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The following points highlight the two families under which sphaeriales has been classified. The families are:- 1. Sordariaceae 2. Xylariaceae.
Family # 1. Sordariaceae:
The Sordariageae are saprophytes, mostly occurring on dung or on decaying plant parts. The distinguishing feature of this family is the somewhat beaked, superficial, separate perithecia. The perithecia are dark-brown or black with membranous or carbonous wall. Their long, club-shaped or cylindric asci are often interspersed – with paraphyses, in most of the species the paraphyses are evanescent.
The ascospores are dark-brown to black and variously ornamented. Conidial stages are infrequent. Sexual reproduction may be by somatogamy or spermatization. The two genera Sordaria and Neurospora are very well-known for their great value as tools for the genetical studies.
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Genus Neurospora of Sordariaceae:
Neurospora is one of the best known and most intensively studied genera in the Ascomycetes. The species of Neurospora, particularly Neurospora sitophila, the red bread mold or commonly known as bakery mold, frequently infest bakeries causing severe damage to the baking industry. It also causes considerable trouble as a contaminant of cultures, in bacteriological and mycological laboratories.
The genus Neurospora for the last few decades is of considerable interest to a large and diverse group of geneticists and biochemists who have established, Neurospora Genetics. Some of the pioneer workers are: Dodge, Lindegren, Beadle and Tatum. Neurospora sitophila, N. crassa, and N. tetrasperma have been widely used as tools for fundamental biological studies.
In the early 1840’s the occurrence of a pink mold on bread was detected in France and in 1-841 a French pharmacist brought the mold to the attention of a student of the fungi, who published a description, a name, and some detailed microscopic drawings. Sometime later, the mold caused such extensive spoilage in army bakeries, that a commission was appointed to examine the matter.
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Subsequently, the mold, was noted not only in Western Europe, but as far away as Brazil and Japan, and in the United States.
The mold was known as Monilia sitophila—monilia,’ from a word necklace, because -it bears spores in series like strings of beads, and ‘sitophila’, meaning bread-loving; and included under the Fungi Imperfecti.
In 1927, perfect stage of the mold was collected in Brazil, Japan, Surinam, and in Louisiana. All of these, and other strains lacking the perfect stage, were gathered together by B. O. Dodge and C. L. Shear and assigned to a new genus Neurospora and to four distinct species.
When Shear and Dodge (1927) described the genus Neurospora, four species were included:
Neurospora sitophila Shear and Dodge, N. crassa Shear and Dodge, N. tetrasperma Shear and Dodge, and N. erythraea (Moll.) Shear and Dodge. But up to 1969 the total number of recognized species of Neurospora is twelve.
Following is a key to the twelve species of Neurospora now recognized:
A. Asci four-spored
B. Mature perithecia mostly 250-300 µ,; conidial masses pale salmon
N. tetrasperma Shear and Dodge
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C. Asci about 250 p long; conidial masses orange
N. erythraea (Moll.) Shear and Dodge
CC. Asci about 190 n long; conidial masses pink
N. toroi Tai
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AA. Asci eight-spored
B. Perithecia embedded in a stroma
N. phoenix (Kze.) Dennis
BB. Perithecia not embedded in an expanded stroma
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C. Heterothallic with imperfect stage
D. Perithecia 200-300 n
N. sitophia Shear and Dodge
DD. Perithecia larger than 300
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E. Ascospores mostly 23 x 13 µ.N. intermedia Tai
EE. Ascospores mostly 27-30 x 14-15 µ
N. crassa Shear and Dodge
CC. Homothallic without imperfect stage
D. Ascospores with.one germ pore N. terricola Gochenaur and Backus
DD. Ascospores with one germ pore at each end
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E. Spores larger, mostly 30-33 x 14-18 µ, ribs and intercostal veins prominent.
F. Spores broad, mostly with continuous, conspicuous ribs
N. dodgei Nelson and Novak
FF. Spores narrow, occasionally with branched or anastomosed ribs N. galapagosensis Mahoney and Backus
EE. Spores smaller, mostly less than 25 µ long, intercostal veins distinct or incospicuous
F. Intercostal veins distinct; spores broad, 14-17 µ in width N. africana Huang and Backus
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FF. Intercostal veins indistinct; spores narrow, 10-15 µ in width- N. lineolata Frederick and Uecker
The species of Neurospora besides being pest in bakeries also live saprophytically on many decaying substrates of both animal and plant origin. Again some species have been obtained from soil.
The somatic mycelium is composed of rapidly growing pigmented hyphae. The colouration of hyphae varying with the substratum, may be red, orange dark-brown, or pinkish-grey. The hyphal cells are multinucleate.
Some of the species of Neurospora reproduce asexually by conidia. Again in others, conidial stage is absent. The vegetative hyphae produce two types of conidia borne on conidiophores.
The two types of conidia are:
large multinucleate macro-conidia produced abundantly on macro-conidiophores having moniliform in arrangement (Fig. 248A), and uninucleate micro-conidia growing singly or in short chains on micro-conidiophores (Fig. 248B). Both types of conidia germinate to form mycelium. The process may be repeated and the fungus can propagate itself indefinitely.
There are both homothallic and heterothallic species of Neurospora. The development of ascocarp is initiated by the formation of a short, stout, specialized hypha arising from an ordinary vegetative hypha growing up ‘at approximately a right angle to it. This hypha, which at first is without cross-walls, coils at the tip to form an ascogonium.
A septum soon develops in the stalk, and one or two branches arising below the septum grow out to form an envelope around the fertile central structure. The entire structure is the primordium of perithecium also known as protoperithecium.
The ascogonium develops into a multinucleate structure bearing a long hyphal branch which functions as a trichogyne. The antheridium is not developed in Neurospora. During ‘fertilization the conidium behaves as a spermatium either directly coming in contact with the trichogyne or germinates producing a germ tube and provides with the male nucleus.
In the heterothallic species the performance of sexual functions is regulated by compatibility factors which prevent self-fertilization. With the disappearance of antheridium development, Neurospora exhibits partial degeneration of sex. Once initiated, growth of the ascocarp is typically uninterrupted. The perithecial wall is composed of several cell layers, the outer portion of which becomes firm and dark-coloured.
An ostiole lined with well-defined periphyses develops in a short beak at the apex of the ascocarp (Fig. 249B). The asci arise from croziers. The young asci grow upward among paraphyses but the latter structures disintegrate as the asci mature and are not evident in mature fruiting bodies. The mature perithecia are dark-coloured, pyriform and beaked (Fig. 249A).
They may or may not be associated with stroma. Asci bear eight ascospores (Fig. 249G). There are both eight-spored and four-spored species of Neurospora. The ascospores of Neurospora are dark-brown or black and possess characteristic wall sculpturing in the form of ribs or veins running roughly parallel to the long axis of the spore (Fig. 249G).
Nelson and Backus (1968) studied the development of ascus in N. crassa and reported that the cytology of ascus development is similar to that of higher Ascomycees.
Life cycle of a heterothallic species of Neurospora is presented in Figure 250.
Some Indian species of Genus Neurospora of Sordariaceae:
Neurospora dodgei Nelson and Novak; N. tetrasperma Shear and Dodge.
Family # 2. Xylariaceae:
The Xylariaceae occur chiefly on wood; they constitute the highest development of the Sphaeriales and are characterized by the free, superficial stroma, though a few, including certain species of Hypoxylon, have stromata partly sunk in the substratum.
The stromata show every variety of form, from a spreading crust, to the almost spherical cushions of Daldinia, Hypoxylon, and the erect, simple branched expansions of Xylaria. The perithecia are arranged just below and at right angles to the surface of the stroma; their development may be preceded by the formation of conidia, which often cover young stromata with a white or brown powder.
Asci are 8-spored. Ascospores are non-septate, dark-brown to black.
Genus Xylosphaera (=XYLARIA) of Xylariaceae:
It is a cosmopolitan genus, but especially well represented in the tropics. It generally inhabits dead wood, rarely dung or dry fruits in wet and shady places.
All the species vary somewhat in shape and size. However, the general habit is fairly characteristic. The stromata are erect, more or less stalked, simple or branched and sometimes forked, more or less Club-shaped, cylindrical or fusiform (Fig. 251A to C), flesh white or pale buff to black. Xylaria polymorpha is commonly known as deadman’s finger for its typical stromata.
Stromata are sometimes partly covered with light- coloured conidia when immature. In some species the stroma becomes hollow at maturity, apex apiculate, free from perithecia (Fig. 251A, G). Perithecia embedded in the stromatic tissue (Fig. 251E, F), are in a single layer usually inserted beneath a black crust of stromatic tissue (Fig. 251G to F).
In a mature stroma, ostioles of the perithecia are visible from outside with unaided eye (Fig. 251G, D).
Perithecia are with protruding tips (Fig. 251G to F); Asci are cylindrical 8-spored and are mixed with paraphyses (Fig. 251F, G). The ascus tip contains a blue-staining apical apparatus pierced by a narrow pore. Ascospores are uniseriate, fusiform, inequilateral (i.e., with one side more strongly curved than the other) and often with a hyaline equatorial germ pore, non-septate, dark-brown to black at maturity (Fig. 251G, H).
From the ascospores develops an extended mycelium. Mycelial hyphae unite into thick strands; these, grow tall and show an intense heliotropism so that even when under bark or tree trunks they easily come to the outer surface. They are first differentiated into a black pseudoparenchymatic rind and a light fibrous core and then gradually develop to the cylindrical, clavate or branched black fructifications.
The growing tip of the stroma remains white for a long time and is covered with a strikingly regular hymenium of palisade-like conidiophores which, if unicellular, cut off ovoid conidia; if they are multicellular, however, at any position they cut off fusiform conidia at their tips.
The conidia form a white coating in marked contrast to the exposed portions of the black stroma, and justify the name “candle-snuff fungus”. After the disappearance of conidia, the stromatal branches swell clavately in the upper part and proceed to form perithecia. If the stroma is sectioned during the conidial stage, nests of small hyphae are found, and form the first indication of perithecia.
Note on of Xylariaceae:
Dennis (1958) claimed that the name Xylosphaera should have priority over Xylaria. But the proposal has not been welcomed by Petrak (1962), Holm and Muller (1965).
Some Indian species of of Xylariaceae:
Xylaria aemulans Starb; X. aspera Massee; X.fimbriata Lloyd; X. obovata Berk; X. tricolor Fr.
