ASCOMYCOTA: PYRENOMYCETES, DISCOMYCETES AND LOCULOASCOMYCETES

In this exercise, we will look at the other classes of Ascomycota - Pyrenomycetes, Discomycetes and Loculoascomycetes. As we examine the different members of these classes, compare and contrast them with each other and the two other classes we studied last week.

III. CLASS PYRENOMYCETES

In the Pyrenomycetes, the asci are borne in a layer or fasicle in the ascocarp. A few species form only a single ascus per ascocarp. The ascocarp is usually an ostiolate, flask-shaped perithecium. The internal tissues of the ascocarp comprise the centrum, and in ostiolate species the ostiole is lined with slender periphyses. The asci are variable in shape and usually contain eight ascospores. In most genera the asci are persistent, with a characteristic apical apparatus through which the ascospores are forcibly discharged. Members of the pyrenomycetes occur in a variety of habitats, but are especially common on dead wood, herbaceous stems, and in soils. Many species are important pathogens of economic crops.

A. ORDER CHAETOMIALES

Members of this order have dark brown to black, ostiolate ascocarps that are covered by characteristic hairs. The centrum in young ascocarps contains lateral paraphyses. The asci are evanescent and the one-celled dark brown ascospores are extruded in a cirrhus. Most species are cellulose decomposers. There is a single family Chaetomiaceae.

Chaetomium globosum - This is one of the most common species and has wavy hairs on its perithecia. Examine cultures under the dissecting scope and observe the hairs. Look for cirrhi of ascospores. Mount young ascocarps in a drop of water, crush gently, and examine asci and ascospores.

B. ORDER SORDARIALES

In some species of this order, the wall is thin and translucent. The ascospores are typically unicellular and dark brown, with a germ pore or a germ slit, but in some genera the ascospores are two-celled especially when they are young. Anamorphs are phialidic or blastogenous. Members of this order are saprobic, often coprophilous, but they also occur in soils and seeds. We will consider two families here, the Melanosporaceae and the Sordariaceae.

1. FAMILY MELANOSPORACEAE

Members of this family do not have paraphyses. Their asci are deliquescent and the ascospores have germ pores.

a. Melanospora - This genus has brown, transparent perithecia with evanescent asci and one- celled, brown ascospores. It usually grows in association with another fungus; it is not parasitic but derives some essential nutrients from it. Mount some perithecia of Melanospora zamiae in water and examine the asci and ascospores.

b. Thielavia - This is a common soil fungus with dark brown, completely enclosed ascocarps and evanescent asci. Mount young ascocarp and look for asci of Thielavia sapilonium. Because of the enclosed ascocarp, this genus has traditionally been placed in the plectomycetes, but recently some workers have moved it to the pyrenomycetes because of the ascocarp morphology and the relatively large ascospores which have a germ pore.

2. FAMILY SORDARIACEAE

Unlike the Melanosporaceae, the members of this family have paraphyses which are filamentous when young and appear vesiculose at maturity. The asci are persistent and the ascospores have germ pores. a. Sordaria - Ascospores of this genus are one-celled, dark brown and smooth. The ascospores are also surrounded by a gelatinous sheath. Mount and crush some perithecia of Sordaria fimicola. Examine the asci and note the characteristic ring at the apex. Examine the ascospores and note the gelatinous sheath and the germ pore in the end of the spore.

Count 10 hybrid asci from the cross you made from last week and score the phenotype of each. Refer to the Student Guide for Sordaria genetics given last week. (This will also be explained in class).

b. Monosporacus cannonballus- This genus has recently been reported to infect cucurbits. It has one ascospore per ascus. Examine cultures on V-8 agar producing perithecia. Mount some perithecia and observe the asci and ascospores.

C. ORDER XYLARIALES

This include members with typically ostiolate perithecia that contain paraphyses. The asci have a well-developed, amyloid, apical apparatus, and the ascospores are one celled, dark-brown, with a germ-slit. The ascocarps are embedded in a true stroma composed entirely of fungal tissue with the ostiole protruding. There is a single family, the Xylariaceae.

Xylaria - Members of this genus form erect, dark-colored stroma on dead wood. Examine dried specimens of Xylaria hypoxylon, then look at prepared slides showing cross sections through stroma. Note the arrangement of the perithecia, asci, and ascospores. The ascospores are one-celled and dark brown.

D. ORDER DIAPORTHALES

The Diaporthales are perithecial fungi having a pseudoparenchymatous centrum and unitunicate asci. The asci have short deliquescent bases and lie free in the centrum at maturity, until they are pushed out of the ostiole. The apex of the ascus has a characteristic thickening which appears as a refractive ring under the light microscope. The perithecia are often black and in many species are embedded in a stroma. The arrangement of the perithecia in a stroma is an important taxonomic character. These fungi are often found on leaves and woody tissues, both as saprobes and parasites. All are placed in a single family, the Diaporthaceae, but some mycologists recognize more than one family.

Cryphonectria - This genus forms fleshy, light-colored stromata on woody stems and twigs. The ascospores are hyaline and two-celled. Examine prepared slides of Cryphonectria parasitica showing sections through a stroma and note the arrangement of perithecia. This species is the causal organism of the disease known as chestnut blight, and was formerly placed in the genus Endothia.

E. ORDER CLAVICIPITALES

These are perithecial fungi with lateral paraphyses and unitunicate asci. The perithecia are produced in a well-developed stroma which is usually light-colored. The asci are long and cylindrical, with a thickened apex, and they contain eight ascospores. The ascospores are filiform, hyaline, and septate. Most species are parasitic on grasses, insects, or other fungi.

Claviceps - In this genus, the perithecial stroma develops as a head on an erect stalk and arises from a dark sclerotium. Examine preserved material of Claviceps purpurea, the cause of ergot disease of cereals. Look at prepared slides showing sections through young sclerotia bearing the conidial state, Sphacelia segetum. Also examine prepared slides showing sections through sclerotia bearing perithecial stromata. Note the perithecia, asci and ascospores.

IV. DISCOMYCETES

The discomycetes are characterized by the possession of an ascocarp, the apothecium, that is open at maturity, exposing the asci. In some species the apothecium is open from the start, while in others it begins as a closed structure that opens at maturity. Typically the apothecium is cup- or saucer-shaped, but it may vary greatly in shape and structure, depending upon the species. The asci are borne in a hymenium, usually interspersed with sterile paraphyses. Asci can be operculate or inoperculate, and the ascospores are forcibly discharged. Discomycetes grow in a wide variety of habitats; saprophytes are found in soil soil and on dead plant debris, while parasites are seen on a variety of plants.

A. ORDER PEZIZALES

The order Pezizales includes all epigean discomycetes with operculate asci. The apothecia are usually fleshy, brightly colored, and relatively large. The asci are borne in a distinct hymenium. The ascospores are all one-celled, hyaline, brown, or occasionally purple. Members of this order are found on soil, wood, dung, and plant debris.

1. FAMILY PEZIZACEAE

The asci of this family have terminal opercula and turn blue when stained with iodine.

Peziza - In this genus, the apothecium is discoid to cup-like, centrally attached, and not gelatinous. Examine prepared slides showing a section through an apothecium.

2. FAMILY HELVELLACEAE

Members of this family differ from the Pezizaceae in that their asci do not turn blue in iodine. In addition, they have tetranucleate ascospores. Their apothecia are also large, usually with a stalk, and often saddle-shaped.

Helvella - In Helvella, the ascocarp is cupulate to saddle-shaped, with a stalk. The ascospores may be smooth or warted. Examine prepared slides of Helvella and note the asci and the ascospores inside. 3. FAMILY MORCHELLACEAE Members of this family have asci that turn blue in iodine and their ascospores may contain 20-60 nuclei. Apothecia are usually large, discoid to stalked and sponge-like. Morchella - The ascocarp has a stalk, with a hollow, sponge-like pileus. Examine preserved specimens of Morchella esculenta, the common morel. Look at prepared slides showing sections through the hymenium.

B. ORDER HELOTIALES

This order contains inoperculate discomycetes with forcible spore discharge in which the apothecia are not formed in a stroma. The asci are more or less clavate and have an apical pore through which the spores are discharged. They are usually found as saprobes or plant parasites, raely on soil or dung.

1. FAMILY SCLEROTINIACEAE

These discomycetes have cupulate or discoid apothecia, arising from a stroma or sclerotium, or from stromatized host tissue, usually brownish and stalked.

Monilinia - In this genus the apothecia arise from stromatized host tissues or mummified fruits. The ascospores are hyaline. The conidial state is Monilia. Examine prepared slides of Monilinia (Sclerotinia) fructicola showing a section through an apothecium.

C. ORDER TUBERALES

In the Tuberales the closed ascocarps are hypogeous and the ascospores are not forcibly discharged. The spherical or cylindrical asci are arranged in an hymenium. There are eight or fewer ascospores per ascusand the number may vary among asci in the same ascocarp. The ascospores are one-celled and hyaline or brown.

1. FAMILY TUBERACEAE

This family is characterized by hollow to chambered ascocarp. The asci are persistent and irregularly arranged or in a distinct hymenium.

Tuber - In Tuber, there is no hymenium, the ascocarp cavities are filled with hyphae, and the ascospores are sculptured. This is the truffle, prized for gourmet eating. Look at preserved specimens and examine prepared slides showing a section through an ascocarp.

V. CLASS LOCULOASCOMYCETES

The Loculoascomycetes are characterized by having bitunicate asci. The ascus wall consists of two layers, a rigid outer layer, the ectoascus, and a thick, extensible inner layer, the endoascus, which extends through the ectoascus at the time of spore discharge. The ascocarp or pseudothecium, begins as a stroma in which the ascogenous system is differentiated. As the asci develop a locule is formed around them. The ascostromata may be uni- or multi-loculate. The uniloculate stromata, the pseudothecium may resemble the perithecium of the pyrenomycetes. In most species the ascospores are forcibly discharged through an ostiole. The ascospores may be multicellular and hyaline or brown.

A. ORDER PLEOSPORALES

The Pleosporales have pseudothecia that have pseudoparaphyses. These fungi commonly occur on plant materials, such as dead leaves, herbaceous stems, branches, and wood, as well as on green leaves and stems.

1. FAMILY VENTURIACEAE

Members of this family have small pseudothecia and ascospores with a single septum and thin walls.

Venturia - the pseudothecia are separate and immersed in the substrate. The ascospores are unequally septate and greenish-yellow to pale olive brown. Examine prepared slides of sections through pseudothecia of Venturia inaequalis, the cause of apple scab. Note the asci with two unequal sized ascospores.

OTHER ACTIVITIES:

Examine your dung cultures and observe macroscopically and microscopically the different fungi growing on them. Try to get an idea of which taxonomic groups appear earliest and which arrive later, as well as which ones are ephemeral and which are long lasting. Record your observations. There will be several references available in the lab to help you figure out what you are seeing. Be sure to moisten the paper towel if they are dry already. We will keep your dung for one more week or even longer. Observations should be made as frequent as possible so as not to miss any species.

TERMS TO KNOW:


Hypogeous/Hypogean - growing below the ground
Epigeous/Epigean - growing above the ground
Hymenium (pl. hymenia) - a fertile layer consisting of asci or basidia
Cirrhus (pl. cirrhi) - a ribbon-like cylinder of spores held together by mucus as it issues from an ostiole
Centrum - (pl. centra) - the totality of structures enclosed by the ascocarp wall
Periphyses - short, hair-like growths in the form of a fringe lining the inside of an ostiole or a pore in a stroma
Paraphyses - sterile, basally attached structures in a hymenium
Pseudoparaphyses - sterile threads attached both to the roof and to the base of a pseudothecium
Stroma (pl. stromata) - a compact somatic structure, much like a mattress, on which or in which fructifications are usually formed
Deliquescent - becoming liquid by absorbing moisture
Evanescent - vanishing or disappearing quickly
Persistent - continue to exist for a long time