In this article we will discuss about phylum Basidiomycota and Ascomycota of fungi. Also learn about their characteristics, reproduction and classification.
Phylum Basidiomycota of Fungi:
Characteristics of Phylum Basidiomycota:
The mushrooms and toadstools with which mycology started, the rusts and smuts, which have been devastating crops since recorded history, the ‘shelf’ and ‘bracket’ fungi that lie attached to the trees sapping their vitality, and all belong to Basidiomycota. These are the highest evolved fungi.
The important characteristics of the phylum are:
1. The basidium,
2. The thallus,
3. The clamp connections, and
4. The dolipore septum
1. The Basidium:
Like the ascus of Ascomycota, the basidium is a diagnostic feature of Basidiomycota. A typical basidium is a club-shaped structure bearing apically four basidiospores on pointed projections called sterigmata. The basidium may be septate, phragmobasidium or unseptate, holobasidium.
In rusts and smuts, the basidia are entirely of different nature. Basidiospores are formed after karyogamy and meiosis of the diploid nucleus in the basidium and are, therefore, uninucleate and haploid. These may be of two or more mating types. On germination, the basidiospores give rise to monokaryotic primary mycelium.
2. The Thallus:
Three types of mycelia viz., primary, secondary and tertiary are found in the Basidiomycota.
The primary mycelium is monokaryotic (i.e., made of haploid uninucleate cells) and is formed by germination of the basidiospores. These are of different mating types. It exists for a very short period and gives rise to the dikaryotic secondary mycelium.
The secondary mycelium consists of dikaryotic (binucleate) hyphae formed by the following methods:
a. Somatogamy between cells of monokaryotic hyphae,
b. Fusion of two basidiospores, and
c. Spermatization of receptive hyphae (in rusts only).
The single binucleate cell formed by fusion of two cells of the primary mycelia or between two basidiospores, divides repeatedly and gives rise to secondary mycelium, e.g. smuts.
In spermatization, which occurs in Puccinia, spermatia reach the female receptive hyphae through wind, insects, etc., and dikaryotize one of its cells. Thus, a binucleate cell is formed. The male nucleus divides and its daughter nucleus migrates into the adjacent cell, which then becomes binucleate.
This division and migration of the male nucleus converts the primary mycelium into a secondary mycelium, the secondary mycelium is multiplied by binucleate spores, e.g., aeciospores, which give rise to more and more thalli of secondary mycelium.
The secondary mycelium forms the major portion of the somatic phase. Thus, the Basidiomycota have an extensive and independent dikaryophase. In Ascomycota, the dikaryophase is of short duration, limited to the ascogenous hyphae, which have no independent existence. The dikaryophase ends with the formation of basidia, which usually develop on the terminal cells of the dikaryotic hyphae.
The karyogamy and meiosis occur in the basidia and four haploid nuclei are formed. The basidiospores, formed from these haploid nuclei represent the haplophase. In rusts and smuts, the karyogamy and meiosis occur in special spores called teliospores, which are formed from cells of the dikaryotic mycelium.
The tertiary mycelium is associated with the basidiocarp formation around the basidia. Its hyphae are also dikaryotic like the secondary mycelium.
3. Clamp Connection:
It is a hook-like structure associated with the conjugate division of the dikaryons in the secondary mycelium. It is a bypass for the nuclei, if they cannot pass through the septal pore, as may happen due to the presence of a dolipore septum. A small outgrowth called ‘clamp- connection’ arises between the two nuclei of a binucleate cell and forms a curved hook.
The two nuclei divide simultaneously. Their spindles are oriented in such a way that the daughter nucleus of the upper nucleus lies in the clamp while that of the lower nucleus comes close to the upper nucleus. The clamp bends and its tip touches the cell; the intervening walls dissolve and the nucleus in the clamp migrates into the cell to lie close to the lower nucleus.
Two septa are now laid down – one longitudinal at the place of the origin of the clamp connection, so as to cut its original connection with the cell, and the other transversely at right angles to the first longitudinal septum. A new daughter cell is formed. The clamp remains permanently attached to the hypha. Its presence indicates that the hypha is dikaryotic.
4. Dolipore Septum:
Electron microscopy has revealed another characteristic feature of Basidiomycota, the dolipore septum. It is a mechanism to stop nuclear movement from one cell to the other.
The septum around the central pore projects laterally to form a barrel- shaped pore. The pore on the two sides is surrounded by a cap-like structure – called the parenthesome or ‘nuclear pore cap’. It is made up of a double membrane and its function is to shut the pore.
The simultaneous occurrence of clamp connections and dolipore septum suggests that the clamp connection is a ‘by-pass’ in hyphae having closed septum.
Reproduction of Phylum Basidiomycota:
Budding, fragmentation, oidia, chlamydospores and conidia are formed by Basidiomycota. However, asexual reproduction is not of much importance in their dissemination.
Budding of basidiospores is quite common. Oidia are formed by separation of cells of primary mycelium. This increases the chances of contact between primary mycelia of opposite strains for dikaryotization. Fragmentation of the secondary mycelium is common. The inoculum for inoculating the compost in mushroom cultivation is obtained by fragmentation of secondary mycelium. Uredospores and aeciospores, produced by rusts, e.g., Puccinia, are conidial in origin.
Distinguishable sex organs are not formed in Basidiomycota except in Puccinia, where spermatia and receptive hyphae are distinctly the male and female structures. The plasmogamy, karyogamy and meiosis, which comprise sexual reproduction, occur at different intervals and at different places.
Plasmogamy occurs when secondary mycelium is initiated. Karyogamy is delayed due to the extensive dikaryophase. Eventually, the karyogamy occurs in the basidia formed by the terminal cells of the dikaryotic secondary mycelium, or in teliospores, and the promycelium.
Segregation of characters occurs during the meiotic division, which follows karyogamy. Four haploid nuclei ace formed. Sterigmata arise as little outgrowths; their tips swell, into each of which a haploid nucleus migrates. The swellings develop into basidiospores.
The teliospores germinate and form a club-shaped promycelium in which karyogamy and meiosis occur. It becomes septate and four cells are formed, each bearing a basidiospore on a short pointed sterigma. The karyogamy and meiosis, sometimes, occur in the teliospore.
Thus, the teliospore and the promycelium jointly perform the function of a basidium. The germinating teliospore is called hypobasidium, and promycelium, the epibasidium. Both jointly form the basidial apparatus. The teliospore before germination is regarded as encysted probasidium.
Discharge of Basidiospores:
In majority of Basidiomycota, the basidiospores are discharged violently. The mature basidiospores are perched asymmetrically on the tip of the pointed sterigmata. The basidiospores have a minute projection – the hilar appendix, near the point of attachment to the sterigma.
Just before a spore is shot out, a minute bubble of fluid, (not a drop) appears at the hilar appendix; it grows to a certain size and its expansion results in explosive throwing of the basidiospores, up to a distance of 0.1 mm. The mature spores are violently discharged in succession and not simultaneously.
About 90 per cent of the investigated genera of Basidiomycota are heterothallic. They may be bipolar (37 per cent) or tetrapolar (63 per cent). Bipolar species produce basidiospores of two mating types, while the tetrapolar species produce basidiospores of four mating types.
The Basidioma (= Basidiocarp):
Basidiomycota, except the rusts and smuts, form the basidia in fruiting bodies, called basidioma (pl. basidiomata.). Sometimes, they are beautiful and called fungus flowers. The basidiomata of mushrooms, formed in circles, got the name fairy rings. The somatic hyphae remain underground.
The basidiomata vary in size, from minute or microscopic to 3 feet or more in diameter. The shapes are fantastic. Mushrooms, shelf fungi, coral fungi, puff balls, stink horns and bird’s nest fungi are important basidiomycota named on the shape of their basidomata. Any one will love them at first sight!
Classification of Phylum Basidiomycota:
The phylum comprises 3 classes:
ii. Ustilaginomycetes and
The Urediniomycetes includes the rust fungi, while the Ustilaginomycetes contains the smuts. The Basidiomycetes includes the former Hymenomycetes and Gasteromycetes. The rust and smut fungi were placed together under Teliomycetes, while the Hymeno-, and Gasteromycetes were put under separate classes.
These changes of the separation of smuts and rusts, despite forming the teliospores, and the grouping together of Hymeno-, and the Gasteromycetes, is based on molecular phylogenetic studies of their 18 S ribosomal DNA gene, which is now considered the most reliable method of finding genetic relatedness.
Phylum Ascomycota of Fungi:
Characteristics of Phylum Ascomycota:
Fungi belonging to phylum Ascomycota are characterized by:
1. An ascus,
2. A septate mycelium,
3. The production of a fruiting body, and
4. The absence of flagellated structures.
Of all, the ascus is the most important and it is this character which makes a fungus an Ascomycete. An ascus is usually an elongated saclike structure which contains normally eight ascospores. The hyphae have septa which are perforated in the centre. Cytoplasmic strands and nuclei freely migrate from one cell to the other. The thallus, except in the yeasts, is filamentous.
Reproduction of Phylum Ascomycota:
The asexual reproduction occurs by:
1. Budding or fission – fission is characteristic of bacteria and among fungi it occurs in “fission yeasts” only,
2. Fragmentation, occurs frequently in all fungi,
3. Chlamydospores, and
The Ascomycota have specialized in conidial production and it is said that conidia have reached their zenith in this group. A variety of conidia are produced in hundred different ways. Conidiophores may lie externally on the hyphae or inside fruiting bodies called pycnidium (pl. = pycnidia) or acervulus (pl. = acervuli).
The conidiophores may be free or aggregated in various ways. When the conidiophores are fused it is called synnema (pl. = synnemata). Less compact fusion is called a coremium. Sometimes, conidiophores arise from a cushion-like stroma (a mass of hyphal tissue). It is called a sporodochium.
A pycnidium is a globular or flask-shaped fruiting body having an ostiole at the top while the acervulus is a disc-like structure usually formed under the epidermis or cuticle of the host plant. The conidia are of various shapes, sizes and colours. Sometimes, the conidial masses give characteristic colour to the fungus, e.g., green-mold Penicillium or black-mold, Aspergillus.
Sexual reproduction occurs by several methods, viz., gametangial copulation, gametangial contact, spermatization or somatogamy.
1. Gametangial Copulation:
The somatic cells sometimes act as gametangia (example yeast) which fuse to form a unicellular diploid cell, which develops directly into an ascus. There is no dikaryophase, i.e., at no stage the cells are dikaryotic.
2. Gametangial Contact:
Morphologically distinct antheridia and ascogonia (sing. = ascogonium, female sex organ) are produced in some Ascomycota. The ascogonia are usually globular and bear a hair-like outgrowth called trichogyne. Sometimes, the ascogonium is cylindrical and without a trichogyne.
The antheridia are usually club-shaped. The male nuclei pass into the ascogonium through a pore formed on the ascogonial wall at the point of contact with the antheridium. In some species, the antheridia, though present, are defunct (non-functional) and the ascogonia form asci parthenogenetically.
Spermatia are minute, conidia-like male cells which are produced on short spermatiophores in the same way as conidia are produced by conidiophores. The spermatia reaches the trichogyne, get attached to it and empty the contents into the trichogyne. The male nucleus migrates and reaches the ascogonium, where it pairs with the female nucleus to form the dikaryon (nuclear pair).
Somatic hyphae of compatible strains fuse and the male nucleus migrates into the ascogonium through the septal perforations. Frequently, undifferentiated somatic cells act as the ascogonium.
Development of Ascus:
The male nuclei, on reaching the ascogonium form dikaryons (pairs of nuclei) with the female nuclei. They don’t fuse immediately to form diploid nuclei as it occurs in Zygomycota. Thus, the karyogamy is delayed. A dikaryophase intervenes between plasmogamy and karyogamy.
Small protuberances appear on the ascogonium (or the somatic cell acting as the female cell), which grow and form ascogenous hyphae. The dikaryons multiply by conjugate (simultaneous) divisions and the daughter nuclear pairs migrate into the ascogenous hyphae.
Later, septa are laid down in the ascogenous hyphae. Each cell of the ascogenous hypha is dikaryotic except the terminal cell which contains a single nucleus. The dikaryophase in Ascomycota is represented by ascogenous hyphae; rest of the hyphae remain monokaryotic and take part in the formation of a thick protective covering around the asci.
Any dikaryotic cell of the ascogenous hyphae elongates and bends to form a hook or crozier-like cell, called crook cell. Its two nuclei divide mitotically in a way that their spindles lie vertically and parallel to each other. Two daughter nuclei, one from each spindle, lie at the bend of the hook while one nucleus lies at the tip and one near the basal septum.
Two septa are laid down to form three cells – uninucleate basal and apical cells, and binucleate penultimate cell. The penultimate cell acts as the ascus initial cell. The basal and apical cells later fuse and the nucleus of the basal cell migrate into the apical cell, through a pore formed at the point of contact.
Thus, the apical cell becomes binucleate and repeats the above events to form a second ascus initial cell. This occurs repeatedly, and several penultimate cells are formed which give rise to asci.
The penultimate, or ascus initial cells, elongate into a club-shaped structure. The two nuclei, which remained as a dikaryotic pair from the time of plasmogamy, fuse to form a diploid nucleus. Thus, the plasmogamy and karyogamy occur at different places; plasmogamy in ascogonium and karyogamy in the ascus initial cell.
The diploid nucleus undergoes meiosis resulting in the formation of four haploid nuclei. Segregation of mating types and other characters takes place during the meiotic division. The four nuclei, by a mitotic division, form eight nuclei.
Ascospores are organized around these eight nuclei by free cell formation. Some cytoplasm accumulates around each nucleus and later a wall is laid down. The cytoplasm left unused is known as the epiplasm, as probably serves as nutrition of the developing ascospores.
The ascus has proved to be a great help to the geneticist in the analysis of segregation of characters during meiosis. The meiotic products are arranged in an ordered fashion and can be taken out for genetic analysis. The segregation of characters, whether it occurs during the first or the second division of meiosis can be known. Because the final division is mitotic, converting the tetrad into 8 nuclei; pairs of ascospores have the same genetic constitution.
In all, there are six possible patterns of segregation of characters; two patterns, if segregation of alleles occurs in the first division of meiosis and four patterns if the segregation occurs during the second division of meiosis. The ratio of types 1 and 2 to the other types depends on the position of the allele on the chromosome in relation to its centromere.
Formation of Ascoma (=Ascocarp):
During the development of asci, the surrounding monokaryotic hyphae organize a thick protective coat around the developing asci. The wall is formed by prosenchymatous or pseudoparenchymatous fungal tissues. The protective coat and the asci are jointly called the ascomata. Three types of ascomata are known, viz. (cleistothecium, perithecium, and apothecium.
Cleistothecium, which is usually globose, is completely closed. The perithecium is ostiolate, that is, it has a pore and is usually flask-shaped. The apothecium is an open, cup-shaped ascoma having an exposed hymenium (layer of asci). The asci are usually elongated and club- shaped, though globose or ovoid asci also occur frequently. These asci, which may be stalked or sessile, are arranged in a layer called hymenium.
Intermingled with the asci in the hymenium are present sterile hair-like structures called paraphyses (sing, paraphysis), which help in liberation and dispersal of the ascospores. The ascus wall may be unitunicate or bitunicate and this is an important character in the taxonomy of Ascomycota. The unitunicate asci have two thin walls which appear as a single wall.
The bitunicate asci have an outer rigid wall and an inner extensible wall. The inner wall on maturity comes out after the rupture 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 in several ways – a pore, a slit, an operculum (lid) or by breaking of the apical wall. In a large number of Ascomycota, the ascospores are released forcibly.
When the ascospores reach a suitable substratum, they germinate and give rise to a monokaryotic mycelium.