Lecture #17

Pyrenomycetes 2 and Loculoascomycetes

Sordariales

• Not an important economic family, but of great value as experimental organisms
• Neurospora is the Drosophila of the filamentous fungus world

The order is characterized by:

• Dark or pallid perithecia or cleistothecia with periphyses and membraneous or leathery peridia, sometimes embedded in a subiculum
• Asci are ovoid to cylindrical and are produced in basal fasicles and usually have a thin refractile apical ring; ascus walls may be evanescent in some species
• Hamathecium is composed of both deliquescent paraphyses and pseudoparenchyma in many species
• Hyaline to dark ascospores are usually one- or two-celled, variously ornamented, and may bear gelatinous sheaths and appendages; germ pores occur in many species
• A number of phialidic anamorphs have been reported and some of these have spermatial function
• Most species are saprobes in cellulosic substrates including dung, wood, and soil
• Many groups to have polysporous asci
• Many cleistothecial species, especially from dung substrates, were described by Cain and Malloch, and these discoveries were the basis for deemphasis of ascocarp type as an evolutionary character at higher taxonomic levels

Sordariaceae

• Persistent cylindrical, basally clustered asci in perithecial forms and clavate asci in cleistothecial forms
• Ascus apex usually has a pore and refractile ring through which the ascospores are forcibly discharged
• Ascospores are typically dark brown, globose to ellipsoid, and one-celled, usually with gelatinous coatings and one or two terminal germ pores
• Ingold (1965) studied ascospore discharge in Sordaria. He found that the perithecial necks are positively phototropic. As the asci mature they swell and fill the upper part of the perithecium. One of the asci stretches and pushes through the ostiolar opening while its base remains attached to the perithecial wall. As the ascus tip protrudes, it discharges all of its spores explosively, collapses, and disintegrates, to be followed by each of the other asci in succession. This method of ascospore discharge is probably not confined to Sordaria, but may be the pattern of other members of this family

  (IMAGES)

• Neurospora sitophila is referred to commonly as the red bread-mold
• Ascospores are dark brown with an ornamentation of nerve-like ribs on the outer wall; ornamentation characterizes the genus Neurospora and gives it its name

Tripterosporaceae

• Similar to Sordariaceae, but tend to have inflated asci with narrow non-functional apical rings or no apical rings
• Brown ascospores are one- or two-celled, with one to four pores and bear gelatinous appendages.

Podospora

• May be the most often encountered member of the family, and is rarely absent from herbivore dung placed in moist chamber
• Podospora includes species with a perithecium, paraphyses among the asci, ascospores with an apical germ pore, an upper dark cell and hyaline lower one, and gelatinous appendages

Chaetomiaceae

• Usually globose or ovoid asci which lack an apical ring and deliquesce within the perithecium or cleistothecium
• Most species have conspicuous hyphal appendages on the ascocarp surface (Gr. chaite = long hair, mane).
• Straw and dung also are common substrates for these fungi.
• Cleistothecial members, Thielavia and Chaetomidium, and Achaetomium distinguished by cylindric asci.

Loculoascomycetes

Characterized morphologically by:

• Production of asci within locules in a preformed stroma (ascostroma) that constitutes the ascocarp
• Bitunicate ascus with two separable wall layers usually functional in ascus dehiscence After plasmogamy takes place, differentiation and deliquescence of part of the stroma leads to the development of a locule surrounding the asci and any hamathecial tissues that may be present.
• Thus, there is no special wall around the centrum structures such as the perithecium wall of pyrenomycetes such as Xylaria that also are produced in a stroma
• True paraphyses, which by definition arise from non-stromatic structures, are not produced in any members of the group, other types of sterile hamathecial structures may be produced. These include pseudoparaphyses and periphysoids
• Ascostromata of loculoascomycetes are quite variable in form and may be multilocular or unilocular. When they are unilocular, it may be extremely difficult to distinguish them from a true perithecium unless development is studied. Such a unilocular ascostroma is called a pseudothecium (Gr. pseudo = false + theke = box).

The ascostroma and its associated structures have been an important characters in delimiting orders of loculoascomycetes, and three basic centrum types have been defined (Luttrell, 1951). These major developmental types are:

1. Globose asci occuring singly in locules that are scattered or grouped in a fertile region in stromatal tissue. Hamathecial tissues are absent, and ascus discharge occurs after the asci break through the the stromal tissues (Elsinoe type of Myriangiales).

   (IMAGES)

2. The Dothidea centrum type is characterized by basal fasicles of asci produced in one to many locules of a stroma. No hamathecial tissues develop within the locules; however, the tissues of the stroma present between the developing locules may become stretched and elongated to resemble true hamathecium elements. An ostiole usually develops in the locule or the asci are released after rupture of the stromatal tissue

   (IMAGES)

3. The Pleospora type, has asci interspersed with pseudoparaphyses in the ascostroma, which is usually a pseudothecium. Pseudoparaphyses originate above the hymenial area and grow downward among the asci to fuse at the base of the locule and sometimes become free at the top.

   Ascus variation provides another taxonomic character

4. Generally "bitunicate" has been used to refer to any ascus that has a clear separation between the wall layers; several specific types are distinguished

5. Fissitunicate type includes
   1. Relatively thin ectotunica (primary or outer ascus wall) and thicker endotunica (secondary or inner ascus wall) with reshaping of the ascus wall after ascosporogenisis to accommodate apical structures; both walls may be lamellate;
   2. Complete separation of the ascus walls during spore release by rupture and collapse of the ectotunica from the apex and elastic expansion of the endotunica through the opening to act as a dispersal tube;
   3. Apical modification within the cytoplasm at the endotunica apex with a series of refractile spirals visible at the light microscope level (nasse apicale, Fr.= basket + apical, Chadefaud, 1982a,b) surrounding an apical or ocular chamber (Baral, 1992; Reynolds, 1989). The apical chamber is the cytoplasm that displaces the apical region of the endotunica; either of these apical modifications may be absent in some asci, and, when present, must be viewed at the right time of development to be seen at all; and
   4. A banded pattern often is seen in the endotunica. Forcible ascospore release is of the "jack-in-the-box" type. An opening develops in the ectotunica and the two wall layers separate completely with great elongation of the endotunica and expulsion of spores (Eriksson, 1981; Reynolds, 1989).

   Semifissitunicate and rostrate asci, occur in some members of the group, and these are distinguished by lack of separation of the two ascus walls during discharge.
   • Sometimes an apical ring is present in the endotunica. In the past the presence of an apical ring, especially one that is amyloid, has been used to distinguish the unitunicate ascus, a diagnosis that is incorrect
   • Other variants either the endotunica or the ectotunica may be deliquescent. We suspect that some species that may have diverged from bitunicate forms have been unrecognized because both ascus walls are evanescent

   • Well known anamorphs

   Alternaria ,Curvularia , Dreschlera

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