importance.
The members of the above-mentioned genera reproduce asexually by formation of conidia. The conidial chains in Aspergillus and Penicillium often form long columns. Conidia are cut off in characteristic manner from conidiophores (Fig. 5.16).
Sexual reproduction of members of Plectomycetes takes place as in other ascomycetes by formation of antheridia and ascogonia. From the fertilized ascus, ascogenous hyphae are produced and asci are formed in the cleistothecia.
Ascospores of Eurotium are eight per ascus and they are typically provided with an equatorial crest. In mature asci, the ascospores lie free within the cleistothecium due to dissolution of ascal wall. The ascospores are liberated by rupture of the cleistothecial covering layer. On germination, ascospores give rise to haploid mycelia of two opposite mating types.
The Discomycetes include apothecium-forming ascomycetes, like Pyronema, Peziza, Ascobolus etc. Some of these fungi, like Morchella and Tuber are highly prized edible ones. Pyronema confluens and Ascobolus sp. are well-studied fungi useful as demonstration materials. Ascobolus grows on decaying cow-dung forming saucer-shaped apothecia in profusion.
Fruit bodies of tuber and morchella are shown in Fig. 5.17:
The pyrenomycetes are perithecia-forming ascomycetes. An important fungus of this class is Claviceps purpurea which infects the cereal crop, rye (Secale spp.) and produces ergots. Other species of the genus attack other grasses, including barley.
Claviceps purpurea causes ergot disease of rye. The cereal grains are replaced by horny structures, called ergots. These ergots are actually sclerotia of the fungus. The ergots are the source of several pharmacologically useful alkaloids of which ergometrine is the most important. On germination, the ergot produces several stalked spherical structures, known as stromata or spheridia.
The spherical head of the stromata contain a series of perithecia in which asci develop. The ascospores are long thread-like. The fungus reproduces also asexually by formation of conidia. The small unicellular conidia are produced in large numbers and they accumulate as dew drops on the surface of the inflorescence. The conidia are disseminated by insects which feed on them.
Sexual reproduction takes place in the usual way through formation of antheridia and ascogonia. Asci are formed from the ascogenous hyphae. The filiform ascospores are extruded from the elongated asci and the spores collect at the ostiole.
The ergots (sclerotia) are produced from hyphal mats invading the ovarian tissue of the host. The ovary is transformed into a sclerotium which in young condition is soft and whitish, but later becomes hard and brownish black. It is a resting structure of the fungus. After shedding, it germinates to produce perithecia with asci and ascospores (Fig. 5.18).
Another genus of academic interest is Neurospora. N. crassa and N. sitophila have been widely used in basic genetical research. Neurospora spp. are common bread-molds and they cause damage in bakery industry. The genus includes both heterothallic and homothallic species. They are rapidly growing pigmented organisms with multinucleate septate hyphae.
Asexual reproduction in some species occurs through two types of unicellular conidia differing in size — uninucleate micro-conidia and rriultinucleate macro-conidia. Sexual reproduction takes place by transfer of a male nucleus from a spermatium to an ascogonium through a trichogyne. A conidium may act also as a spermatium. The ascocarps developing from ascogenous hyphae are perithecia and the asci are elongated structures with eight ascospores.
Morphological characteristics of Neurospora are shown in Fig. 5.19:
Taxonomic Group # 7. Basidiomycetes:
Basidiomycetes are characterized by formation of basidia and basidiospores. They constitute a large fungal group comprising some 12,000 species. In most of them, male and female sex organs are absent and plasmogamy generally takes place through fusion of vegetative hyphae leading to a dikaryotic phase. Karyogamy is postponed till the formation of basidia.
In basidia the paired nuclei fuse to form a diploid nucleus which undergoes immediate meiosis to form haploid nuclei which pass into the basidiospore. The haploid basidiospores germinate to produce a monokaryotic mycelium. In holo-basidiomycetes represented by mushrooms, toadstools, puff-balls, bracket fungi etc., basidia are formed in well-differentiated fruit bodies, called basidiocarps. In these fungi, the basidia are aseptate. In other basidiomycetes, known as Phragmobasidiomycetes, represented by the rust fungi, smuts and some other groups, the basidia are septate, either longitudinally or transversely.
There are also some unicellular basidiomycetes. The best known among them is a yeast-like fungus, known as Sporobolomyces. The single cell of this fungus forms basidiospores. Another example of such basidiomycetes is the perfect stage of a human pathogen, Cryptococcus neoformans.
The perfect stage is known as Filobasidiella neoformans (Fig. 5.20):
The structure of well-differentiated basidiocarps in members of Holobasidiomycetes is highly variable. In mushrooms and toadstools, the basidiocarps are provided with a thin or fleshy stalk, called a stipe bearing in mature stage an umbrella-shaped cap, called a pileus. The pileus may be thin or fleshy having an upper sterile tissue and bearing gills on the undersurface.
The gills are lined with basidia together with sterile hyphal structures. The basidia project at right angles to the vertically hanging gills. Each basidium is a club-shaped structure bearing generally four basidiospores on sterigmata. The basidiospores are ejected forcibly and they fall downward on the ground where they germinate to produce monokaryotic somatic mycelium.
Most of the basidiomycetes are heterothallic, and dikaryotization by hyphal fusion occurs only when compatible monokaryotic hyphae meet and fuse with each other. Fruit-body formation under suitable environmental conditions is possible only from the dikaryotic mycelium.
The basidiocarps of puff-balls are usually closed structures consisting of a sterile covering, called peridium containing inside the fertile tissue, the gleba. The gleba is divided into regular or irregular chambers lined with basidia. On maturity, gleba as well as the peridium are disorganized and become degenerated to release the basidiospores. Some genera of puff-balls and related fungi include Lycoperdon, Scleroderma (both have edible species), Phallus etc. In Phallus, the gleba comes out on maturity piercing the peridium.
In the bracket-fungi, the basidiocarps may be sessile or stipitate. The stipe may be centrally or laterally placed. Many of these fungi grow on felled logs of trees and cause wood-rotting. The basidiocarps may be woody or fleshy. The undersurface of the fruit-bodies may be smooth or, more often, is provided with pores, pegs or teeth.
The basidia-forming tissue (hymenium) in the forms with pores or teeth is within these vertical pores or teeth. The pores may be circular, hexagonal or irregular depending on the genus. Some representative genera of bracket-fungi include Polyporus, Polystictus, Fomes, Ganoderma, Hexagonia, Trametes etc.
Some typical basidiocarps and spore-bearing tissues are schematically shown in Fig. 5.21:
The phragmohasidiomycetes are provided with septate basidia. The basidia may be longitudinally septate as in Tremella, or transversely septate as in the rusts, smuts and related fungi. Rusts and smuts include some important pathogenic species infecting crop plants. Rusts, smuts and related genera do not form basidiocarps. But other members of phragmohasidiomycetes, like Tremella, Auricularia etc., form basidiocarps.
The cause of black-rust of wheat, Puccinia graminis tritici is an obligate parasite which requires two different hosts, viz. wheat and berberry to complete its life cycle (heteroecious). Other rust fungi, like Uromyces dianthae, Cronartium ribicola etc. are also heteroecious. These rust-fungi are also called long-cycled. There are also autoecious rust fungi which complete their life-cycle in a single host plant e.g. Uromyces phaseoli, Puccinia helianthi etc.
Puccinia graminis tritici produces as many as five different types of spores on wheat and berberry. These are spermatia (n) and aeciospores (n + n) on berberry, and uredospore’s (n + n), teleutospores (n + n) on wheat. The teleutospores after they are shed, germinate to produce basidiospores (n). Karyogamy takes place in teleutospores and is followed by meiosis.
The haploid nuclei pass into basidiospores. Spermatia are formed in pycnia on berberry leaf. They are transported to the receptive flexuous hyphae projecting from the pycnial opening. The spermatial nucleus passes down the flexuous hypha and pairs with the female nucleus to form a dikaryotic cell. Dikaryotic hyphae developing from this cell produce large number of aeciospores in specialized structures, called aecia. Aeciospores infect wheat plants and cause the black-rust disease.
The black-rust of wheat is spread from one plant to another by single-celled bi-nucleate uredo-spores formed in uredosori. Towards the end of the growing season, thick-walled two-celled, bi-nucleate spores, called teleutospores are formed in teleutosori. Teleutospores are shed and they germinate to produce basidiospores which infect berberry.
The life-cycle of P. graminis tritici is represented in Fig. 5.22 and the various spore-forms in different rusts Fig. 5.23.:
The smut fungi, like Ustilago, Tilletia etc. are also without basidiocarps. In these fungi, dikaryotic chlamydospores are produced which are equivalent to teleutospores of rust fungi. The chlamydospores germinate to produce basidia which are transversely septate in Ustilago and non-septate in Tilletia.
Basidiospores are borne directly on the basidia without sterigmata. The pair of nuclei in the chlamydospore fuse and undergo reduction division as in other basidiomycetes. These fungi infect many important cereal plants, like wheat, barley, maize etc. and they grow as intercellular parasites. However, they are not obligate parasites.
The ehlamydospores and basidiospores of Ustilago and Tilletia are shown in Fig. 5.24:
Taxonomic Group # 8. Deuteromycetes (Fungi Imperfecti):
These fungi do not represent a homogeneous group, like the others, but they are an assemblage of forms for which sexual reproduction is unknown. These fungi are considered as imperfect, because their perfect stage i.e. sexual reproductive phase is not known.
Therefore, they have been assorted to a Form class which includes many form genera. Placement of any fungus in this class is a stop-gap arrangement, because as soon as the sexual stage of any species of this group is discovered, it is transferred to its legitimate taxonomic position.
For example, the perfect stage of Helminthosporium oryzeae, the fungus causing brown-spot disease of rice has been found to be Cochliobolus miyabeanus which is an ascomycete. Similarly, perfect stages of Penicillium and Aspergillus are species of Eurotium which are also ascomycetes. The perfect stage of Cryptococcus neoformans, a human pathogen is a basidiomycete, Filabasidiella neoformans.
Deuteromycetes generally form well-developed septate mycelia with multinucleate cells. Some of them, like Candida albicans form rudimentary pseudo mycelium. The imperfect fungi generally reproduce by formation of conidia. Some of them do not form any spores (mycelia sterilia), but may produce sclerotia which are compact masses of sterile hyphae.
Sclerotia are resting structures and help the fungus to survive under incongenial environmental conditions. In general, propagation occurs by conidia which are formed in the aerial hyphae, often from specialized conidiophores. In some forms the conidiophores arise in groups in specialized structures, known as acervulus, sporodochia, perithecia etc.
Although sexual reproduction is absent in Deuteromycetes, genetic recombination is known to occur spontaneously through fusion of genetically dissimilar nuclei. The phenomenon is known as para-sexuality. In the absence of true sexuality, para-sexuality produces the genetic variabilities necessary for evolution of new strains.
Many members of this class are well-known pathogens mainly parasitizing plants, but also some can infect humans. Some examples of destructive plant pathogens are Alternaria solani (cause of early blight of potato), Colletatrichum falcatum (red-rot of sugar cane), Fusarium udum (wilt of pigeon pea), Cercospora personata (tikka disease of ground nut), Helminthosporium oryzeae (brown-spot of rice) etc. Among the human pathogens are Candida albicans, Histoplasma capsulatum, Trichophyton gypseum, T. mentagophritis, T. purpureum, Cryptococcus neoformans, Coccoides immitis etc.
Conidia and conidiopheres of some imperfect fungi are shown in Fig. 5.25: