In this article we will discuss about the range and development of ascocarps.
The fructifications or fruit bodies or ascocarps of the Ascomycetes are structures containing asci surrounded by, or enclosed within, protective tissue—peridium.
Their general structure, conditions of growth and shape are extremely variable. Ascocarps may occur singly or in groups, and may be superficial, erumpent (bursting through the substratum) or embedded in the substratum or host tissues. The origin and distribution of hymenium are also different in them.
The hymenium may consist of asci only or the asci may be interspersed with sterile hyphal elements, paraphyses (sing, paraphysis) (Figs. 205E, F and 206B) arising in the ascogenous layer.
The paraphyses may be simple or branched, sometimes coloured, sometimes inflated at the apex (capitate), or at intervals along their length (moniliform), and with or without septation. Paraphyses are said to space out the asci and to secrete mucilage which is concerned in spore discharge mechanisms. The fructifications may be flat expansions, crusts, rounded, cup-like, flask-like, alveolar, or erect club-like, etc.
But the most common and widely encountered forms are:
(i) A more or less rounded structure, completely closed, and having hymenium enclosed within the wall, a cleistothe- dum (pl. cleistothecia) (Fig. 205A, B);
(ii) A saucer- or cup-shaped ascocarp with wide open hymenium, an apothecium (pl. apothecia) (Fig. 205G);
(iii) A flask-like ascocarp designated as a perithecium (p1. perithecia) (Fig. 206A to C) perforated at the apex by a pore known as ostiole. Sometimes the ostiole is prolonged into a beak —rostrum and usually it is lined with hair-like hyphal structures, periphyses (sing, periphysis) (Fig. 206B) directed towards the opening and assisting in expelling asci or ascospores.
A perithecium has its own distinct peridium of specialized tissue and may or may not be embedded in a stroma and is thus designated as stromatic (Fig. 246G) or non-stromatic perithecium (Fig. 206A). In addition to these forms of ascocarp, there are others which are not so widely encountered.
They may be black, carbonous or leathery, boat-shaped or branched, etc. Some of them are designated as:
(i) A hysterothecium (pl. hysterothecia) (Fig. 206F), an elongated ascocarp closed when young, but opens at maturity by a long slit following a line of dehiscence parallel to the long axis extending almost the entire length of the ascocarp; and
(ii) A thyrkn thecium (pl. thyriothecia), also known as catathecium or catothecium (Fig. 206E), is an inverted saucer-shaped ascocarp having wall more or less radial in structure. The hysterothecium is often considered to be an intermediate form between an apothecium and a perithecium.
Again there are ascocarps that are developed as locules or cavities, especially ascigerous cavities, in a stroma where asci are directly borne. These locules are also designated as pseudothecia (sing, pseudothecium) or pseudoperithecia or as ascostromata (sing, ascostroma) (Fig. 206D). A pseudothecium may be uni- or multilocular in which asci are borne in the base of the locule. The asci may or may not be mixed with hyphal structures which are attached both to the roof and to the base of the locule.
These hyphal structures having no free ends are known as pseudo- paraphyses (sing, pseudoparaphysis), also designated as paraphysoids. The cavity of a pseudothecium may or may not be developed before the formation of asci and paraphysoids. In a pseudothecium, there is no special wall similar to what is present in a perithecium.
Only a cavity is present within the stroma in which asci are located. The development of a regular ostiole is absent in a pseudothecium, instead a pore or slit is formed at a point above the asci where the stromatic tissue is dissolved. Through this pore the ascospores escape.
Besides the above forms, the fructifications may be hypogean and closed, as in the genus Tuber (Fig. 207A). Ascospores are liberated by the decay or by the breaking of the fructifications by animals.
The development of fructifications in the Ascomycetes, up to the present time, has been closely studied in a comparatively few species. In general, die process is initiated by the stimulus produced during sexual reproduction.
In some Ascomycetes the ascocarp tissues are differentiated around the sex organs after plasmogamy, while in others the ascocarp tissues form first, forming a stroma within which the sex organs appear later.
In a fructification the hymenium and sterile tissue originate from different hyphal elements which in some cases, may be very well differentiated from the., very inception or at best in a very early stage of the fructification. Whereas, in others they are not clearly distinguishable.
Usually the development of hymenium and the steriletissue is simultaneous. Immediately after plasmogamy, the ascogonium gives rise to structures known as ascogenous hyphae from which asci are developed. The ascogenous hyphae are generally broader than the associated vegetative hyphae and have thinner walls, and dense easily stained contents.
They grow out radially to form a plate or hollow disk in the lower part of the fructification, but they may also ramify throughout the central tissue of the ascocarp. Along with this process, profuse growth of sterile hyphae takes place from the adjacent hyphal cells.
In a mature fructification the elements producing asci and the sterile hyphae become so intimately united and interwoven with one another that it is often difficult to separate or distinguish them.
But most fructifications differentiate into two, and in more highly developed ones three, Hyphal systems:
(i) Tissues derived from the ascogonium comprising of ascogenous hyphae and asci;
(ii) Vegetative protective tissue developed from the surrounding mycelium; and
(iii) Secondary protective tissue formed under stimulus from the ascogonium. All the tissues and structures enclosed by the peridium or other boundary layer of the ascocarp are known collectively as the centrum. Thus the centrum includes sex organs, ascogenous hyphae, asci and sterile tissues. The structural details and development of three widely encountered forms of fructification are discussed separately.
I. Cleistothecium:
A cleistothecium is a rounded, completely closed ascocarp which has no natural opening but brusts irregularly at maturity, or along sutures in the peridium. The cleistothecial wall may (Fig. 205A) or may not (Fig. 205B) be provided with outgrowths on its external surface. These outgrowths are known as appendages. The appendages when present serve character of taxonomic interest.
In a cleistothecium the hymenium may be distributed at different levels or at a particular area within the ascocarp. The number of asci in a cleistothecium may be one or more (Fig. 243).
Again the asci may be persistent or evanescent. The number of asci in a cleistothecium and whether the asci are persistent or evanescent are also characters on the basis of which genera of certain fungi are separated. The mature ascospores are released from a cleistothecium by external influences which cause the rupture or decay of its wall.
In Podosphaera, the process of cleistothecium development is very simple. Two hyphal branches put out short protuberances at the same time and are delimited by antheridial branch. The antheridial branch remains closely applied to the ascogonium and its upper extremity bends over and covers the apex of the ascogonium, and is delimited by a transverse wall, forming a short nearly isodiametric cell, the antheridium.
After plasmogamy the ascogonium divides by transverse wall into two cells, an upper which becomes the solitary ascus and subsequently produces eight spores, and a lower which is a stalk cell (Fig. 208E) bears the ascus.
The wall of the cleistothecium also begins to develop at the same time with the ascus. Tubular outgrowths (Fig. 208B) appear close round the base of the ascogonium on the hyphae which bear it and the antheridial branch.
These tubular outgrowths grow up round the ascogonium and in close contact with it and in close lateral contact with one another and with the antheridial branch, till they all meet together above the apex of the ascogonium.
Each one of the tubular outgrowths then divides by one or two transverse walls, so that the ascogonium is surrounded by an envelope formed of a single layer of many cells (Fig. 208G & D).
The cells then increase in size in the surface-direction, their walls thicken and assume a dark-brown colour and they thus form the outer wall of the cleistothecium from the outer surface of which number of hyphae with delicate ramifications are developed which ultimately mature into appendages (Fig. 208F).
Branches from the inner surface of the cells of the outer wall ramify and develop into a dense parenchyma-like weft resulting in the formation of the inner wall of the cleistothecium.
II. Apothecium:
An Apothecium is a cup- or saucer-shaped ascocarp in which the hymenium remains wide open. The hymenium is uniformly continuous and is developed lining the wide open surface of the apothecium (Fig. 205D). It is made up of asci and paraphyses (Fig. 205E & F). The paraphyses may be as long as the asci, longer or somewhat shorter. They are extremely variable in shape and may be septate or aseptate.
The tips of the paraphyses are usually unbranched. In cases where the tips are branched, the tips of the branches may unite above the asci forming a layer known as epithecium (pl. epithecia). The rest of the tissue of the apothecium is composed of interwoven hyphae.
The thin layer of interwoven hyphae immediately below the hymenium is the hypothecium (Figs. 205D, E and 209) whose hyphal elements are rather less dense than the rest of the tissue of the apothecium. The fleshy part of the apothecium which supports the hypothecium and the hymenium is the excipulum (Fig. 209).
The excipulum consists of two parts:
(i) The ectal excipulum is the outer layer of the apothecium, and
(ii) The medullary excipulum, forms the inner portion (Figs. 205D and 209).
Korf (1958) defined in details the types of tissue found in the excipulum (Fig. 209). According to him, the ectal excipulum is short-celled, in which the individual hyphae are not recognizable; and the medullary excipulum is with long cells, in which. The component hyphae are visible (Fig. 205D).
The short-celled tissue he subdivided into three types as follows:
(a) Cells globose with intercellular spaces—textura globuloaa;
(b) Cells polyhedral by mutual pressure, no intercellular spaces—textura angularis;
(c) Cells rectangular in section— long-celled tissue he divided into four subgroups. Long-celled tissue of medullary excipulum may have- (i) hyphae running in all directions;
(a) Hyphal walls not united, usually with distinct interhyphal spaces— textura intricata;
(b) Hyphal walls united, with distinct interhyphal spaces—textura epidermoidea; and (ii) hyphae are more or less parallel:
(c) Hyphae with strongly thickened walls, cohering—textura oblita;
(d) Hyphae without thickened walls, not cohering —textura porrecta (Fig. 210).
Since the hymenium is wide open, the discharge of ascospores from the asci is rather simple and often clouds of spores are seen on the surface of mature apothecium.
The apothecia are white or brilliantly coloured red, yellow, or orange. Some may be brown. A few are black. The apothecia may be stipitate or non-stipitate, that is, with or without any stalk or stipe respectively.
Besides the typical cups or saucers, there are various other forms of apothecia:
(i) Apothecium with a thick stalk and a cap or pileus on which the pitted or ridged hymenium looks like a sponge (Fig. 205G);
(ii) Apothecium having heavily ridged short or long stalk and the pileus may be quite large being saddle- shaped (Fig. 205H) or convoluted and brain like;
(iii) Aothecium may be tongue-, club-, or fan-shaped with long stalk (Fig. 239A to D);
(iv) Flat, circular, black, tar-like stroma which bear radiate wrinkles under which apothecia are formed (Fig. 205 I).
The development of apothecium in Pyronema confluens was imperfectly described by De Bary for the first time in 1863. Tulasne also added some information after De Bary. Later on Kihlmann studied in further details. The details of gametangial contact; plasmogamy; development of dikaryons and ascogenous hyphae; crozier formation; karyogamy; meiosis; and formation of asci and pores (Figs. 192 & 195).
Simultaneous with all these processes from gametangial contact to the development of ascospores, the vegetative cells at the base of the ascogonium and antheridium put out a growth of vegetative mycelium which surrounds the ascogenous hyphae and ultimately the asci (Fig. 211). The two elements form a compact structure to constitute the apothecium.
Corner (1929) defined three types of development of apothecia:
(i) Angiocarpic development, in which the tissues of the excipulum initially grow over and protect the developing hymenial layer;
(ii) Gymnocarpic development, in which the hymenium is exposed throughout its development; and
(iii) Hemiangiocarpic development an intermediate condition in which the hymenium is partially covered. However Corner did not attribute any significance to these types of development.
III. Perithecium:
The perithecia vary in shape from globose to pear-shaped or elongated to flask-shaped with long or short neck. They may be associated with stroma and are known as stromatic perithecia (Fig. 246G) or free of any stromatic connection, non-stromatic perithecia (Fig. 249B). Again, the stroma of stromatic perithecia may be only of fungal hyphae or it may be composed of a mixture of fungal hyphae and host tissue.
The stroma may not necessarily surround the perithecia, but may instead be reduced to a shield-like clypeus around the ostiole, or to a subiculum of stromatic interwoven hyphae below the perithecium.
A perithecium is furnished in the full-grown state with a schizogenous pore, ostiole through which asci or ascospores are discharged (Figs. 206G & E, 249B). It is a conspicuous feature of the perithecium. The ostiolar canal is lined by short hair-like growths known as peri- physes (sing, periphysis) (Figs. 206B & C, 249B).
A perithecium, stromatic or non-stromatic has a definite wall of its own with a true ostiole. In a stromatic perithecium, the differentiation of wall may not be sometimes very distinct.
The perithecial wall is usually formed of a dense hyphal weft or pseudoparenchyma (Fig. 206B). The outer surface of a non-stromatic perithecium may (Fig. 206A) or may not (Fig. 249A & B) bear numerous long hairs which when present have taxonomic importance.
In a perithecium, the hymenium either occupies a narrow bit of surface opposite the ostiole (basal portion of the inner surface of the wall) on which asci grow as a small tuft parallel and erect towards the ostiole, or is spread over the entire inner surface of the wall, and the asci then converge radially towards the middle line of the perithecium.
The asci are inserted in the delicate tissue lying inside the perithecium. They fill the inner space of the perithecium or at least the largest part of it, excepting the neck. All the space not occupied by them is filled with branches of the hyphae which grow out from the inner layer of the wall. Some of these branches which are basally attached to hymenium and lie between the asci are termed paraphyses (sing, paraphysis) (Fig. 206B).
Depending on fungi, the asci of perithecium may be evanescent or persistent. In case of evanescent asci the ascus wall with maturity of asci breaks down to form a mass of gelatinous material in which lie embedded the ascospores. This mass of gelatinous material with ascospores is discharged from the perithecium by hydrostatic pressure and remains at the mouth of the ostiole.
The ascospores are then transferred to distant places by the visiting insects which are attracted by the gelatinous material.
Whereas, in case of perithecia bearing persistent asci, the ascospores when mature, are forcibly expelled from them. The ascospores borne in the perithecial ascocarps are one to more than one-celled and are extremely variable in shape and size.
The perithecium of Venturia inaequalis is initiated by the development of a coiled hypha (Fig. 212A & B) arising within a stroma. The hyphal cells at the periphery of the stroma are Uninucleate, and their walls become thickened, whereas, the cells of the inner hyphae remain thin-walled and multinucleate.
One of these thin-walled cells produces a chain of cells, the ascogonium, each cell of which is bi- to quadri-nucleate.
The apical cell of the ascogonium becomes clavate giving rise to a well-defined trichogyne (Fig. 212D). Meanwhile, near the developing ascogonium another hyphal tip thickens and becomes lobate, and multinucleate. This is the antheridium (Fig. 212D). Its growth continues until the lobes contact the trichogyne and become closely applied to it (Fig. 212C).
A pore in the adjoining walls then forms, and the antheridial content empties into the trichogyne. Septa in the ascogonial chain are then, dissolved, whereupon the antheridial nuclei migrate to the ascogonium and become associated in pairs with the ascogonial nuclei (Fig. 212E to G).
Immediately after this, ascogenous hyphae arise as outgrowths from the ascogonium. Groziers are formed at the tips of the ascogenous hyphae (Fig. 212H). A binucleate penultimate cell forms, and the two nuclei fuse promptly to form the diploid stage. The ascus elongates, and three successive divisions of the diploid nucleus occur producing 8 haploid nuclei, the first of which is the reduction division.
The cytoplasm is delimited around each of the eight nuclei, and eight uninucleate haploid ascospores are formed. Meanwhile the uninucleate cells of the hyphae multiply to form the wall of the developing perithecium and the nurse tissue for the developing asci and paraphyses. Ultimately a perithecium with well-developed perithecial wall bearing ostiole and containing asci and paraphyses is developed (Fig. 212H).