In this article we will discuss about how does fungi in class Ascomycetes reproduce. Learn about:- 1. Reproduction of Genus Uncinula 2. Reproduction of Genus Eurotium 3. Reproduction of Genus Eupenicillium 4. Reproduction of Genus Peziza.
Reproduction of Genus Uncinula:
Large number of short, erect, hyaline conidiophores arises from the mycelium and cut a row of oval, hyaline conidia. A conidiophore has a stipe of one or more cells, a generative cell and one or more maturing conidia. The enormous number of conidia forms a powdery coating on the leaves – hence the name powdery mildew.
The conidia have high water content and need no free-water for germination. Maximum germination occurs in remarkably low humid atmosphere. It is interesting that free-water is inhibitory for germination and sometimes lethal to the conidia.
The conidia, largest in fungi (20-50 µm), are wind-disseminated. On reaching a suitable host, these germinate and form hyphae which lie externally on the leaf surface. The hyphae form haustoria which penetrate only the epidermal cells and derive nutrition without causing any visible damage to the host cells.
Coloured cleistothecia appear on the superficial mycelium at the end of the host season. Sex organs are borne on adjacent hyphae. A globular, single-celled and uninucleate ascogonium is formed by swelling of one of the cells.
The antheridium, which is also uninucleate, is stalked and is thinner than the ascogonium. Plasmogamy occurs when the intervening walls dissolve. Pairing of nuclei takes place. The ascogonium elongates and becomes 2-3 septate. Ascogenous hyphae arise from the central cell and form asci without formation of croziers. The ascus formation is accompanied by the development of the peridium from the surrounding somatic hyphae.
The peridium is 2-3 layers thick; the innermost layer consisting of thinner nurse cells (used up by developing asci). Appendages with coiled tips, a characteristic feature of Uncinula, arise from the outer most peridial layer. Appendages with coiled tips, and the many asci are the two diagnostic features of this genus.
Cleistothecia are minute, round, black structures, which represent the resting stage of the fungus. At the return of suitable weather, this breaks by the pressure generated due to swelling of asci after absorbing moisture. The asci spread out like a fan, and finally, when the ascus wall bursts, the ascospores are liberated. The ascospores germinate only on the host surface to produce the superficial mycelium.
In India, the cleistothecia of Uncinula have not been reported and, therefore, the recurrence of the disease is supposed to be through conidia only.
Reproduction of Genus Eurotium:
Conidia are produced in enormous quantity. The mycelium starts forming conidiophores very early. The long and erect conidiophores arise from a particular thick-walled foot cell. It forms a terminal swollen vesicle. Bottle-shaped structures, called conidiogenous cells, arise from all over the surface of the vesicle and produce chains of conidia at their tips.
When there are two layers of conidiogenous cells, the lower is called supporting cell. (The terms phialide, metula and sterigma are obsolete and not used now.) The neck of the conidiogenous cell is called the spore-producing tube. A portion of protoplasm with a nucleus inside the neck is delimited by a septum.
The protoplasm rounds off, develops a wall which fuses with the wall of the tube and becomes a conidium. The basal portion of the tube elongates and cuts off another conidium, and in this way conidia are produced at the tip of the conidiogenous cells. The conidia, which remain in chains, are globular, multinucleate and have a thick and rough wall.
The colour of the conidiophores and the conidia are responsible for the colour of the colony, which may be green (A. flavus), yellow (A. ochraceus), brown (A. tamari), black (A. niger), white (A. candidus), etc. The colony colour is an important criterion in the identification of species. The conidia, which are ‘dry’, are easily detached and disseminated by wind.
Degeneration of the sex organs, especially of the antheridium, is common. The antheridia and ascogonia are produced closely on the same hypha.
Both are multinucleate, helical and elongated structures which coil around each other. Antheridia may be functional or defunct. Pairing of nuclei, derived from antheridia or ascogonia, (or more commonly only from ascogonium), occurs only in the ascogonium. Ascogenous hyphae arise from the ascogonium and pairs of nuclei migrate into them.
The ascogenous hyphae grow to unequal lengths and produce asci at different lengths. Simultaneously, with the development of the asci, hyphae arise from the base of the ascogonium and collect around the sex organs and form a characteristic simple astomatous (non ostiolate) ascoma.
The pseudoparenchymatous wall of the ascoma—peridium and the asci, are jointly called as cleistothecium (pl. cleistothecia). The cleistothecia, which are spherical and small, (100-200 µm), contain large number of minute, 8-spored, globose asci. These are evanescent, and dissolve leaving the ascospores free inside the cleistothecium.
The cleistothecia open by natural withering or by rupturing from internal pressure created by maturing ascogenous hyphae. The ascospores have a distinct central peripheral groove which divides the ascospore into two equal halves. The ascospores resemble pulley wheels. The wall of the ascospore is sculptured. On germination, the ascospores produce germ tubes which give rise to the mycelium.
Reproduction of Genus Eupenicillium:
The dominance of the conidial state is greater in Eupenicillium than in Eurotium The characteristic conidial apparatus, which resembles a brush, is called penicillus (penicillium= small brush). Long septate conidiophores can arise from any cell of the hypha and not from a thick-walled foot cell, as in Eurotium.
The conidiophores branch once or twice at two- third of its total length. These branches are called branch cells (rami) and supporting cell, which finally bear the bottle-shaped conidiogenous cells. Conidia are produced in the neck region of the conidiogenous cell, as described for Eurotium. Long chains of conidia are formed, with youngest conidium at the base.
These are globose to ovoid and look like glass beads, under the microscope. As in Eurotium, the coloured conidia, green, blue or yellow, give the characteristic colour to the colony which helps in identification of the species. In some species, the conidiophores unite to form coremia (sing, coremium).
The uninucleate mycelium forms a swollen cylindrical ascogonium which is uninucleate in the beginning but by repeated nuclear divisions comes to have 64 nuclei. An antheridial branch, which originates on a separate hypha, coils around the ascogonium and cuts a terminal antheridium.
The wall at the place of contact of the two sex organs dissolves, but nuclear migration from the antheridium has never been observed. The male nuclei can be seen in the antheridium even after the formation of ascogenous hyphae. Sometimes, the antheridial branch appears late, and by the time it is formed, the ascogonium is in advanced stage of fruiting.
Thus, the female nuclei in the ascogonium pair and form the dikaryons, which by conjugate divisions form several dikaryons. Septa are formed in the ascogonium. Ascogenous hyphae arise from the binucleate cells of the ascogonium and produce asci at different levels. The somatic hyphae, in the meantime, grow and surround the ascogenous hyphae and asci, form a two-layered wall (peridium).
Ultimately, a cleistothecium is formed. On the basis of determinate or indeterminate growth of the ascoma, Pencillium is called by different generic names. If it continues growing in size (indeterminate growth), the fungus is called Talaromyces, if it stops growing (determinate growth) after attaining a definite size, the fungus is called Eupenicillium.
The asci are globose and evanescent; liberate the ascospores in the cleistothecium. The ascospores are freed after the decay of the cleistothecial wall. The ascospores, as in Aspergillus, resemble pulley- wheels and on germination form a germ tube, which finally develops into the somatic mycelium.
Reproduction of Genus Peziza:
It is rare. However, in P. repanda and P. vesiculosa, Oedocephallum, an anamorphic fungus, is found to be the conidial stage. But its role in spread of the fungus is negligible.
Somatogamy between hyphae of opposite strains (in heterothallic species) or between two cells of the same hypha (in homothallic species), results in plasmogamy and formation of a dikaryotic cell. Sex organs are not formed at all. Ascogenous hyphae arise from the dikaryotic cell and the fruiting body is organized around the developing asci. Eventually, a cup-shaped apothecium is formed.
The apothecium consists of three parts:
(a) The hymenium
(b) The hypothecium, and
The hymenium, which consists of a layer of asci and paraphyses, lies at the inner surface of the cup. The asci are club-shaped or cylindrical having eight, obliquely arranged ascospores. The paraphyses, which are found intermingled with the asci, may be longer, equal, or shorter than the asci. Sometimes, the tips of the paraphyses branch, which unite to form a layer called epithecium, covering the hymenium.
The hypothecium is a thin layer of interwoven hyphae lying below the hymenium. The main body of the apothecium is called the excipulum. It may be divided into an outer ectal excipulum and an inner medullary excipulum. The ectal excipulum is made up of pseudoparenchymatous tissue, while the medullary excipulum is made up of proenchymatous tissue.
The asci in the hymenium are erect at the bottom of the cup and curved on the side walls. This arrangement of asci at different angles is to facilitate upward vertical discharge of the ascospores. The ingenious device is the result of positively phototropic nature of asci and the paraphyses. The ascospores are ejected by ‘puffing’ in instalments. When the mature asci are in a stretched condition, the operculum is thrown open even by a little jerk of wind. The ascospores are violently ejected in large numbers and form a miniature cloud.
When the ascospores, reach the humus-rich soil or moist wooden log, they germinate and form the haploid mycelium, which bear the cups that crowd on manure heaps.