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In this article we will discuss about the classification of phylum Basidiomycota.
1. Class Urediniomycetes:
The class Urediniomycetes includes the rust fungi, that cause rust diseases of plants, and symbiotic fungi that parasitise insects parasitising plants and form intricate fungal structures called ‘fungal gardens’.
The two types of fungi are placed in two orders, Uredinales and Septobasidiales, respectively:
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Order Uriedinales:
The order includes the rust fungi, which are characterised by the formation of teliospores that originate from the terminal cells of the dikaryotic hyphae. On germination, these give rise to basidiospores formed on short, pointed sterigmata and -discharged explosively.
These fungi lack the dolipore septa and clamp connections, and do not form basidioma. The genera of the rust fungi are identified by the structure of their teliospores. There are five families in the order. We shall study the family Pucciniaceae, which includes the genus Puccinia.
Family Pucciniaceae:
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Genus Puccinia:
Puccinia is an obligate parasite and is extremely host-specific. Attempts to isolate and grow the fungus in the laboratory, which started at the beginning of this century, after several failures, have succeeded to a great extent. Williams, Scott and Kuhl, the Australian scientists, succeeded in 1966 in growing P. graminis tritici on a non-living mineral medium.
But in their culture the organism behaved inconsistently and erratically. It never formed the various spores in the sequence they are formed on natural hosts. This could be due to the change from parasitic to saprobic mode of life in the culture tube. The organism, in nature however, lives only as an obligate parasite.
The fungus causes ‘rust’ disease of several economically important plants.
Some of the important species are as follows:
1. P. graminis- It has 6 sub-species or formae specialis, which attack only one particular host of Graminae; but basidiospores of all subspecies infect Berberis, the alternate host.
P. graminis tritici infects wheat.
P. graminis avenae infects oats.
P. graminis hordei infects barley.
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P. graminis secalis infects rye.
Even these sub-species consist of physiological races which grow only on a particular variety of the host. The physiological races are designated by numbers, e.g., P. graminis tritici 138.
2. P. malvaceareum causes hollyhock rust.
3. P. coronata causes crown rust of oats.
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4. P. artirrhini causes snapdragon rust.
Physiological Races:
P. graminis infects several hosts belonging to the family Graminae. The uredospores of P. graminis, which infect wheat, fail to infect another host – say oat or barley. Similarly, uredospore produced by P. graminis on barley, fails-to infect any other host. Erikson, (1891) a Swedish botanist, designated each isolate by a third name (trinomial nomenclature) and called it sub-species or formae specialis.
Thus, the name P. graminis tritici was given to the isolate that infects wheat; P. graminis hordei for the isolate infecting barley and P. graminis avenae for the isolate infecting oat. Later, it was found by Stakman and Levine of the U.S.A. (1922) that in P. graminis tritici, the isolate that infects one variety of wheat fails to infect another variety of the host.
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These are called physiological races. Over 200 physiological races of P. graminis tritici are known. The physiological races are designated by numbers, e.g., P. graminis tritici 138. The work of identification of the physiological races is very important. Before developing a resistant variety, a knowledge of the exiting races is essential.
It is a difficult job and is done only by specialists. But we can see how it is done. For delineating the races of Puccinia graminis tritici, uredospores obtained from different areas, are inoculated on differential varieties (these are 12 in number for wheat rust fungus).
The infections produced, on the basis of number and size of pustules formed, are graded on a standard chart as follows:
0. Immune – no pustules produced, no sign of infection.
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1. Very resistant – only faint chlorotic spots surrounded by necrotic tissues appear.
2. Moderately resistant – minute scattered pustules appear.
3. Moderately susceptible – medium size pustules and chlorotic spots appear.
4. Very susceptible – medium to big size confluent pustules formed.
X. Heterogeneous – size of pustules vary, but mainly mixture of type 1 and 4.
Small variations in the grades are indicated by one or two plus or minus signs, e.g., 2–, 2—, 2+, 2++ etc. The identification of physiological races is a difficult task; the fluctuating effect of the environmental factors adds to the complexity. Thus, certain races produce the X-reaction under some conditions but not under others.
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Fungi are a treacherous lot. Their physiological races can be further demarcated into biotypes by adding another variety to the standard differentials.
Knowledge of all the existing physiological races is essential before work on breeding for resistance is undertaken. Mehta, in 1931, started the work on examining the physiological races in India which was continued mainly by Vasudeva, Prasada, Gokhale, and Uppal.
New races continue coming up in the nature’s ‘field laboratory’ by the genetic recombinations and mutations. Once they arrive, they multiply unchecked, as other races are ineffective on the hitherto resistant host. The resistant host, thus, selects the new race and brings about, what is aptly called, the man-guided evolution. Thus, the plant breeders have to be on the alert.
2. Class Ustilaginomycetes:
Ustilago causes smut diseases of several economically important plants. Some of the important species and the diseases they cause are – U. tritici (loose smut of wheat), U. nuda (loose smut of barley), U. avenae (loose smut of oats), U. maydis (smut of maize), U. scitaminea (smut of sugarcane), and U. occidentalis (smut of cyanodon).
In nature, all the species, except U. maydis, live only as parasites on their specific hosts and are incapable of saprobic existence. U. maydis, however, grows profusely on manure heaps. As will be evident later, this species is different in many ways from rest of the species of Ustilago. Though incapable of saprobic existence, Ustilago is not an obligate parasite because it has been cultured in the laboratory.
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In all smut diseases, except the smut of maize, the hyphae become systemic in the host tissue, but the sori of teliospores are formed only in certain parts of host, usually the ovary or the inflorescence. The sori are covered by a host membrane. This is a distinguishing feature of this genus.
In loose smut of wheat, barley and oat, the ears are transformed into a black mass of spores, which after the rupture of host membrane, become free and are blown by the wind in such huge amounts that something like a smut cloud can be seen. In smut of sugarcane, the whole floral axis becomes a black, sooty whip-like structure due to its transformation into smut spores. The smut of maize is different and smut sori are formed on any part of the host.
Except U. maydis, which grows well saprobically, all other species grow only parasitically inside their host tissues.
The mycelium is of two types. The primary mycelium, consisting of uninucleate cells, is formed by the germination of basidiospores and is of very short duration; even absent in some cases. The secondary mycelium is dikaryotic, i.e., the hyphae consist of binucleate cells. It extends practically through the entire life.
It grows intercellularly and draws nutrition from host cells through haustoria (except U. maydis). The mycelium grows extensively and is present in every part of the host. Eventually, at the end of the host season, the hyphae accumulate in the part where smut sori are to be formed. The binucleate cells round up to form thick-walled teliospores.
In nutrient solutions, the mycelium breaks into yeast-like cells which show characteristic ‘budding’. This is known as the Torula or yeast stage. This phenomenon has earlier been seen in Mucor and Rhizopus.
Asexual Reproduction:
Budding of basidiospores is the most common method of asexual reproduction. Fragmentation, and in some species, conidia are the other means of asexual reproduction.
Sexual Reproduction:
Sex organs are absent. But the sexual reproduction, represented by karyogamy followed by meiosis, does occur and brings about the genetic recombination. The function of sex organs is taken up by somatic cells which transform into teliospores. Plasmogamy, karyogamy, and meiosis are separated in space and time, that is, they occur at different places and at different times.
Plasmogamy is brought about by the fusion between two uninucleate cells of opposite mating types, which may be the basidiospores or cells of the primary mycelia. Most of the smut fungi are heterothailic. The karyogamy is delayed until teliospores germinate and form a promycelium. The diploid zygote nucleus undergoes meiosis to form four haploid nuclei, two of each mating type.
The promycelium is divided into four cells by horizontal septa. A bud arises from each cell into which one nucleus passes. The bud develops into a basidiospore and is pinched off. In some species the promycelium may continue forming the basidiospores. In some species, e.g., U. maydis, the basidiospores themselves by budding form sprout cells (= daughter basidiospores or secondary sporidia). In U. tritici there are no basidiospores; the cells of the promycelium form infection threads which fuse to establish a dikaryotic hypha that grows and develops into the secondary mycelium.
3. Class Basidiomycetes:
Order Agaricales:
Family Agaricaceae (Genus Agaricus):
Agaricus has long been taught in mycology classes as the typical example of a mushroom and gill fungi. A. campestris is the most important species which is cultivated extensively under the industrial name A. bisporus. In culture, the physiological and morphological characters show marked differences to justify the change in name.
The fungus grows in lawns, fields and forests round the year. The extensive mycelium remains hidden in the soil; only the fruiting body is visible.
The mycelium, which remains underground and grows saprobically, is the dikaryotic, secondary mycelium. It is formed by somatogamy between monokaryotic primary mycelia of different mating types formed by germination of the basidiospores. The primary mycelium is of short duration. The fruiting body, which forms the magnificent umbrella above the ground, is made up of dikaryotic hyphae called tertiary mycelium. The fruiting body is ephemeral and lives only for a few days.
The secondary mycelium, however, is perennial and continues growing for several years and forms fruiting bodies year after year. The fruiting body, or the basidioma, has a stalk (stipe) and a circular cap (pileus). A skirt-like ring of tissue, called annulus, surrounds the stipe a little below the pileus.
The gills (= lamellae) can be distinctly seen if the stipe is removed and the pileus is inverted. The pileus on the under surface, bears numerous, vertically-hanging gills, which converge from periphery towards the stipe. The gills are of different lengths and bear basidia all over the surface. The basidia produce basidiospores in astronomical numbers.
The fruiting bodies of Agaricus campestris form fairy rings. The umbrella-shaped basidiomata, the fungus flowers, formed in circles on the ground, provide a beautiful spectacle which could be described as fairies dancing in a circle.
The fruiting bodies being ephemeral, the pleasant sight is short-lived. But the mycelium persists in the soil and continues growing as a circular colony, in the same way as a circular fungus colony grows in a Petri dish in the laboratory. As the outer edge of the colony grows, older parts in the centre die and degenerate. The next time when the fruiting bodies are formed, the ring is bigger in diameter. The dead remains of the fruiting bodies make the soil more fertile and rings of greener grass are formed where once stood the umbrellas.
Asexual Reproduction:
A. campestris produces only chlamydospores; conidia and oidia are not known.
Sexual Reproduction:
The sexual reproduction is represented by karyogomy and meiosis, which occur in basidia, borne on the gills. The nuclei of opposite mating types come together and form dikaryons after the fusion of the monokaryotic primary hyphae. The dikaryons multiply by conjugate divisions in the extensive, secondary mycelium.
Ultimately, club-shaped basidia develop from the terminal cells of these secondary hyphae. The binucleate cell enlarges and becomes broader and club-shaped. Karyogamy and meiosis occur resulting in the production of four haploid nuclei. Segregation of sex occurs during the meiosis and the nuclei formed are of two or more mating types, depending on the type of the heterothallism (bipolar or tetrapolar).
The nuclei later migrate into the sterigmata and from there into the basicfiospores, perched asymmetrically on the tips of the sterigmata. The basidiospores, on germination, form primary mycelia, which by anastomosis and plasmogamy, establish the secondary mycelium. The basidiospores, which are borne asymmetrically on sterigmata are discharged forcibly by water drop method.
The basidiospores are produced in enormous numbers. If the earth is not covered by mushrooms, this is because the chances of failure of spore germination are also equally great. Spores ‘rain down’ at a rate of half a million spores per minute during the two or three days of the existence of the sporophore (fruiting body).
If the pileus is detached from the stipe and placed on a paper, a spore print can be obtained showing radiant lines between the spore masses which represent the spaces between the gills. The spore print is of great help in identification of species.
The basidiospores are violently discharged by the ‘water bubble method’. The trajectory (path) of the released spores is a sporobola. Spores are shot horizontally up to the middle of the space between the gills which then fall down vertically.
The distance between the gills far exceeds the distance to which basidiospores are shot. Even a little tilt of 5° of the gills from the exact vertical position may obstruct the free downward fall of the spores. But this is not allowed to happen. The gills are positively geotropic and any deviation from the vertical position is immediately corrected by growth movements at the place of attachment of the gills with the pileus.
Family Lycoperdaceae:
Genus Lycoperdon (The Puff Ball):
The puff-balls are globose to pyriform in shape, depending upon the length of the stalk. These are found growing in pastures on ground in woods or on tree stumps. The basal part is sterile. When young, it is surrounded by fragile spines which soon fall off or get rubbed off.
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The outer layer of the peridium withers and a pore is formed on the inner membrane at the top of the fruiting body. This membrane acts like a bellows, puffing out spores through the pore when some object strikes the membrane. The fertile portion, gleba, is of lacunose type, i.e., consists of cavities, each lined with a hymenium. All the species are edible.
The order has 3 families. We shall study the family Geastraceae to which belongs the genus Geastrum, the ‘earth star’. (The family, along with Lycoperdaceae was put under order Lycoperdales, which is now derecognized.)
Genus Geastrum:
The earth stars differ from the ‘puff-balls in that the outer peridium splits radially and opens out like rays of a star. Such fruit bodies lying on the ground gave the name ‘earth star’. A pore is formed on the inner peridium at the top .The spores are released through this pore.
The order includes the polypores, the bracket and shelf fungi that cause severe damage, especially to forest trees. There are 6 families. We shall study genus Polyporus of the family Polyporaceae.
Genus Polyporus:
This is the largest genus of Polyporaceae and its several species are important wood-rotting fungi. P. sulphurens causes wood rot of oaks and other trees; P. squamosus causes ‘heart-rot’ of a number of forest trees; P. betulinus causes ‘heart-rot’ of birch trees. Destruction of the heart wood makes the trees hollow which eventually die. Some, for example, Polyporus schweinitzii, attack the lower part of the trunk and the roots (butt-end rot), making the trees liable to snap off in gales.
The mycelium ramifies within the substratum. It consists of septate dikaryotic hyphae which usually show clamp connections. The fruiting body (the basidioma or sporophore) develops as a laterally-stalked, fan- shaped, bracket which may be 20 to 40 cm in diameter and 2 to 3 cm thick. A vertical section of the basidioma shows the following zones – (a) pileus surface, (b) context, (c) tube layer, (d) pore surface and (e) hymenium.
The pileus surface may be smooth or encrusted. The context is white below, which lines the tube layer. The tube layer consists of vertically- placed tubes which open below at the under surface of the fruiting body called the pore surface; hence the name Polyporus.
The tubes are lined internally by hymenium, which consists of basidia and sterile structures, like paraphyses, cystidia and setae – all placed at right angles to the length of the tube. The basidiospores are shot into the cavity of the tubes, which then fall below through the pores and get disseminated by wind. Spore output is enormous. A single fruiting body may produce a billion spores.
The species may be annual or perennial. The annual forms produce new basidiomata each year. The perennial species add a new layer of tubes each year.
